Sunday, July 11, 2010

Lead-Acid Battery Overdischarge Protector

Don't really have a schematic for this small project. I had an MC34161 (datasheet PDF link) glorified comparator voltage monitor IC in an old inverter PSU that I decided to re-use to protect a single SLA battery against over-discharge. Only one voltage monitor channel is used, and the chip is configured to be an overvoltage detector. The open collector output it pulled high with a random resistor and it's connected to the gate input of a random N channel FET. The MC34161 is calibrated with a precision trim-pot (not the one in the picture, I replaced that because the threshold would crawl 0.5V just by blowing on it :) ) to detect overvoltage at around 10.6V. At which point it'll switch the FET enabling the output terminals. If the voltage sinks below 10.6V it'll disconnect the battery from the output. (overvoltage condition disappears)

I actually haven't tested whether the chip's built-in hysteresis is enough to prevent oscillation yet (large load on battery -> battery discharges below 10.6V -> battery is disconnected -> battery voltage rebounds ? -> ? above hysteresis threshold -> repeat until something melts/breaks/explodes)
This is obviously something to avoid. It's probably not a issue for me because it doesn't matter for the devices I'll be powering off this battery. (I'd just realize what's going on and power them off) but it's something to keep in mind in case anyone builds something similar based on this chip.

UPDATE: Ok, you probably shouldn't build this unless you use an extremely low RDS-on FET or something. I'll be modifying this to work as an undervoltage detector and to use a bistable relay when I have the time. This'll also have the advantage of solving the hysteresis issue (the relay will just switch off when the undervoltage detector triggers it). If I set up the chip as a dual detector I could use it to switch the relay back on when the voltage rises above say.. 13 volts. I'll either do that, or just add a "reset protection" button if I'm lazy. (I'll probably just do that)

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