LiIon/LiPoly Backpack Add-On for Pro Trinket/ItsyBitsy – Adafruit 2124
If you have an ItsyBitsy or Pro Trinket you probably know it’s the perfect little size for a portable project. This LiPoly backpack makes it really easy to do! Instead of wiring 2 or 3 boards together to make a charging system, this little PCB sits on top of the PCB and allows a LiPoly/LiIon battery to plug in. When the Pro Trinket or ItsyBitsy USB port is powered, the battery is recharged automatically. Unplug the USB port and the microcontroller will switch over to the rechargeable battery.
Ideally, this backpack is for use with the 3.3V Pro Trinkets & ItsyBitsy, so that the battery voltage (3.7V-4.2V) will get regulated down to 3.3V on the board. However, many times, you can run a 5V microcontroller + accessories off of the ~4V from the LiPoly battery with no problem. (Technically its overclocking but we’ve never seen that affect the microcontroller itself, AVRs are happy to overclock without complaint).
Solder the backpack on with the extra-long header so that it can still plug into a breadboard, or clip the long header leads short afterward for a slim package. There are two LEDs – one red and one green. While charging, the red LED is lit. When the battery is fully charged and ready for use, the green LED turns on. Seriously, it could not get easier.
If you want to add an On/Off switch, we also made that really easy. The two 0.1″ holes with a box around them are the battery output line. Carefully cut the trace between them with a hobby knife and replace it with two wires from a switch like this slide switch, for example.
Charging is performed in three stages: first a preconditioning charge, then a constant-current fast charge, and finally a constant-voltage trickle charge to keep the battery topped up. The charge current is 100mA by default, so it will work with any size battery and USB port. If you want you can easily change it over to 500mA mode by soldering closed the jumper on the bottom, for when you’ll only be charging batteries with 500mAh size or larger.
Pro Trinket Backpack Dimensions:
- Board Dimensions: 15mm x 17mm x 2mm / 0.6″ x 0.7″ x 0.08″
- Height w/ JST: 7mm / 0.3″
- Weight: 1.3g
- MCP73831 (Datasheet)
- Always carefully check the voltage and polarity of any battery before connecting to this backpack.
- Uses the 5V input via the Micro-B USB connector on the Pro Trinket or ItsyBitsy
- For charging single Lithium Ion/Lithium Polymer 3.7/4.2v batteries (not for older 3.6/4.1v cells)
- 100mA charge current, adjustable to 500mA by soldering a jumper closed
- For the Itsy nRF52840, this backpack covers the reset/user buttons (you could use 3 wires instead of having it sit on top.
- You may get an off-white or a black JST connector. It’s normal for the red and green LED’s to be very faintly glowing when the battery is attached but not charging, there is a 500uA reverse-leakage current from the Schottky diode from the battery that back powers the charger chip.