Adafruit 328 Lithium-Ion Polymer 2500mAh 3.7V rechargeable battery

Lithium Ion Polymer, 3.7v, 2500mAh Lithium ion polymer (also known as 'lipo' or 'lipoly') batteries are thin, light and powerful. The output ranges from 4.2V when completely charged to 3.7V. This battery has a capacity of 2500mAh for a total of about 10 Wh. If you need a smaller battery, we also have a 1200mAh model

The batteries come pre-attached with a genuine 2-pin JST-PH connector as shown and include the necessary protection circuitry. Because they have a genuine JST connector, not a knock-off, the cable wont snag or get stuck in a matching JST jack, they click in and out smoothly.

The included protection circuitry keeps the battery voltage from going too high (over-charging) or low (over-use) which means that the battery will cut-out when completely dead at 3.0V. It will also protect against output shorts. However, even with this protection it is very important that you only use a LiIon/LiPoly constant-voltage/constant-current charger to recharge them and at a rate of 1200mA or less.
Like most lipos, the batteries we sell do not have thermistors built in. This is why we suggest charging at 1/2C or even less - 1200mA max in this case. Even 500mA is a good charge rate, which is how much you can get from a USB port.