Low cost Lithium Polymer batteries have revolutionized electronics - they're thin, they're light, they can be regulated down to 3.3V and they're easy to charge. On your phone, there's a little image of a battery cell that tells you the percentage of charge - so you know when you absolutely need to plug it in and when you can stay untethered.The Adafruit MAX17048 LiPoly / LiIon Fuel Gauge and Battery Monitor does the same thing. Connect it to your Lipoly or LiIon battery and it will let you know...