Cell phone batteries for projects

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Cell phone batteries for projects

Post by gordyn » Wed Sep 01, 2010 6:47 pm

I recently picked up some Motorola BT50 3.7V li-ion packs for various applications and was wondering if anyone has had experience repurposing cell batteries. After disassembling one pack I noted the charging circuit contained within each however was puzzled by what appeared to be 4 contacts on the external of the pack. One on each side is clearly labeled positive and negative, but the two in the center remain a slight mystery. Any ideas how these extra contacts can be used as well as best practice to charge?

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Re: Cell phone batteries for projects

Post by MrAl » Wed Sep 01, 2010 7:48 pm


Those extra contacts could be for a thermistor that is used to sense temperature.

The way to charge Li-ion is to provide a current limited by the max charge rate, then when cell voltage gets up to about 4.15v cut back the current. It's called constant current followed by constant voltage charging.

The thing about Li-ion is that the can not be allowed to go above 4.200 volts or the cell gets damaged. Due to meter tolerances and other variables 4.15v is good compromise.

I have a cell like that that i used for a head lamp, but it only had two wires coming from it.
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