How to monitor 12 V LiPo charging from Arduino 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?
If you want to buy something off the shelf that will provide power for an Arduino, try a power bank - they have USB out, and are also charged via USB. I found that I had to put a 100 Ohm resistor to stop mine going to sleep, but it's +5V native.

A 100 ohm resistor (0.25 watt at least!) draws 50mA, so a 5200mAH powerbank like http://www.dx.com/p/universal-5200mah-external-li-ion-battery-charger-power-bank-w-led-indicator-usb-cable-pink-362725#.VQneCnWjlIc would leak it empty in 104 hours. If you use at least 50mA at all times, then this isn't needed - I think an Arduino draws 35mA already, so you should get away with a 300 Ohm resistor, drawing 16mA. Some experimentation might be needed.