What's the safest way to determine a lithium-ion polymer battery's discharge and charging state using an Arduino?
I see a ton of 12 V LiPo batteries like this being sold on eBay and elsewhere.
They have a built-in charger, and I'd like to use one to power an Arduino, but the Arduino needs to know "when" the battery needs to be charged, and "when" the battery is fully charged. Unfortunately, the battery doesn't expose any pins or LEDs or other outputs that I could tap into.
I've found a few LiPo charging circuits specifically designed for the Arduino, but they all can only handle 3.7 V cells.
I'd like to avoid having to disassemble the battery or modify it to allow me to monitor its state. Is there an easier way, like connecting a battery lead to an analog pin and measuring voltage, or using a coulomb counter?