This little backpack is very "short & sweet" when it comes to pinouts:
- 5V - this is the pin that comes from the USB port power, with 5V DC to charge the battery.
- G - this is the common ground pin, shared with the battery and charger
- BAT - this is the voltage out from the battery, which will range from 3.2V when the battery is dead to 4.2V when it's charged.
The default current rate is 100mA, which is safe for any battery 100mAh or greater capacity. If you're going to be using larger batteries, you may want to bump up the charge rate for faster charging. Easy! Just solder the jumper on the bottom of the board as shown here:
There's two LEDs on the backpack
- CHG red LED - This LED is lit when the battery is being charged.
- DON green LED - this LED is lit when the battery is done being charged (charge complete)
When the backpack is installed on the Pro Trinket, but not charging, you may notice these LEDs faintly glowing. That's a side effect of a slight current leakage through one of the protection diodes on the Pro Trinket. It's not harmful at all and you can basically ignore it.
If you want to have an on/off switch for your project, it's a quick mod. The line from the battery to the VBAT pin has a 0.1" breakout with a thin trace in between. Cut the trace between the two outlined pins and solder a switch in place. The switch is current-carrying so make sure it can handle the current draw you're expecting