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 400mAh for a total of about 1.9 Wh. The batteries come pre-attached with a genuine 2-pin 25mm long JST-PH connector as shown and include the necessary protection circuitry. Because they have a genuine JST connector, not a knock-off, the cable won't snag or get stuck in a matching JST jack, they click in and out smoothly.
The size of this battery was customized so that it tucks perfectly between the headers on a Feather, and you can even sandwich it between a Feather and FeatherWing. As seen above, its a perfect fit for a Feather Huzzah ESP8266 and an Adafruit FeatherWing OLED! Just remember to use the normal-height female headers or stacking headers, not the shorty ones.
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 400mA or less. We suggest Adafruits Micro Lipo charger, which has a 100mA default rate. You can also set the Micro Lipo to 400mA rate for a faster charge. All of our Feathers charge at a safe rate.
Like most lipos, the batteries we sell do not have thermistors built in. This is why we suggest charging at 1 C or even less - 100 to 400mA is a good rate, and available from any USB port if using a USB-powered charger
Additional safety notes: Do not use a NiMH/NiCad/lead-acid charger! Also, do not abuse these batteries, do not short, bend, crush or puncture. Never charge or use unattended. Always inspect batteries and surrounding circuitry constantly for any damage, loose wiring, or possibility of short circuits. Polarity matches all Adafruit LiPoly/LiIon chargers and boards, other brands may have reverse polarity and can destroy your battery. As with all Lithium ion polymer batteries and with any power source - they should be used by experts who are comfortable working with power supplies.