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:
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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!
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.
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.
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| 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:
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| 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.
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| 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.
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?
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| 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.
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This page was stolen from "ka7oei.blogspot.com"
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