Charging LiFePO4 batteries from a vehicular electrical system - the problems and a solution.

Figure 1:
The Renogy RNG-DCC1212-20 - an isolated
and current-limited battery charger, intended
for use with vehicle electrical systems.
Click on the image for a larger version.

There are times - usually on a camping or road trip - where I would like to charge my LiFePO4 batteries en-route, from the vehicle.  The "need" is largely the result of having one of those coolers with a built in compressor:  It runs about 10-30% of the time at normal room temperature and pulls up to about 3.5 amps when doing so - but it's also advantageous to be able to keep a battery topped off in the event that you didn't start the trip with a fully-charged battery in the first place.

To do this, one may be tempted to connect the battery directly to the vehicle's electrical system, as might have done in days past with a lead-acid battery.

DO NOT do this with any lithium battery - at least not directly.

In short, you cannot and should not parallel a LiFePO4 battery with an existing charging system intended for lead-acid batteries.  The biggest issue with doing so is that unlike a lead-acid battery, a LiFePO4 battery will attempt to charge with all available current, likely resulting in blown fuses, heated wires and burnt-out alternators. A secondary issue has to do with the BMS (Battery Management System) of the LiFePO4 simply disconnecting abruptly when the battery is fully-charge, potentially causing voltage spikes capable of damaging vehicle electronics and possibly, the BMS itself.

See the section "Why you need to treat LiFePO4 batteries differently" in the "tl;dr" section near the end of this article (link) for more details as to the problems that can occur.

A solution

The solution to the issues noted above lie largely in limiting the charging current.  One way to do this would be resistively - perhaps with the use of intentionally small-gauge wire and/or resistor or incandescent automobile headlamp in series.  This will, by its nature, generate heat as it's inefficient - and it can generate quite a bit of heat (potential fire risk here!) - but this is the way one might have accomplished this in years past.

This sort of limiting may occur unintentionally if one charges via, say, a cigarette lighter/accessory plug connected with light-gauge wire, but this is sort of a "kludge".  One issue with this is that it can cause frequent blown fuses as the current isn't regulated and if the user attempts to circumvent this by using a higher-current fuse, damage to the electrical system (or even fire) can result.  If the connection is made to a power source that is switched on/off with the ignition, a connected battery can "back feed" the electrical system which can result in the battery being discharged when the vehicle is off or, worst case, damage to both the vehicle and battery.

These days one would use a current-limited and regulated voltage DC-to-DC converter with its power source connected as close to the vehicle battery as possible.

One of the many devices out there that will fit the bill is the Renogy RNG-DCC1212-20 (pictured above), available at the time of the original posting of this article for around US$100:  Using DIP switches, the type of battery (lead-acid, Lithium-Ion or LiFePO4 - I configured for the latter) may be selected along with the charging profile/voltage - and the device will limit the maximum charge current to just 20 amps, selectable to 10 amps with the addition of a jumper wire to the "LC" terminal.  What this means is that no matter the charge state of the LiFePO4 battery, the current being pulled from the vehicle's electrical system will be limited - very useful if one expects to avoid blowing fuses, destroying alternators, or burning up wiring.  (Note:  I have no vested interest in Renogy, they just happen to make one of the readily-available devices that is appropriate for this task.)

Additionally, it's rated to operate from between 8 and 16 volts while maintaining a constant output voltage (once the output current has dropped below limiting) that is independent of the voltage from the vehicle's electrical system. The Renogy is also an isolated DC-DC converter in that there is no electrical connection between the input and output terminals:  By being isolated, circulating currents (through the chassis or other "sneak paths") can be completely avoided which may be helpful for some sensitive equipment and/or to minimize/eliminate alternator "whine".

This particular unit is rated for up to 20 amps output current.  Rated at about 90% efficiency, it will take more power on its input connections than it will output, producing a bit of heat (which is why it has internal fans).  Also note that the current pulled by the unit will vary depending on the voltage input despite the fact that the output voltage and current - and overall power consumption - may remain constant.

For example, let's say that the unit is outputting 20 amps at 14.5 volts, representing a LiFePO4 battery that is nearly fully-charged representing an output power of 290 watts:  Assuming 90% efficiency, the unit will actually consume 322 watts with the difference (32 watts) as heat.   At an input voltage of 12.0 volts,  322 watts is 26.8 amps, but at 14.0 volts, 322 watts is just 23.0 amps.  The fact that it can pull more current from the source supply than it is outputting - particularly when the input voltage goes down - must be taken into account when sizing the wire and selecting the fuse rating.

You can't just connect it and walk away!

The Renogy has a "D+" terminal that, when connected to a voltage source, will activate it.  The intent is that this wire is connected to some part of the vehicle's electrical system that is likely to be on when the engine is running to charge the battery - such as the "accessory" circuit.  The reason for this is that the Renogy itself has no useful low-voltage disconnect:  If you connected it to the vehicle's electrical system with the engine off, it will happily attempt to charge the battery to which it's connected - and if the battery being charged is a large, discharged LiFePO4 battery, it will likely run the vehicle's battery down completely in doing so.

For a permanent installation in a truck, van or RV, finding a wire that is only active when the engine is running (or, perhaps, the ignition is just "on") makes sense - but in my case I have no need for a permanent installation of the unit - plus, I don't have room to mount the unit and am unwilling to connect/disconnect an "ignition on" wire from the electrical system every time I install/remove it.

One way around this would be to monitor the battery voltage:  If it's above about 13.2-13.5 volts, one can be assured that the engine is running, but it will drop fairly quickly when the engine is off as the lead-acid starting battery's voltage drops.  Unfortunately, the Renogy's only means of low-voltage cut-off is set to 8 volts (a very "dead" 12 volt battery!) which requires that I come up with another way of enabling/disabling the device. 

Another issue is that whenever  the unit is on (the D+ line is active) but unloaded (e.g. no battery connected to the output) it consumes about 250 mA at 14 volts - increasing to over 500mA at 10 volts - and more than this if its cooling fans are running:  This sort of load may run a battery dead in a few days at best, so there had to be a way of completely disabling it and eliminating current draw.

A voltage-controlled switch

In poking around, I noted that without the "D+" line connected to a voltage source, the Renogy drew no detectable current meaning that I could leave the high-current input leads connected full-time:  By switching just the D+ lead I could enable/disable the device as needed without the need of a heavy-duty relay.  (Judging by the "clunk" that one hears when applying power to the D+ line, the Renogy probably has such a relay built into it.)

As the D+ line itself drew very little current (only about 3 milliamps) and anything above about 4 volts seemed to reliably trigger it, it would take almost nothing to drive it so the circuit could be very simple as the diagram below shows:

Figure 2:
Schematic diagram of the low-voltage cut-off circuit with hysteresis.
This circuit provides an "on/off" control of the converter to the "D+" line based on the
voltage at the "V+" and "V-" connections.
Click on the image for a slightly larger version.

How it works:

The "V+" and "V-" lines are connected across the unit's input terminals to monitor the voltage applied to it.  Resistor R1 scales the input voltage to a lower value to apply to the top of R2, a 10-turn trimmer potentiometer, that is used to divide the voltage down to the 2.5 volt threshold of U1, a TL431 "programmable Zener" via its "reference" terminal.  Capacitor C1 connected across the top of R2 provides a degree of filtering to reduce the probability of the circuit from responding to noise on the electrical system.

Figure 3:
The prototype, built on a scrap of proto board.  This uses
uses through-hole components, but could have been built to be
much smaller using surface-mount devices.  The capacitor has
been lifted up to allow a better view of the components.
Click on the image for a larger version.

Resistor R3 limits the current into U1 and R4 limits the current into Q1 while R5 keeps the emitter-base voltage of Q1 high when U1 isn't conducting, turning it off, resulting in no voltage on the "Out" lead and in this state, with the Out lead connected to the Renogy's "D+" connection, the unit would be powered down and draw no current.  Resistor R6 offers protection to the circuit in case the "Out" terminal is momentarily shorted to ground.

If the voltage on the reference terminal on U1 exceeds 2.5 volts, it turns on, pulling the bottom of resistor R3 toward ground, turning on Q1 and causing the "Out" lead to go high, enabling the Renogy via the "D+" line. When this voltage goes high, resistor R7 feeds back a slight amount of current into the junction of R1/R2, very slightly increasing its voltage, lowering the circuit's turn-off voltage slightly but leaving the turn-on voltage unchanged:  The value of 270k shown causes this voltage difference between "on" and "off"  to be about 0.9 volts while a value of 680k results in a threshold difference of about 0.3 volts.

This threshold difference between turn-off and turn-on (a.k.a. hysteresis) is very important to the stable operation of this circuit.  If the voltage applied to the circuit were just above the threshold (by a fraction of a volt) the "Out" lead would turn on and activate the Renogy.  When this happened, the Renogy would start drawing current, causing the voltage to drop slightly through wire losses and load on the electrical system - but if this voltage dropped below the threshold, the "Out" lead would turn off again and the current consumption would stop, causing the voltage to rise again and turn it back on, causing an endless "on-off" cycle.  

By adding such hysteresis - and making sure that the voltage drop under load was comfortably less than the hysteresis amount - the unit will reliably turn on at the high voltage threshold and will not turn off until/unless the voltage drops below the low voltage threshold.  It is also imperative that this unit be connected as close to the battery (with appropriate fusing!) with as short and heavy leads as practical:  Too-light wiring will cause the voltage to drop under load, possibly causing it to trip out due to low voltage - only to be re-enabled immediately (e.g. the "on/off" cycling mentioned above.)  The need to minimize voltage drop is one reason why the power source should be connected as near the battery/alternator as practical.

Figure 4:
The completed unit in heat-shrink tube.  There are no
exposed electrical connections - just the adjustment at the end.
Click on the image for a larger version.

Enabling the Renogy by voltage detection alone isn't quite as reliable as having a connection to the ignition circuit of the vehicle, but it will work "well enough" and prevent the vehicle's battery from being flattened by the unit staying on all of the time, when the engine is off.

Figure 3, above shows the prototype unit, built on a small piece of prototype board.  R2, the 10 turn potentiometer is the blue device on the far right with U1 being the black object to the left of it with Q1 being on the far left.  In this photo, capacitor C1 is bent up, out of the way to allow a view of the components underneath where it will be laid over.

Figure 4 shows the same circuit covered with some yellow heat-shrink tubing to hold the components together and to protect it from external short circuits.  The end of the adjustment resistor, R2, protrudes from the end of the tubing so that it is accessible.

Installing within the unit

Figure 5:
The circuit within the converter.  The DC output
terminals (to the battery being charged) are in
the lower part of the image.
Click on the image for a larger version.

Not wanting to have any more of a maze of wires outside the device than necessary I installed the circuit inside the Renogy unit itself as seen in Figure 5.  Using some "Shoe Goo", a strong rubber adhesive (do not use "hot melt" glue!) the encapsulated board of Figure 4 was mounted in the upper-right corner of the "output" side of the unit, set back by about 3/8" of an inch (10mm).  The location is such that the voltage threshold adjustment is accessible via one of the ventilation holes:  Setting it back prevents it from obstructing air flow and makes the precise alignment between the screw of the potentiometer and the hole less critical.

The "V+" and "V-" wires from the circuit are soldered directly to the bottom of the board on the DC input terminals and the "out" terminal of the circuit (the blue wire in Figure 5) is routed through another hole near the green "D+" and "LC" terminals.

Figure 6 shows how these wires are routed.  In addition to the connection to the "D+" terminal from the circuit, another wire and a switch was added that optionally connects the "LC" terminal to the "D+" to set the Renogy to the "Low Current" mode by pulling it high when the switch is closed - in this case, limiting the maximum charge current to 10 amps, which may be useful if you are connecting the unit to a current-limited power source (e.g. "cigarette lighter" plug) that cannot supply the 25-ish amps current input that the unit may draw when charging at 20 amps output.

Figure 6:
Looking on the "output" side of the Renogy, this shows how
the "out" wire from the circuit routes out of one of the air
to the "D+" terminal.  Also shown is a switch that optionally
connects the "D+" to "LC" terminal for just 10 amp max.
Click on the image for a larger version.

This "modification" - since it does not involve drilling any holes - is "reversible" if desired as the circuit and wiring could be easily removed.

In-vehicle testing and use

High/low voltage turn-on/turn-off

Prior to testing the modified unit in my vehicle I set the "cut-in" voltage to about 13.65 volts which resulted in a disconnect voltage of around 12.7 volts - a voltage below which a 12 volt lead-acid battery will quickly drop when charging is stopped.  As expected, the unit did not get turned on until a few seconds after the engine was started, the voltage rising due to charging by the alternator:  If the battery had been heavily discharged and a lot of accessories were running (headlights, blower, wipers) it may take longer than this for the voltage to rise above the threshold.

The voltage dropped below the 12.5-12.7 volt shut-off threshold within a few 10s of seconds of turning off the engine with the entire unit drawing only about 0.5mA (all of that being from the added circuit) in that state - far lower than the vehicle's own quiescent current, and probably lower than the vehicle battery's self-discharge rate.  So far, I have found no tendency for the unit to cycle on and off while the engine is running - even if the headlights, heater blower and windshield wipers are on.  (As noted in a sidebar below, cycling did start to occur, but this was traced to the adjustment potentiometer setting having drifted upwards by about 0.4 volts, likely due to vibration.)

Of course, the voltage thresholds mentioned above are only valid for a healthy (and properly functioning) conventional charging with lead-acid batteries as part of the chassis electrical system:  If your vehicle somehow has a different type of electrical system than the conventional "alternator + lead acid" configuration it'll be up to you to determine how and even if a solely voltage-referenced on/off system like this can be done.

RF Noise generation

Being an amateur radio operator, I was concerned that this unit might produce an excess of radio frequency interference as it contains a high-power oscillator in its power converter.  While visual inspection of the Renogy (with its end covers removed) showed that it does have some filtering of its own in the form of series inductors and capacitors across the input/out leads and to the metal case (to suppress common-mode and differential RF energy) it would be unusual for even a well-designed commercial device to go to extremes in reducing radio frequency energy to the point of extinction. 

Using a "Tiny SA" Spectrum analyzer I connected directly to the input and output leads - using a 0.002uF capacitor to block DC and protect the analyzer - I measured the amount of RF energy being differentially emitted from the unit.

This measurement is important in that if the instantaneous RF voltage on the output leads is different than on the input leads, the in/out cables will necessarily conduct RF energy to the outside world, into whatever is connected at both ends, including the wiring itself, which may radiate like a dipole antenna and/or conduct radio-frequency current through the unit and into other wiring and/or equipment.  A plot from the spectrum analyzer showing the produced RF energy up to 10 MHz is shown below:

Figure 7:
The spectrum of RF energy as measured directly between the voltage in and out terminals across the range of 0-10 MHz with no filtering.  If a receiver's input terminals were connected directly to the DC terminals, the signal level at 40 meters (7 MHz) would be bit more than "10 over S-9.

Without any added filtering, I tested it in my vehicle - powering the 100 watt HF transceiver directly from the Renogy (with no battery) - something that I probably would not ever do in normal use:  If the converter does have the tendency to produce RF interference, connecting the radio directly to it and putting conducted RF energy on its power leads - and its chassis - would represent a "worst-case" scenario.  On 40 meters (7 MHz) and 12 meters (24 MHz) I could just hear the switching frequency's harmonics near the noise floor which indicated that it was pretty quiet - but not completely so.

Since the spectral switching components were just audible I decided to add a modicum of filtering on both the DC input and output leads - four bifilar turns of #12 AWG (e.g. the input/output power cables) each on their respective T140-43 ferrite cores as seen in Figure 9.  In most situations I would prefer to include bypass capacitors in the mix (see figure 4 in the article "Reducing QRM (interference) from a Renogy 200 watt (or any other!) portable solar panel system" - link) to (significantly!) improve performance, but I decided that even a modest reduction in conducted emissions would likely reduce them to the point of inaudibility.

A spectrum analyzer plot of the noise generated by the unit with the added filtering using just the bifilar-wound T140-43 cores is below:

Figure 8:
The spectrum of RF energy as measured between the in/out terminals with the bifilar inductors between the measurement point and the converter - also over the range of 0-10 MHz.  If a receiver's input terminals were connected directly to the DC terminals the signal level at 40 meters (7 MHz) would be a bit less than "S-9" - for a reduction of about 15dB, or nearly  3 "S" units.

As can be seen Figure 8, the bifilar chokes alone reduced conducted RF by a significant amount above a few MHz, but from as noted in the linked article mentioned above, the addition of the capacitors would have improved the attenuation of the conducted RF energy by another 20 dB or so, but including capacitors is a bit awkward as it involves baring wires and adding additional jumpers.  One issue related to lacking capacitors is the response peak around 2 MHz - likely due to a broad resonance of the bifilar inductors themselves - but this effect diminishes quickly as frequency increases on amateur bands likely to be used in a vehicle.  While not shown in any of the included plots, between 10 and 30 MHz the attenuation afforded by the bifilar chokes, alone, remains at 20dB or better for much of that range.

Note:  At HF, a simple "snap on" choke with a single wire running through its center will not offer enough impedance to provide good attenuation - particularly below 20 MHz.  As the choking inductance is proportional to the square of the number of turns through the ferrite device (e.g. 16-fold with four turns) it is only by being able to put multiple turns through it that we can effectively attenuate frequencies in the HF spectrum.

Figure 9:
The Renogy charger with 5-turn bifilar-wound 12 AWG
chokes wound on the DC input and output leads.  For best
results, always place the inductors as close to the noise-
generating device as practical.  Not visible is a fuse on the
input lead to provide protection to the device and wiring.
Click on the image for a larger version.

If interference from this device were to persist after adding the bifilar inductors, I will go through the trouble of adding the aforementioned capacitors.

Can it be scaled up?

The Renogy RNG-DCC1212-20 is "only" a 20 amp converter/charger, but higher current devices are made by Renogy and others.  While I don't own a higher-current Renogy device, those units seem to operate in exactly the same way:  The "D+" terminal may be used to power it on/off and the "LC" terminal, when pulled high, sets the output current to half of the unit's rating.

If RF interference is considered to be an issue, the higher-current units would require proportionally larger wires and likely larger ferrite cores (say, FT240-43) to accommodate a reasonable number of turns of that larger wire.

I cannot speak to how other brands or dissimilar models from Renogy might be powered down via their equivalent of the "D+" terminal to minimize quiescent current consumption:  That must be left as an exercise by the reader.

Real-world useage Shortly after originally posting this article I went on a rather long road trip.  I had along with me three 100 aH LiFePO4 batteries and I was using them to power not only my refrigerator/cooler, but also my 100 watt HF transceiver. The reason for powering the transceiver from the batteries was due to not wanting to pull more than about 30 amps from the connection to the battery, which itself is fused for 40 amps:  At full charge current, the Renogy could pull about 26 amps from the vehicle to deliver 20 amps to the battery, but the addition of the transceiver would have added another 20 amps, peak to this, the the desire to "average" out the current.  To monitor, I put a voltage and current meter on the "vehicle" side of the Renogy. For the most part, things work perfectly:  I heard no QRM (interference) from the Renogy across the HF spectrum and the 20 amp charge current was more than enough to keep up with the loads, recharging the batteries within an hour or so even after running the refrigerator for a couple of days, in the car. The one issue that I had was that at night, with the headlights and with the heater running was that when the vehicle's engine cooling fan would kick on, the electrical system voltage would drop just enough that my circuit for the Renogy would drop off - then the voltage would increase and would kick back on, repeatedly cycling.  At the time I simply flipped the switch (see Figure 6) to the "10 amp" position to reduce current and the related I*R drop:  This lower current was still more than enough to maintain the batteries - even with the refrigerator and the HF transceiver running/being used.  This issue was later found to be due to the voltage threshold having drifted upwards by about 0.4 volts - likely due to mechanical vibration of the potentiometer.

Conclusion

This unit - and the modification - have worked as expected:  The unit gets turned on and off with the running of the engine automatically with no connection required other than that of power.  When traveling, 20 amps is enough to provide a reasonably fast charging rate to a modest bank (say, 200aH) of LiFePO4 batteries while even the "Low Current" 10 amp limit is more than enough to keep the batteries topped off with a moderate load such as a refrigerator-type cooler or a 100 watt HF amateur transceiver occasionally used for transmitting.

With the added filtering using the ferrite cores on which multiple turns are wound, no interference from the Renogy is audible on the HF transceiver in the vehicle.

 * * * * *

The TL;DR part

Why you need to treat LiFePO4 batteries differently

In the "old days" of lead-acid batteries, you could probably get away with putting it in parallel with the vehicle's electrical system - possibly with the use of an "isolator" (e.g. diode, FET pack, a relay or contactor that connected it in parallel with the starting battery when the engine is running) to prevent the drain on the auxiliary battery from depleting the vehicle's starting battery when the engine was off - but this CANNOT and SHOULD NOT be done with LiFePO4 batteries.

A LiFePO4 battery will attempt to pull "infinity" current when charging

The reason for this has to do with a fundamental difference between the two chemistries.  A healthy lead-acid battery is somewhat self-limiting in the amount of charging current it will take - at least when it's nearly fully-charged:  The charge current will gradually taper off as it asymptotically approaches full-charge.  Additionally, on a typical lead-acid battery the internal resistance of the battery and evolution of gasses at the plates often leads to intrinsic current limiting.

A healthy LiFePO4 battery is closer to that of an "ideal" battery in that unlike a lead-acid battery, where the current will gradually taper off as it approaches "full-charge" voltage (which isn't well defined in that chemistry), a LiFePO4 battery will attempt to consume as much current as it can until it is fully charged.  Practically-speaking, the current is actually limited by internal resistance of the battery - which can be in the milli-Ohm range - and the resistance of the wiring between the voltage source (the alternator) and the battery - and since heavy-gauge wire is typically used, this current can be very high.

In the case of a large (100aH or bigger) LiFePO4 battery, it's likely capable of consuming as much current as the alternator will put out - and this could easily exceed its actual ratings.  Short-term overcurrent conditions on an alternator - such as those that might occur immediately after starting the engine, particularly if accessories (lights, wipers, heater) is on - are tolerated, but they cannot withstand a continuous overload - such as that which might occur with a discharged LiFePO4 battery - without overheating - particularly in hot weather and/or if the vehicle is moving down the road quickly and providing air movement.

Another potential issue with a LiFePO4 battery has to do with its BMS (Battery Management System).  If the charge current exceeds the rating of the BMS, it will disconnect to prevent overcurrent that could damage the cells by charging them too vigorously.  At best, this would cause the BMS to disconnect/reconnect the battery (called "load dump", which is a problem as noted below) and at worst it could cause overheating and damage to the BMS - not to mention the alternator and other vehicle systems as well.

The dangers of alternator "load dump"

Another issue with LiFePO4 batteries that does not exist with Lead Acid is that they can abruptly "dump" their load (e.g. disconnect).  While a lead-acid battery's charge current will gradually taper off, if a LiFePO4 battery attains full charge, its BMS (Battery Management System) will abruptly disconnect the battery once any of its individual cells get to full voltage - something that can happen if the cells are all fully-charged and the current is minimal (the preferred situation) or if high current is still flowing, perhaps due to too-high charging voltage - a much worse case.  The result of an abrupt drop of a large current flow is that the voltage from the alternator will briefly skyrocket, its voltage regulator unable to compensate quickly enough.

While this can happen in a vehicle using a lead-acid battery when a load is suddenly removed (e.g. fan cycling, headlights being turned off) a healthy lead-acid battery is quite good at suppressing such voltage spikes and protecting the attached electronics as it functions much like a large capacitor - but voltage transients high enough in voltage to cause damage can still occur, perhaps cumulatively, particularly if the lead acid battery's condition is poor:  If there is no lead acid battery at all to buffer such transients (e.g. only a LiFePO4 battery) such a voltage spike can damage other devices connected to that power source as described in the example below.

(Note:  As the BMS "disconnect" voltage of a four cell LiFePO4 battery is typically around 14.6 volts, a "load dump" may not regularly occur in many automotive applications as the voltage may never get that high - at least under typical conditions.)

Lead Acid and LiFePO4 batteries don't use the same voltages

A third issue is that the full-charge voltage of a typical "12 volt" LiFePO4 battery is 14.6 volts, precisely, whereas a lead-acid battery is quite forgiving, allowing anything between 13.5 and "14.something" volts as a full charge.  The implication of this is that a vehicle's electrical system is not precise enough to either avoid under-charging (e.g. too low voltage, preventing full charge) or over-charging (e.g. causing the BMS to connect/disconnect/reconnect).

Maintaining a precise voltage near the maximum voltage of a "12 volt" LiFePO4 battery (14.4-14.6 volts) for extended periods (a few hours) - at least occasionally - is also necessary for the BMS (Battery Management System) equalize the individual cells within the battery.  Failure to do this every so often will allow individual cells to drift apart in their charge states as inevitably, one or more cells will discharge more quickly - and if never fully recharged, those cells will seem "weaker" and the battery will appear to lose capacity.

"Equalization" as done by the BMS of a LiFePO4 battery is typically done by "leaking" current across fully-charged cells to top off those that are not - but this will only happen effectively at/near the battery's maximum voltage.  Depending on the degree of this "inequality", it may take hours of holding the battery at this high voltage to fully equalize the battery's cells.

Note that the equalization mechanism for LiFePO4 cells is NOT compatible with that which might be done for Lead-Acid - see the battery's manual or other references for the technical details.

Real-world case

I've seen the above issues play out on a friend's RV:  The original "chassis" battery to run the engine and charge the engine starting battery was augmented by a second and completely separate "coach" alternator which was dedicated to charging the LiFePO4 battery bank and running the devices in the living quarters (lights, TV, pumps, microwave oven, inverter, etc.)  In this case, the secondary alternator was adjusted to produce higher voltage than would be necessary for lead-acid batteries to allow full charging of the LiFePO4 system.

Built by Thor onto a Mercedes chassis,  several alternators were destroyed (one of them lasting only minutes!) by overheating due to the the lack of current-limiting in the battery-charging regimen:  One of them lasted longer than the rest only due to several of the rectifier diodes going open-circuit almost immediately, crippling the ability of the alternator to produce output, limiting current - but putting very high ripple voltage/current onto the coach battery's electrical system.  Additionally, equipment connected to that circuit (namely a $1200 amateur radio transceiver) was destroyed by the high-voltage spike when a "load dump" occurred at the instant that the LiFePO4 battery disconnected  upon full charge do to the intrinsic inability of the alternator's voltage regulator to act quickly enough.  

It is fortunate that this vehicle had two separate alternators so the integrity of the "chassis" electrical system responsible for powering the vehicle itself was spared any problems - and no damage to its components (engine and transmission computers, etc.) was possible.  Without a functioning "coach" alternator to recharge the LiFePO4 battery he was still able to make his trip, but had to stop every couple of days and camp somewhere where he could plug into a mains outlet and use the onboard charger to top it off.

Ultimately this friend ended up taking his rig to a company that specialized in RV power systems and the system was upgraded and reconfigured - at significant expense - to avoid the issues noted above.  A quick perusal of online RV forums will reveal many similar stories - some being a result of the manufacturers apparently being unfamiliar with the requirements of LiFePO4 batteries, and others from individual owners' botched retrofits.

* * * * *

This page stolen from ka7oei.blogspot.com

[END]

An RV "Generator Start Battery" regulator/controller for use with a LiFePO4 power system

I was recently retrofitting my brother's RV's electrical system with LiFePO₄ batteries (ReLi³on RB-100's).  This retrofit was done to allow much greater "run time" at higher power loads and to increase the amount of energy storage for the solar electric system while not adding much weight, not needing to vent corrosive fumes.  (These types of batteries, LiFePO₄,  are very safe - e.g. they don't burst into flame if damaged or abused.)

While I was doing this, I began to wonder what to do about the generator "start" battery.

Charging LiFePO4 batteries in an RV The voltage requirements for "12 volt" Lead-Acid batteries are a bit different from those needed by LiFePO4 "12 volt" batteries: Lead acid batteries need to be kept at 13.2-13.6 volts as much as possible to prolong their life (e.g. maintained at "full charge" to prevent sulfation). LiFePO4  batteries may be floated anywhere between 12.0 and their "full charge" voltage of around 14.6 volts - but they will not be fully recharged unless they are held near the high end of this range. Routinely discharging lead-acid batteries below 50% can impact their longevity - and they must be recharged immediately to prevent long-term damage. LiFePO4  batteries may be discharged to at least 90% routinely - and they may be left there, provided their voltage is not allowed to go too low. Lead acid batteries may be used without any management hardware:  Maintaining a proper voltage is enough to ensure a reasonable lifetime. LiFePO4 batteries must have some sort of battery management hardware to protect against overcharge and over-discharge as well as to assure proper cell equalization.  Many modern LiFePO4 batteries (such as the "Rel3ion" devices used here) have such devices built in. Conventional RV power "converters" are designed to apply the proper voltage to maintain lead-acid batteries (e.g. maintain at 13.6 volts.) Because LiFePO4 batteries require as much as 14.6 volts to attain 100% charge (a reasonable charge may be obtained at "only" 14.2 volts) connecting them directly to an existing RV with this lower voltage means that they may never be fully-charged!  Modern, programmable chargers (e.g. inverter-chargers, solar charge controllers) have either "LiFePO4 " modes or "custom" settings that may be configured to accommodate  the needs of LiFePO4 batteries.  While the lower voltage (nominal 13.6 volts) will not hurt the LiFePO4 batteries, they likely cannot be charged to more than 40-75% of their rated capacity at that voltage.  (approx. 13.6-13.7 volts is the lowest voltage were one can "mostly" charge a LiFePO4 battery.) Because of Peukert's law, one can only expect 25-50% of the capacity of a lead-acid battery to be available at high amperage (e.g. 0.5C or higher) loads. With LiFePO4 batteries, more than 80% of the battery's capacity can be expected to be available at similar, high-amperage.  What this means is that at such high loads, a LiFePO4 battery can supply about twice the overall power when compared with a lead-acid battery of the same amp-hour rating.  At low-current loads the two types of batteries are more similar in their available capacity. In short:  Unless an existing charging system can be "tweaked" for different voltages and charging conditions, one designed for lead-acid batteries may not work well for LiFePO4 batteries.  In some cases it may be possible to set certain "equalize" and "absorption" charge cycle parameters to make them useful with LiFePO4s, but doing this is beyond the scope of this article.

Originally the RV had been equipped with two "Group 24" deep-cycle/start 12 volt batteries in parallel (a maximum of, perhaps, 100 amp-hours, total, when brand new, for the pair of "no-name" batteries supplied) to run things like lights, and the pump motors for the water system, jacks and slide-outs and as the "start" battery for the generator.  Ultimately we decided to wire everything but the generator starter to the main LiFePO₄ battery bank.

Why?

Suppose that one is boondocking (e.g. "camping" away from any source of commercial power) and the LiFePO₄ battery bank is inadvertently run down. As they are designed to do, LiFePO₄ battery systems will unceremoniously disconnect themselves from the load when their charge is depleted to prevent permanent damage, automatically resetting once charging begins.
 
If that were to happen - and the generator's starter was connected to the LiFePO₄ system - how would one start the generator?

Aside from backing up the towing vehicle (if available), connecting its umbilical and using it to charge the system just enough to be able to get the generator started, one would be "stuck", unable to recharge the battery.  What's worse is that even if solar power is available, many charge controllers will go offline if they "see" that the battery is at zero volts (e.g. when they are in that "disconnected" state) - even if the sun is shining, preventing charging from even starting in the first place!

What we needed was a device that would allow the starting battery be be charged from the main battery, but prevent it from back-feeding and being discharged.


Note:

It is common in many RVs for the generator to not charge its own starting battery directly, via an alternator.  The reason for this is that it is assumed by the makers of the generators and RVs that the starting battery will be charged by the towing vehicle and/or via the RV's electrical system via its AC-powered "voltage converter", powered from "shore" power or via the generator's AC output.

But first, a few weasel words:

• Attempt to construct/wire any of the circuits only if you are thoroughly familiar with electronics and construction techniques.
• While the voltages involved are low, there is still some risk of dangerous electric shock.
• With battery-based systems extremely high currents can present themselves - perhaps hundreds or even thousands of amps - should a fault occur.  It is up to the would-be builder/installer of the circuits described on this page - or anyone doing any RV/vehicle wiring - to properly size conductors for the expected currents and provide appropriate fusing/current limiting wherever and whenever needed.  If you are not familiar with such things, please seek the help of someone who is familiar before doing any wiring/modifications/connections!
• This information is presented in good faith and I do not claim to be an expert on the subject of RV power systems, solar power systems, battery charging or anything else.
• You must do due diligence to determine if the information presented here is appropriate for your situation and purpose.
• YOU are solely responsible for any action, damage, loss or injury that might occur.  You have been warned! 

Why a "battery isolator" can't be used:

If you are familiar with such things you might already be saying "A device like this already exists - it's called a 'battery isolator'" - and you'd be mostly right - but we can't really use one of these devices because LiFePO₄ batteries operate at a full-charge voltage of between 14.2 and 14.6 volts, and the battery isolator would pass this voltage through, unchanged.  If you apply 14+ volts to a "12 volt" lead-acid battery for more than a few days, you will likely boil the away electrolyte and ruin it!

What is needed is a device that will:

• Charge the generator start battery from the main (LiFePO₄ ) battery system
• Isolate it from the main battery, and 
• Regulate the voltage down to something that the lead-acid chemistry can take - say, somewhere around 13.2-13.6 volts.

In this case the main LiFePO₄ battery bank will be maintained via the AC-powered (generator or shore) charging system and/or the solar power converters at its normal float voltage, so it makes sense to use it to keep the start battery fully-charged.

The solution:

After perusing the GoogleWeb I determined that there was no ready-made, off-the-shelf device that would do the trick, so I considered some alternatives that I could construct myself.

Note:  The described solutions are appropriate only where the main LiFePO₄ bank's voltage is just a bit higher (a few volts) than the lead-acid starting battery:  They are NOT appropriate for cases where a main battery bank of a much higher voltage (e.g. 24, 48 volts, etc.) is being used to charge a "12 volt" starting battery.

Simplest:  "Dropper diodes":

Because we need to get from the nominal 14.2-14.6 volts of the LiFePO₄ system down to 13.2-13.7 volts it is possible to use just two silicon diodes in series, each contributing around 0.6 volts drop (for a total drop of "about" 1.2 volts) to charge the starting battery, as depicted in Figure 1, below.  By virtue of the diodes' allowing current flow in just one direction, this circuit would also offer isolation, preventing the generator's battery from being discharged by back-feeding into the main battery.

To avoid needing to use some very large (50-100 amp) diodes and heavy wire to handle the current flow that would occur when the starter motor was active - or if the start battery was charging heavily - one simply inserts some series resistance to limit the current to a few amps.  Even though this would slow the charging rate somewhat, the starting battery would be fully recharged within a few hours or days at most - not a problem considering the rather intermittent use of the starting battery - more about that later.

Figure 1.
This circuit uses a conventional tungsten-filament "1157" tail/turn signal bulb (NOT an LED replacement!) with both filaments tied together, providing more versatile current limiting.  Please read notes in the text concerning mounting of the light bulb.
The diodes (D1 and D2) should be "normal" silicon diodes rather than "Shottky" types as it is the 0.6 volt voltage drop per diode that we need to reduce the voltage from the LiFePO₄ stack to something "safe" for lead-acid chemistry.  If one wished to "tweak" the voltage on the starting battery, one could eliminate one diode or even replace just one of them with a Shottky diode to increase the lead-acid voltage by around 0.2-0.3 volts.
The use of a current-limiting device like a tungsten light bulb allows lighter-gauge wire to be used to connect the two battery systems together.
Click on the image for a larger version.

In lieu of a large power resistor, the ubiquitous "1157" turn signal/brake bulb is used as depicted in Figure 1.  Both filaments are tied together (the bulb's bayonet base being the common tie point) providing a "cold filament" resistance of 0.25-0.5 ohms or so, increasing to 4-6 ohms if a full 12 volts were placed across it.  The reason for the use of a light bulb will be discussed later.

Although not depicted in Figure 1, common sense dictates that appropriate fusing is required on one or both of the wires, particularly if one or more of the connecting wires is quite long, in which case the fuse would be placed at the "battery" end (either LiFePO₄ or starting battery) of the wire(s) to provide protection should a fault occur between that source and the charge controller:  Fusing at 5-10 amps is fine for the circuit depicted.

This circuit is "good enough" for average use and as long as the LiFePO₄ bank is floated at 14.2 volts with occasional absorption peaks at 14.6 volts, the lead-acid starting battery will have a reasonably long life.

A regulator/limiter circuit:

As I'm wont to do, I decided against the super simple "dropper diode and light bulb" circuit - although it would have worked fine - instead, designing a slightly fancier circuit to do about the same as the above circuit, but have more precise voltage regulation.  While more sophisticated than two diodes and a light bulb, the circuit need not be terribly complicated as seen in Figure 2, below:

Figure 2:
The schematic diagram of the slightly more complicated version that provides tight voltage regulation for the starting battery.  As noted on the diagram, appropriate fusing of the input/output leads should be applied!
This diagram depicts a common ground shared between the main LiFePO₄ battery bank and the starting battery, usually via the chassis or "star ground" connection.  In the as-built prototype, Q2 was an SUP75P03-07 P-channel power MOSFET while D1 was an MR750 5 amp, 50 volt diode. A circuit board is not available at this time.
NOT SHOWN is the fusing of the input and output leads, near-ish their respective batteries/source connections, with 10 amp automotive fuses.
Click on the image for a larger version.

How it works:

U1 is the ubiquitous TL431 "programmable Zener".  If the "reference" terminal (connected to the wiper of R5) of this device goes above 2.5 volts, its cathode voltage gets dragged down toward the anode voltage (e.g. the device turns "on").  Because R4, R5 and R6 form an voltage divider, adjustable using 10-turn trimmer potentiometer R5, the desired battery float voltage may be scaled down to the 2.5 volt threshold required by U1.

If the battery voltage is below the pre-set threshold (e.g. U1 is "seeing" less than 2.5 volts through the R4/R5/R6 voltage divider) U1 will be turned off and its cathode will be pulled up by R2.  When this happens Q1 is biased on, pulling the gate of P-channel FET Q2 toward ground, turning it on, allowing current to flow from the LiFePO₄ system, through diode D1 and light bulb "Bulb1" and into the starting battery.

By placing R1 and R2 on the "source" side of FET Q2, the circuit is guaranteed to have two potential sources of power:  From the main LiFePO₄ system, through D1, and from the starting battery via the "backwards" intrinsic diode inside Q2.  The 15 volt Zener diode (D2) protects the FET's gate from voltage transients that can occur on the electrical system.

Figure 3:
The completed circuit, not including the light bulb, wired on a small
piece of perforated prototype board.
A printed circuit board version is not available at this time.
Click on the image for a larger version.

Once the starting battery has attained and exceeded the desired float voltage set by R5 (typically around 13.5 volts for a "12 volt" lead-acid battery) U1's reference input "sees" more than 2.5 volts and turns on, pulling its cathode to ground.  When this happens the voltage at the base of Q1 drops, turning it off and allowing Q2's gate voltage, pulled up to its source by R1, to go high, turning it off and terminating the charge.

Because the cathode-anode voltage across U1 when it is "on" is between 1 and 2 volts it is necessary to put an additional voltage drop in the emitter lead of Q1, hence the presence of LED1 which offsets it by 1.8-2.1 volts.  Without the constant voltage drop caused by this LED, Q1 would always stay "on" regardless of the state of U1.  Capacitor C1, connected between the "reference" and the cathode pins of U1 prevent instability and oscillation.

In actuality this circuit linearly "regulates" the voltage to the value set by R5 via closed loop feedback rather than simply switching on and off to maintain the voltage.  What this means is that between Q2 and the light bulb, the voltage will remain constant at the setting of R5, provided that the input voltage from the LiFePO₄ system is at least one "diode drop" (approx. 0.6 volts) above that voltage.  For example, if the output voltage is set to 13.50 volts via R5, this output will remain at that voltage, provided that the input voltage is 14.1 volts (e.g. 13.5 volts plus the 0.6 volts drop of diode D1) or higher.

Because Q2, even when off, will have a current path from the starting battery to the main LiFePO₄ bank due it its intrinsic diode, D1 is required to provide isolation between the higher-voltage LiFePO₄ "main" battery bank and the starting battery to prevent a current back-feed.  Were this isolation not included, if the main battery bank were to be over-discharged, current would flow backwards, through FET Q2, from the generator starting battery and discharge it, possibly to the point where the generator could not be started.

Again, D1's 0.6 volt (nominal) drop is inconsequential provided that the LiFePO₄ bank is at least 0.6 volts above that of the starting battery, but this will occur very frequently if the charge on that bank is properly maintained via generator, solar or shore power charging.  A similar (>= 5 amp) Shottky diode could have been used for D1 to provide a lower (0.2-0.4 volt) drop, but a silicon diode was chosen because it was on hand.

Testing the device:

Assuming that it is wired/built correctly, connect a variable power supply to the input lead to simulate the LiFePO₄ battery bank.  Setting the voltage a volt or two higher than the expected float voltage (e.g. 14.5-16 volts) adjust R5 to attain the desired start battery float voltage (13.50-13.7 volts is recommended - I use 13.55 volts) as measured on either side of "Bulb1".  Adjust the power supply voltage up and down a bit (e.g. below 12 volts and up to 17 volts) and if working correctly, the output voltage from the circuit should be rock-steady as long as the input voltage is about 0.6 volts above the set output voltage.

Now short the output leads (e.g. the "positive" output lead should be going through "Bulb1") and the light bulb should illuminate fully - assuming that your variable voltage supply is capable of supplying the 3-ish amps needed for the lamp.  Measuring directly at the circuit board's "ground" (common "battery negative") terminal and at the connection between Q2 and "Bulb1" you should still have the voltage set by R5 within a few hundredths of a volt.

Note:  If you were to measure connect the negative lead of the voltmeter to the power supply or the shorted output leads the measured voltage would be a bit lower owing to voltage drop along the wires.

Shorting the output leads and measuring the voltage as done in the previous step demonstrates two important design points:

• That the voltage at the output of Q2 remains steady from no-load to maximum current conditions.
• That the light bulb is properly acting as a current limiting device.

While doing this "short circuit" test, make sure that the heat from the light bulb rises away from the circuit board itself and that the means of mounting it is capable of withstanding the bulb's heat without burning or melting anything.

Connecting the device:

On the diagram only a single "Battery negative" connection is shown and this connection is to be made only at the starting battery.  Because this circuit is intended specifically to charge the starting battery, both the positive and negative connections should be made directly to it as that is really the only place where we should be measuring its voltage!

Also noted on the diagram is the assumption that both the "main" (LiFePO₄ ) battery and the starting battery share a common ground, typically via a common chassis ("star") ground point which is how the negative side of the starting battery ultimately gets connected to the negative side of the main LiFePO₄ bank:  It would be rare to find an RV with two battery systems of similar voltages where this was not the case!

Finally, it should go without saying that appropriate fusing be included on the input/output leads that are located "close-ish" to the battery/voltage sources themselves in case one of the leads - or the circuit itself - faults to ground:  Standard automotive ATO-type "blade" fuses in the range of 5-10 amps should suffice.  In order to safely handle the fusing current and to minimize voltage drop while charging the connecting wires to this circuit should be in the range of 10 to 16 AWG with 12-14 AWG being ideal.

What's with the light bulb?

Figure 4:
The circuit  board mounted in an aluminum chassis box along with the
light bulb.  Transistor Q2 is heat-sinked to the box via insulating hardware
and the board mounted using 4-40 screws and aluminum stand-offs.  The light
bulb is mounted to a small terminal lug strips using 16 AWG wire soldered
to the bulb's base and the bottom pins:  A large "blob" of silicone (RTV)
was later added around the terminal strip to provide additional support.
Both the bottom of the box (left side) and the top include holes to allow
the movement of air to help dissipate heat.  Holes were drilled in the back
of the box (after the picture was taken) to allow mounting.
This box is, in this picture, laying on its side:  The light bulb would be
mounted UP so that its heat would rise away from the circuitry via
thermal convection.
Click on the image for a larger version.

The main reason for using a light bulb on the output is to limit the current to a reasonable value via its filament.  When cold, the parallel resistance of the two filaments of the 1157 turn-signal bulb is 0.25-0.5 ohms, but when it is "hot" (e.g. lit to full brilliance) it is 4-6 ohms.  Making use of this property is an easy, "low tech" way to provide both current limiting and circuit protection and, when the filament is cold (e.g. charging battery "mostly" charged), increase the amount of charging current that can flow.  Taking advantage of this changing resistance of a light bulb allows higher charging current that would be practical with an ordinary resistor.


In normal operation the light bulb will not glow - even at relatively high charging current:  It is only if the starting battery were to be deeply discharged and/or failed catastrophically (e.g. shorted out) that the bulb would begin to glow at all and actually dissipate heat.  

Limiting the charging current to just a few amps also allows the use of small-ish (e.g. 5 amp) diodes and reduce the heat that could be dissipated during regulation allowing the use of an aluminum box as Q2's heat sink, but more importantly it allows much thinner and easier-to-manage wire (as small as 16 AWG) to be used since the current can never be very high in normal operation.  Limiting the charging current is just fine for the starting battery due to its very occasional use:  It would take only an hour or two with a charge current to top off the battery after having started a generator on a cold day!

As noted on the diagram and in previous text the light bulb must be mounted such that its operating temperature and heat dissipation at full brilliance will not burn or melt any nearby materials as the glass envelope of the bulb can will easily exceed the boiling temperature of water!  With both the "simple" diode version in Figure 1 and the more complex version in Figure 2 it is recommended that the bulb is mounted above the circuitry to take advantage of air convection to keep the components cool as shown in Figure 4.  If a socket is available for the 1157 bulb, by all means use it, but still heed the warnings about possible amount of heat being produced.

In operation:

When this circuit was first installed, the starting battery was around 12.5 volts after having sat for a week or two (during the retrofit work) without a charging source and having started the generator a half-dozen times.  With the LiFePO₄ battery bank varying between 13.0 and 14.6 volts with normal solar-related charge/discharge cycles, it took about 2 days for the start battery to work its way up to 13.2 volts, at which point it was nearly fully charged - and then the voltage quickly shot up to the 13.55 volts as set by R5.  This rather leisurely charge was mostly a result of the LiFePO₄ bank spending only brief periods above 13.8 volts.

Even though this doesn't very quickly charge the battery under normal conditions, as we'll see below, this isn't really important.

How much of the starting battery's capacity is being used?

If one were to assume that the generator was set to run once per day and pull 100 amps (a current likely seen on a very cold day!) from the battery for 5 seconds this would represent (100 amps * ( 5 sec/3600sec )) = about 0.14 amp-hours: - This happens to be about the same amount of energy as is contained in 4 fresh hearing-aid batteries or about 1/10th of the capacity of a single AAA cell!

From this we can see that this "100 amps for 5 seconds" is an average current of just over 5 milliamps (1/200th of an amp!) when spread across 24 hours - a value likely comparable the self-discharge rate of the battery itself.   By these numbers you can see that it does not take much current at all to sustain a healthy battery that is used only for starting!  Because this battery is never used for running things like lights or motors, it really never gets abused by being deeply discharged.

A standard group 24 "deep cycle starting" battery was used since it and its box had come with the RV.  In this particular application, for generator starting only, a much smaller battery - such as one used for starting 4x4s or motorcycles - would have sufficed and saved a bit of weight and space.

The advantage of the group 24 battery is that it, itself, isn't particularly heavy and it is readily available in auto-parts, RV and "big box" stores everywhere.  Because it is used only for starting the generator, it need not have been a "deep cycle" type, but rather a normal "car" battery - although the use of something other than an RV-type battery would have necessitated re-working the battery connections as RV batteries have handy nut/bolt posts to which connections may be easily made.


Final comments:


There are a few things that this simple circuit will not do, including "equalize" the lead acid battery and compensate for temperature - but this isn't terribly important, overall in this application.


Concerning equalization:

Even if the battery is of the type that can be equalized (many sealed batteries, including "AGM" types - those mistakenly called "gel cells" - should never be equalized!) it should be remembered that it is not the lack of equalization that usually kills batteries, but rather neglect:  Allowing them to sit for any significant length of time without keeping them floated to above 2.17 volts/cell (e.g. above 13.0 volts for a "12 volt" battery) or, if they are the sort that need to be "watered" and not keeping their electrolyte levels maintained.  Failure to do either of these will surely result in irreversible damage to the battery over time.

It is also common practice to adapt the float voltage to the ambient temperature, but even this is not necessary as long as a "reasonable" float voltage is maintained - preferably one where water loss is minimized over the entire expected temperature range.  Again, it is more likely to be failure of elementary battery maintenance that will kill a battery prematurely than a minor detail such as this.

Practically speaking, if one "only" maintains a proper float voltage and keeps them "watered" the starting battery will likely last for at least the 3-5 year expected lifetime, particularly since, unlike battery in standard RV service, this starting battery will never be subjected to the deep discharge cycles which can really take a toll on a lead-acid battery.  While an inexpensive, no-name "group 24" battery, when new, may have a capacity of "about" 50 amp-hours, it won't be until the battery has badly degraded - probably to the 5-10 amp-hour range - where one will begin to notice starting difficulties.

Important also is the fact that the starting battery in this RV is connected to part of the main LiFePO₄'s battery monitoring system (in this case, a Bogart Engineering TM-2030-RV).  While this system's main purpose is to keep track of the amount of energy going into and out of the main LiFePO₄ battery, it also has a "Battery #2" input connection where one can check the starting battery's voltage - always a good thing to do at least once every day or two when one is "out and about".

Finally, considering the very modest requirements for a battery that is used only for starting the generator, it would take only a very small (1-5 watt) solar panel (plus regulator!) to maintain it.  While this was considered, it would have required that such a solar panel be mounted, wires run from it to the battery (not always easy to do on an RV!) and everything be waterproofed.  Because the connections to the main battery bank were already nearby, it was pretty easy to use this circuit, instead.

[End]

This page was stolen from "ka7oei.blogspot.com"

A latching low-voltage disconnect for 12 volt lead acid and lithium batteries (non auto-resetting)

Figure 1:
The as-built and working prototype constructed.
This version does NOT automatically reset itself - by design.
Click on the image for a larger version.

There are two things that you don't want to do with any rechargeable battery on a routine basis:

• Overcharge it.
• Overdischarge it.

While the above are true for lead-acid batteries, they are particularly true of Lithium-Ion chemistries, but for different reasons.

With Lead-acid batteries:

• Lead-acid batteries - particularly the "flooded cell" types (e.g. those to which you can add water) can handle quite a bit of overcharging as long as the electrolyte level is maintained.  "Sealed" batteries (e.g. AGM, or those that many mistakenly called "gel" cells) can handle some overcharging, but only to an extent before their pressure vents release accumulated gasses, reducing the amount of usable electrolyte, which is why they should never be "equalized".
• Lead-acid batteries can also handle being run (almost) completely down - as long as you don't keep them in that state for very long (a few days at most - as little time as possible) and don't do it very often.  In other words, if you run an otherwise healthy lead acid battery completely dead and immediately recharge it, little actual damage is likely to have been done other than taking a bit of life off it farther down the road.  In cold-weather environments, while the degradation (primarily sulfation) is dramatically slowed, extremely deep discharge also reduces the specific gravity, raising the electrolyte's freezing point, increasing the possibility of the battery being damaged/destroyed at very low temperatures if it does freeze.

 With Lithium-ion batteries:

• If a battery is overcharged, it will start to chemically decompose.  Gross overcharging - while tolerated at least briefly by lead-acid batteries - may result in a lithium-ion battery venting and/or exploding, possibly catching fire.
• If a battery is over-discharged it will chemically decompose, often with the contained lithium changing into a more volatile - and not useful - state.  Severe over-discharging (e.g. below 2 volts per cell - this voltage varies depending on chemistry) can mean that the battery can never safely be charged again or, in some cases - if the voltage is only allowed to get down to this general area and not lower (again, the voltage varies according to chemistry) some special charging precautions are required (e.g. a specific, very low-rate trickle charging regimen) to "recover" this battery.
• There are also some specific temperature-related restrictions with lithium-type batteries regarding their use and charge/discharge and these are noted by their respective manufacturers.

Avoiding over-discharge:

The avoidance of overcharging is usually pretty easy:  Just use the appropriate charging system - but over-discharge is a bit more difficult, particularly if the battery packs in question don't have a "protection board" with them.

Lead acid batteries (almost) never come with any sort of over-discharge protection - one must usually rely on the ability of the device being powered (e.g. an inverter) to turn itself off at too-low a voltage and hope that the threshold is sensible for the longevity of a 12 volt battery system.  For low-to-moderate loads (e.g. 1/10th "C" or so) a pretty safe "dead battery" voltage for a 12 volt lead-acid battery is around 11.7 volts - or somewhat higher for heavier loads.  Again, after disconnect, it is not a good idea to keep it in a discharged state for any longer than possible.

Many larger (e.g. >10 amp-hour) lithium-iron phosphate (LiFePO4) do not routinely come with "protection" boards unless it is ordered specially or includes some sort of "Battery Management System":  Batteries in this category can include the "Lead Acid" replacements sold for use with motorcycles and off-road vehicles  and some of the "raw" LiFePO4 batteries available from many vendors, such as the 20 amp-hour modules made by GBS.

While it is also important to equalize LiFePO4 batteries when charging (refer to this post - Lithium Iron Phosphate (LiFePO4) batteries revisited - Equalization of cells - link) the more immediate danger in routine use is accidental over-discharge.

A simple "latching" low-voltage disconnect circuit:

Again, for lithium batteries one may install "protection" boards that prevent accidental over-discharge and, in some cases, provide charge equalization - but such things are much rarer for lead-acid batteries, but such a circuit is quite simple and is applicable to either Lithium or Lead Acid batteries.

Note that this particular circuit does not automatically reconnect the battery again after its voltage has been restored by charging (directly at the battery, anyway - and this is the "latching" part) and this was intentional - both to keep the circuit as simple as possible and because it draws attention to the system on which it is used when it trips out.

Figure 2:
Schematic diagram of the low-voltage disconnect circuit.
Not shown is overcurrent protection (e.g. fusing) that should be present on the output of the battery - see text below. 
If desired, LED1 can be placed in series with R2 which could be changed to 2.2k and R7 be omitted as an indicator that the circuit has actually latched, not just that there is voltage present on the Load+/- terminals.
Click on the image for a larger version.

These days it is rather easy to construct a low-voltage disconnect circuit using readily-available components:  The diagram of one such circuit may be found in Figure 1.

How it works:

The RESET button is pressed, applying a positive voltage to the gate of N-channel power MOSFET, Q1, turning it on, which then connects the "BATT -" output terminal to the "LOAD -" terminal.

If the voltage at the "Ref" terminal of U1 is above 2.5 volts, as determined by the voltage divider consisting of R4, R5 and R6, the cathode of U1 is connected to its anode (e.g. "Load -"), pulling the base of Q2 down, making it negative with respect to its emitter, R2 limiting Q2's base current to a safe value and providing enough current for U1 to function, and turning it on.  With Q2 turned on, Q1 is "latched" on, even when the RESET button is released.

If the voltage at the "Ref" terminal of U1, representative of the voltage across the LOAD terminals, drops below 2.5 volts, U1 turns off and the base of Q2 gets pulled positive to the emitter voltage by R1, turning it off.  With Q2 turned off resistor R8 pulls the gate of Q1 down to its source, also turning it off and disconnecting the load.  Because of the "latching" effect, once this has happened the load will never be turned on again until the RESET button is pressed.  This happens because with Q1 turned off, U1 is without voltage (e.g. "Load -" rises to the same voltage as "Load +) and can never turn Q2 (and thus, Q1) back on again.  Even though pressing and holding the RESET button will connect the load even if the voltage is below the threshold, until the voltage rises above the threshold the circuit will not stay "on" once the RESET button is released.

To accommodate a range of voltages, U1's "Ref" terminal is connected across the output (Load +, Load -) with R4 and R6 to "scale" the range of potentiometer R5 to have a threshold in the 8-16 volt range:  Without R4 and R6 the usable range of R5 would be compressed to a very small portion of the overall rotation and make adjustment touchy, but with these resistors setting R5 at mid-rotation yields a threshold of around 11 volts.

Note the presence of capacitors C1 and C2:  C1 provides a bit of filtering of the sampled output voltage to prevent brief current transients that might momentarily drag the voltage down below the threshold, "falsely" causing an undervoltage condition from being detected.  Similarly, C2 slows the "fall" time of Q1's gate voltage, preventing it from shutting off instantly in response to a brief spike of current - and it also provides some degree of protection of Q1's gate in response to possible voltage transients.

While not explicitly tested, the presence of C1 and C2 should provide a modicum of RFI protection:  If your environment includes high RF fields - such as powering a 100 watt amateur transceiver - this could be considered in testing and the construction/layout, knowing that such a transceiver can also impose very brief, high-current loads on the battery can causing momentary brown-outs due to I*R drops in the wiring and battery which could also trip this circuit.

Finally, the combination of R7 and LED1 provide an indication of power-on to the user - see the note on modification of this circuit, below.

Additional circuit notes:

The "high voltage" limitation of this device is primarily that of the gate voltage rating of Q1.  Most power FETs are rated for only +/- 20 volts gate-to-source voltage which means that it is suitable for no more than a "12 volt" bus (e.g. 10-16 volts or so):  If a higher operating voltage is required it will be necessary to add additional circuitry around the FET's gate to keep its voltage safely below its rating.  For an example of such circuitry see this article:  A Simple, effective, yet Inefficient Solar Charge Controller - link and taking note of components D1, R7, R8 and C4 surrounding Q3 in Figure 3 on that page.

If a lower cut-off than 9 volts  (or higher than 15) is required it will be necessary to recalculate the values of R4 and R6 (in Figure 2, above) to appropriately scale the adjustment range.

It should also be noted that if voltages below 10 volts are routinely required one should pay close attention to the saturation (e.g. "full on") gate voltage required for the FET that you plan to use:  Typical FETs do not achieve their lowest resistance until 8-10 voltage of gate-source voltage is present but there are "logic level" FETs available that will be fully "on" at around 5 volts.

Finally, there is a slight modification to the circuit depicted in Figure 2 that could be made:  Place LED2 in series with R2 and decreasing the value of R2 to 2.2k or so, omitting R7 entirely.  This modification not only saves a few milliamps of "on" current, but it also provides an indication of when the circuit is actually latched in its "on" state - particularly useful if the load has its own, separate power source which would cause LED1 to illuminate no matter the state of the disconnect circuit if wired according to Figure 2.

Construction:

None of the components are critical, save the possible exception of R4, R5 and R6 which are selected to scale the adjustment range of R5:  While it is the ratios of these components that are important (e.g. one could use 4.7k, 1k and 1k for R4, R5 and R6, respectively) going much higher than the stated values may violate the minimum reference current specifications of U1 resulting in temperature/device variations of the set voltage thresholds.

The TL431 (U1) is a rather ubiquitous chip, found in practically every PC-type power supply made in recent years and is available in single quantities for well under $1.

Q2 may be practically any silicon PNP transistor with a rating of at least 30 volts while Q1 may be any N-channel MOSFET with a voltage rating of at least 30 volts and a current rating of at least 3 times the current that you plan to draw and an "ON" resistance of a fraction of an ohm.  For the prototype I used an F15N05 FET - a 15 amp, 50 volt device, more than adequate for the 3 amp load that was to be used, but one could use as "large" a power FET as you wish.  For "12 volt" operation make sure that the FET that you choose has at least a 20 volt gate-source voltage rating.  Higher-current FETs include the IRFZ44 (50 amp max.) and the PSMN2R7-30PL (100 amp max.) to name but two out of hundreds of possibilities.  If even more current is required one can parallel multiples of the same-type FET as needed, potentially providing many hundreds of amps of capacity, provided the wiring is appropriately considered.

Device layout is not critical aside from the use of appropriately heavy conductors to the source and drain leads of Q1 to carry the current.  For most applications a heat sink is not even required for the FET - particularly if one chooses a device with milli-Ohm range "on" resistance but there is never any harm in doing the calculations yourself to verify that this is true in your case with the FET that you choose.  Note that the "Batt+" and "Load+" lead is straight-through and the wire connecting this circuit to that "through" connection may be of light gauge:  The only caveat is that it is recommended that the connection to this circuit be connected closer to the "BATT+" terminal than the "LOAD+" terminal to minimize the resistance of that connecting wire which could cause the circuit to sense a slightly lower voltage than is actually present.

Finally, note that this circuit works by disconnecting the "BATT-" from the "LOAD-":  Your battery's negative terminal must be completely isolated from the load for this circuit to work properly and protect your battery!

(Comment:  It is possible to reconfigure this circuit to disconnect in the positive lead, but this requires the use of a P-channel power FET:  A not-yet-built or tested circuit design is available on request.)

Adjustment and Operation:

For proper set-up an adjustable power supply is required and the procedure is as follows:

• Set the power supply to a volt or two higher than the desired drop-out voltage.
• Adjust R5, the potentiometer so that the wiper is closest to R6 to set the drop-out voltage to maximum (e.g. highest voltage measured betweenU1's REF terminal and LOAD- while the RESET button is being pressed).
• Connect the device to the power supply using the BATT- and BATT+ connections.  No load is required for testing.
• Press and release the RESET button:  The LED should stay on, but if not, check the adjustment of R5 to verify that it is providing the maximum voltage to U1's REF terminal.  If this checks out, check for proper resistor values of R4, R5 and R6 as well as proper wiring of U1.  Note that the circuit will not stay on if U1's REF terminal is below 2.5 volts.
• Lower the power supply to the desired drop-out voltage.  The LED should stay on, but if not check the setting of R5.  (Remember that the useful range of R5 with the specified values of R4 and R6 is in the 8-16 volt area.)
• Slowly rotate R5 until the LED just turns off.
• Increase the power supply voltage slightly, press and release the RESET button and verify that the LED turns on and then goes off again when the voltage drops below the threshold, repeating the above steps as needed.

If a device is connected that has a high "starting" current it is possible that - particularly if the battery is weak or near the cut-off voltage and/or the cut-off device is located at the end of a long run of rather small-gauge wiring - it will drop-out before the voltage gets to the pre-set threshold.  If this happens and it is not practical to move the device closer to the battery or increase wire size to minimize lead resistance one can increase the value of C1 (to as much as 47uF) to slow the response time, allowing a momentary "brown out" to occur without tripping the device.  Note that with such a capacitor it will take longer to respond to such changes, but this should not be an issue from the viewpoint of protecting the battery.  The value of C2 can also be increased, but not much more than 1 uF should be used as this will excessively slow the "turn off" time of Q1, causing it to spend more time out of saturation and potentially dissipating more heat in the process.

Additional comments:

In this particular application Anderson Power Pole (tm) connectors were used on the input and outputs allowing this device to be easily removed from the circuit and configured as needed.

This device should also not be left connected to a battery in long-term storage as it draws several milliamps when it is in its "ON" state due to the LED and the current consumption of Q1/Q2 and associated resistors, R4-R6 and U1.

When in its "OFF" state its current consumption is negligible (likely in the nanoamp range) so if it is left connected and the battery gets drawn down, it will still do its job, disconnecting the load - and itself - from the battery and protecting it.  Note that if the load is "back-fed" from another source - say an AC/solar charger or power supply - and the voltage rises above the threshold, this will have the same effect as pressing the RESET button, turning the circuit on.  Again, if the voltage is back-fed, the LED will be drawing a few milliamps whenever voltage is present whether the circuit is "on" or not - unless the modification noted above is made.

It is recommended that one NOT attempt to charge the battery "through" this device - at least at higher currents:  In theory it should work, but the current will flow backwards through the FET.  The reason for this is that while a FET that is turned "off" has an intrinsic "backwards" diode, it will drop 0.5-0.8 volts across this diode causing the FET to dissipate far more power than it would if it were actually "on".  If the charge rate is limited to a rather low current - perhaps less than 3-5 amps - the amount of heat dissipated by the FET should be tolerable.

Until the voltage rises above the cut-off threshold the FET will exhibit this 0.5-0.8 volt drop, but above this - when the circuit turns the FET on - this diode drop will largely disappear.   If you do this it would be a good idea to test it at your intended charge current in the worst-case scenario (e.g. highest current and adjust R5 so that the circuit will not trigger "on" during this charge, forcing the "diode drop" across Q1 to exist) and note if additional heat-sinking of U1 is needed.  Note:  If this is done, the "LED1-R2" modification noted above is recommended so that the LED will properly show the state of the circuit.

Not shown - but recommended - is the use of some sort of fuse or other overcurrent protection on the output of the battery.  It is recommended that the fuse rating be no higher than a third of the current rating of the FET to increase the chance that the FET will survive the surge current required to blow the fuse in the event of a dead short on the output.

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This page stolen from "ka7oei.blogspot.com".