Electronic – Heating a li-ion battery for use in sub-zero temperatures

battery-chargingoperational-amplifiersolar celltemperaturethermistor

I have a design that I'm working on that is to be outdoors year round, with a 1 watt solar panel trickle charging a 6 Ah li-ion battery during the day.

I expect there to be about 3 to 4 months of subzero temperatures during the winter given the local climate.

I understand that charging lithium polymer batteries when they are at sub-zero temperatures is a bad idea, since it will cause lithium plating, thereby compromising the battery's future performance.

So, when the battery is too cold to charge (below freezing), my charge controller (MCP73871) automatically stops charging the battery.

In such a case, I would like to be able to take the solar energy that isn't being used and send it to some nichrome wire i'll have wrapped around the battery. I'm thinking that some sort of configuration like this would work… any thoughts?

schematic

simulate this circuit – Schematic created using CircuitLab

Best Answer

Not a bad idea. I disagree with the other comments about using the MCU to control this. Why waste the quiescent current? Periodic brief wake ups do have a real effect over time. Just add some hysteresis to the comparator so that it's not oscillating. You could consider having the output of the comparator connected to an interrupt pin on the MCU if needed.

Also, it's probably just a drawing mistake, but I noticed that your FET is in backwards. You should flip it around so that the source goes to ground.

edit: typo

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