Lead acid batteries get damaged by over-discharge somewhat like lithium polymer batteries.
I need a circuit that will cut off the load from the battery when it drops below about 10v. That is the easy part.
The hard part is having a circuit that consumes a miniscule current and wait until the battery's voltage exceeds around 13v, (which would indicate that it is being charged,) at which point it would re-engage the load.
The best idea I have is to use a low-quiescent current voltage regulator, specifically the TS2950CT50 +5v which has a quiescent current of around 75uA. I would use a mid-range PIC16F MCU that has an internal voltage reference (and either ADC or comparator) that goes into standby mode, periodically checking if the voltage exceeds 13.5v. The MCU could consume as little as a couple of uA.
While 75uA isn't too bad, I was wondering how I could achieve a lower standby current.
Another idea which I'm not certain about at all, is operating a zener diode below its breakdown voltage, relying on a slight increase of leakage current to trigger a MOSFET circuit. Leakage for zener diodes seem to be 1uA per volt, so that would in theory only consume 10uA. But since zeners don't have a great accuracy at the best of times, let alone over different temperatures, with aging, EMI or what have you, could I use this reliably?
At a push, it would be permissible to have the over-discharge protection deactivate when the battery's voltage reaches anywhere between 12.5 and 13.5v.
Because I'm a masochist, I'd prefer not to use an IC which is dedicated to this purpose (not that I've found one). What other solutions are there?