Tuesday, August 9, 2016

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.


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.


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


  1. Is it possible for you to post a PCB of the circuit with corresponding parts placement guide? Please..

    1. The circuit is simple enough that I haven't gotten around to designing a board for this but it would be an excellent "first" project for someone who has not done that sort of thing.

  2. nice idea...but is it temperature stabilized to work from -20 to +60C ?

    1. The temperature stability depends on the components used, specifically the resistive divider consisting of R4, R5, R6 and the TL431.

      Because R4-R6 is ratiometric rather than absolute, the temperature coefficient of each of the components should (mostly) cancel out - but one could use high-stability resistors such e.g. metal film, specialized wirewound for the fixed and variable.

      The TL431's itself is rated for less than 20mV of change over a range of -40 to +70C (approximately 100mV at 12-15 volts). It is likely that the combination of the TL431 and the resistors of R4-R6 will increase the actual amount of drift, depending on the tempco of the resistors themselves and whether or not their sign is the same or opposite that of the '431.

      Then again, -40 to 60C is quite a range in which to store a battery - unless it's under the hood of a car all year round in Scandinavia, perhaps...

    2. Thank you very much for your very helpful and detailed response to my question....

  3. Is there a way you could do without the RESET button? Perhaps using the action of unplugging and then plugging back in the battery (such as after it has been charged) as the "reset"?

    1. Yes, probably: I haven't tried it, but a 10uF (or so) capacitor across the RESET switch would probably do the trick. With the 100k resistor (R8) it may take several seconds to discharge the capacitor before a reset is possible.

      Note that this would operate due to a voltage *rise* on the battery side, so if the battery power were "dirty", a capacitor would allow it to reset on its own when a glitch happened.

      In practice this will likely not happen if a battery is actually being used as a healthy battery should not allow/cause much of a positive dV/dT.

      One possible point of concern might be that this added capacitor may "fly" under certain conditions, putting up to twice the power supply voltage on the gate of Q1 and damaging it by going above its nominal 20 volt rating, but this could be prevented by putting a 15-18 Volt Zener diode (e.g. higher than the battery voltage but lower than 20 volts) in parallel with R8.

    2. Cool, thanks. I will definitely give this a try.
      What type of conditions would cause the capacitor to "fly"?

    3. If the voltage from the battery side were very "dirty" (e.g. glitches, etc.) then the sum of the voltage across the battery and the capacitor could exceed the rating of the FET. In theory, this could also happen if one had disconnected the power to reset it and then the reconnection was not "clean" (e.g. on/off transients while reconnecting the terminals.)

      In further thinking, I would recommend the 15-18 volt Zener (1/2-1 watt) if one is adding the capacitor - just to be safe.

    4. Got it, thanks! I don't have a 15-18 V Zener handy, but I am testing without and things are working as expected. I will incorporate the Zener ASAP.

  4. Hey, trying to simulate your circuit and the FET never stops conducting, regardless of input voltage. Here is the simulation... any ideas why this is happening?


    1. Hi Tom,

      If the REF terminal of the TL431 exceeds the 2.5 volts of the chip's internal reference, it will conduct and pull the output low (toward "ground".) The analog to this (pun intended) is that it would be an inverting comparator: Your original simulation had it as non-inverting (e.g. it went high when >2.5 volts) - if I'm reading/running the simulation correctly, that is.

    2. Hey I'm kinda trying to use the components that were at hand in the Falstad simulation ecosystem to replicate the TL431 functionality. As I understand it the equivalent circuit basically boils down to the NPN transistor whose base is being driven by an op-amp whose rails are connected to the cathode and anode (collector and emitter) pins. The inverting terminal of the op-amp is connected to a 2.5 V (w.r.t the anode) reference voltage. This has the effect of making the TL431 appear as a forward-active NPN transistor when a voltage above 2.5 v is applied to the ref pin. That's what I'm simulating with the setup in the simulation I link to above. Because Falstad doesn't have a proper op-amp (that you can connect the rails to whatever), I have opted to use 0/15V output op amp driving a SPDT relay that (when closed) connects the collector of an NPN to its base and otherwise (when open) connects the emitter to the base.

      When the input battery voltage drops sufficiently to cause the voltage on the REF pin of the TL431 to drop below 2.5 V (when the battery voltage is below 11.2V in my simulation), the TL431 stops conducting and the FET gate capacitor begins discharging through the 100 K gate bleed-down resistor. Once the gate voltage approaches the FET's gate threshold voltage, the current passing through the FET resistance begins building until it has caused the voltage "upstream" at the REF pin of the TL431 to rise to 2.5V, thus causing the TL431 to conduct and in turn re-charging the gate capacitor on the FET.

      In other words, the circuit stays "on" when battery voltage drops, but modulates its resistance...

      Note that if you remove the capacitor between the FET gate and ground, the circuit behave as you have described above, however this is unrealistic since there will always be some stray capacitance left over from the conductors and the gate capacitance of the FET itself.

      Note also that when you run the simulation there is a push button in the top left-hand corner you need to hold down to get the FET to start conducting initially. You can vary the voltage of the battery with the "Voltage" slider on the right-hand toolbar.

      Here's an updated simulation matching numbers I just gave:


    3. Here is a more up-to-date simulation: http://tinyurl.com/yc5oxajp

      There is no op-amp in the Falstad simulation engine that I can use to replicate the way the op amp is set up internally to the TL431, so I had to connect it to a relay that connects the NPN emitter/collector to the base.

      Basically, my TL431 model is a NPN transistor that gets its collector connected directly to its base whenever the voltage on the "REF" pin is above 2.5V, and connects the emitter to the base otherwise.

      The problem with my simulation circuit is that when the battery falls below the threshold voltage (you can cause this to happen with the "voltage" slider on the right-hand side of the simulation), the TL431 stops conducting which stops the PNP transistor from conducting which causes a high Z between the gate of the FET and the battery voltage (so far so good). The problem is that since there is a capacitance on the FET gate, the FET doesn't turn off instantaneously. As the gate of the FET discharges to ground, the FET drain/source resistance begins building, which in turn forces the voltage on the "REF" pin back up above 2.51, which in turn causes the PNP to start conducting, which in turn re-charges the FET gate, and the process repeats endlessly.

      If the gate capacitance is taken out of circuit, the simulated circuite behaves as you have described above. However in the real world there will be a capacitance associated with the gate of the FET (and conductors leading to it) therefore the FET will not switch off instantaneously and therefore the problem I just described will occur.

      However, when I build the circuit with a breadboard I find that it does indeed work as you described, despite the simulation not working. My working theory right now as to why the real-world breadboard circuit is not oscillating when the simulation is is that the

    4. Well, just tested the circuit in Multisim and it seems to be working. I guess the FET switches fully off before REF voltage gets above 2.5

    5. Ok, I simulated it in Multisim and it appears to be working! Must just be an artifact in the Falstad simulation engine



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