# Cut-off circuit for 12V battery

I have a simple arduino driven charger for solar energy to pass into 12V lead-acid battery. However, to use the stored energy I need an analog circuit to cut off discharge at 11.5V. I think a voltage divider ( or 10V zener ) to a N-MOSFET (IRF630), or via a small signal N-MOSFET (2N7000) to a P-MOSFET (IRF9630), plus some extra bits, may be sufficient to switch a relay. I would prefer ditching the relay but then I'd need to heavily heatsink the MOSFET.

My main concern with the relay method is that the relay itself pulls about 0.33W when normal-closed (to clarify, this is also when I discharge the battery), which is a waste. Is there a best practice for cutting off 12V batteries? I have found many on google, of course, but most are 'instructable' type methods, others are rather bulky.

• A firmly on FET won't need a heatsink for reasonable currents. Make a latching circuit from a P power FET and an N signal FET. Consumes no current off, very little when on, easy to switch between states. Power the voltage comparator from the load side of the FET. Jul 17, 2017 at 11:59
• @Neil_UK That sounds great. I'll look into it. Jul 17, 2017 at 12:05
• @Neil_UK Can I get by reliably with a voltage divider in this application? Jul 18, 2017 at 16:53
• a voltage divider would be part of the solution Jul 18, 2017 at 17:32
• sigh! see my answer. Jul 19, 2017 at 5:53

M1/IC1 form the latching 'thyristor type' power switch. It draws no power when off, and little when on.

simulate this circuit – Schematic created using CircuitLab

The TLV431 (not the right symbol, this was close) is designed as a 3 terminal shunt regulator, that maintains the sense voltage at 1.24v. You could think of it as a high gain NPN transistor, with a 1.24v base threshhold voltage.

Consider what happens at the switch point. The sense voltage is around 1.24v, the supply current is passing through 200uA so there's 6v on its supply. This is within its normal operating conditions. Any tiny change in output voltage in either direction will result in switching off, or staying well latched.

I've tested the performance of the TLV431 open loop, and it's well behaved. The cathode stays above 1v, even when ref is well above 1.24v, and it comes out of 'saturation' with no hysteresis. With a cathode resistor of 33k, and 12v supply, it goes from full on to full off in about 4mV of ref terminal movement.

Note that the TLV431 is an unusual device in that the same part number from different suppliers has different specifications. The Texas device only works to 6v, so cannot be used here. The On Semi, Diodes Inc and Zetex devices are specified to 16v, so should be OK. There may be other manufacturers I have not named.

I've shown the components I use to toggle the circuit on and off. Note that for toggling off, the load voltage has to collapse far enough while C1 is holding the FET off. It has only the time constant C1.R5 to do this. If the load has a large reservoir capacitor and a low load, the time constant may need to be increased further.

This is an updated circuit several years later, and contains a number of improvements.

• It won't switch on if the battery is below threshold
• It will support much higher voltages, limited by Q1 and M1
• It is integrated with the power-on LED, to save current
• It has a fast load discharge option when turning off

simulate this circuit

In order to inhibit switch on if the battery is low, the ON switch connects the input voltage to the voltage measuring divider chain via D3, and conveniently the ON LED to show that something is happening. If the voltage is high enough, IC1 turns on, using Q1 as a cascode to get the on signal current up to M1, which turns on, connecting the load, and maintaining the input voltage via D2. The ON button can now be released. D1 limits the gate voltage for high voltage battery inputs. D2 prevents the ON switch from powering the load directly.

TLV431 only goes to 6 V or 16 V (see above, though I think TI have now released a higher voltage one), TL431 goes to 36 V. If 36 V is enough, then if using TL431 Q1 can be omitted and R8 increased, however it requires a higher reference input current drive than TLV431.

I'm using a blue or white ON LED for double duty as a regulator, the slope resistance is an order of magnitude better than a Zener at this voltage. Choose R7 for a few mA if it's needed as an ON indicator, or for 100 uA if it's not.

When coming to turn the circuit off, large capacitors in the load may hold the load voltage above the battery minimum threshold for long enough to be tedious, pressing the off button and waiting for it to stop. R11 allows you to actively load the output when turning off, choose a smaller value of R11 to discharge faster. The maximum current you can draw from the load is limited by R11 pulse load handling, D2 and the off switch current handling.

Use of D2 and D3 reduces the threshold accuracy slightly, but I don't think significantly. R9 and R10 are shown for approximately 11 V threshold, about the minimum for nominally 12 V lead acid batteries.

• No, you can use any part you like, that will fit with your rails and budget. You mentioned 339 in a comment above. That's a quad, the 393 is its dual equivalent, which will use less power, but with those resistor values, the odd mA probably won't fuss you. You could increase the values of the resistors, those are only suggestions, though keep the ratios (mostly). Now normally, we'd advise against using an op-amp in a comparator position. Here, why not? As soon as it begins to chatter at the threshhold, the supply will turn off, job done! Even the rightly hated 741 would work here! Jul 19, 2017 at 7:44
• @user2497 I've added a bit to my answer. Note the caveats about the supplier of the part, and the fact I haven't tested it yet. Jul 19, 2017 at 11:00
• @user2497 I've had a further thought about the second circuit. I've still not built one (you're getting real-time thinking here), but I've worked out what the minimum addition should be such that it regulates properly all the time, so I'm much happier that it should work as expected. In case you wonder why I'm spending this effort, I need exactly this circuit for my own purposes, so it's nice to use your question to kick me into designing it! Jul 19, 2017 at 15:39
• @user2497 IRF9530 is the default PFET that comes up in the schematic editor, use any type with suitable specs. That part # is max 0.3ohm RDS so you can do a lot better. Don't understand 'before or after', R4 is a potentiometer, to allow you to alter the trip voltage with a knob. If you don't want a knob, remove R4, and adjust the values of R2/3/99 to set the trip point. The comparator is available in leaded or leadless, take your pick. Jul 20, 2017 at 5:25
• @user2497 Tested the 431, works fine without the 99 components, updated the post. It's easy enough to modify R2/3/4 to reduce the range of adjustment, or for different cells. Jul 21, 2017 at 20:24