I live in Wisconsin and I am homeless. I have 12 NiMH AAcells. I have a seat warmer that plugs into the 12v cigarette lighter. I still have a little bit of cash to make this work. I want to use the batteries to power the heating pad. I have to travel to recover the heating pad. I havent cracked open the controller. It has a three way switch on it for low, med, and high settings. Is it bad to have so many batteries in series? Will the resistors in the controller keep the batteries from getting too hot?

 

The cells are small and can't deliver enough current for a long enough time to be useful so having enough voltage or a smart controller become irrelevant. There's a reason why the pad is designed to attach to a big generator and battery system such as a car.

 

I have a sleeping bag designed for use down to 30ºf. The bag goes inside of a bivy sack, it is a waterproof nylon shell for the bag. I lay down a ThermaRest inflatable mattress in the sack before I put the bag down.

The cells are 1.35Ah a piece, 16.2Ah total. If the pad is 4A, with a smart controller amd thermostat couldn't I have 4 hours of heating time?

I do agree that the system is missing a wind turbine, but a charge controller is an even bigger endeavor. The 16.2Ah might also not be adequate for all conditions throughout a Wisconsin Winter, but AA cells are cheap and i can always get a better sleeping bag.

 

The batteries won't over heat because flow is controlled, correct?

 

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The cells are 1.35Ah a piece, 16.2Ah total. If the pad is 4A, with a smart controller amd thermostat couldn't I have 4 hours of heating time?
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Unfortunately the Ah of cells do not add when they are used in series since all the cells are generating the same current. So your total Ah is the same as that of one battery or 1.35Ah. They won't last long with a 4A load (perhaps about 15 minutes), but longer at the lower heat settings, of course.

The batteries won't over heat because flow is controlled, correct?

Yes, the flow is controlled by the resistance of the pad and depending upon the setting of the controller. But a 4A load could cause the batteries to get hot.

 

Each cell can theoretically provide 1.2 x 1.35 = 1.6 Watt-hrs of energy, so total energy is 12 x 1.6 = 19 W-hr. If that were eked out over 4 hrs you would get ~5 Watts of power. In reality, you wouldn't want to run the cells into the ground, so usable power would likely be only ~3W. Would that be enough for a heating pad?

 

Thanks for the replies. I like the AA cells because of the commercially available chargers. I might be ableto make some equipment that could be handed to anyone in my position.

@crutschow I realize now that making series increases the voltage and making parallel cells increases the amperage. This makes sense. I just read that I should avoid parallel batteries to prevent fires. I will have to test my load on the series.

@alec t I am considering making an ATmega8 to control the voltage. I might be able to scrape together some dough for an arduino.

I dont know a programming language off the top of my head. I know I want a while loop that will take temperature data, then choose how much resistance to apply to the voltage for 3 minutes by closing a switch Then Open the circuit for 2 minutes.

12 batteries is already pushing it if you ask me, but I think this could be useful. Good insulation always works, but this device could make bags more comfortable by keeping the feet warm.