I have the battery charger product https://www.adafruit.com/product/2124 and a teensy 4.0, that I would like to use to power it. what, if any pin would I connect from bat, to the teensy? Its my understanding that the Lion backpack is a voltage regulator, and thats it. it takes Vin, ground, and Vout. I understand the 5v output goes to the 5volt pin of the teensy. the gnd goes to gnd. should Vin go someplace? Or should I leave it off? Please assist.

  • the charger does not have a Vin pin
    – jsotola
    Commented Feb 27, 2020 at 19:45

1 Answer 1


The output voltage of the charger board is marked with BAT. You should connect it to the Vin pin of the Teensy, which then will convert the batteries 3.7-4.2V down to 3.3V.

Of course you need to connect the grounds of the charger board and the Teensy (on the charger board ground is marked with G).

The 5V pin on the charger board is no output. The board does not give you 5V. This is actually the pin for the charging voltage, which is 5V. Here it depends on how you want to charge the battery. If you use an external source, you can simply connect it's 5V to the board. If you want to use the USB jack of the Teensy, you can connect the boards 5V to the Teensys 5V. But: Be sure to cut the little trace between Vin and VUSB/5V at the back of the Teensy. You don't want to connect the battery voltage with an USB power source.

  • I was going back and forth on that. I didnt want to smoke my teensy experimenting, and I couldnt find clear statements on that. I hooked it up that way, and it totaly worked though. A side note, the teensy has a tutorial on cutting vin away from a pad connected to VUSB. VUSB has a through hole via though. I cut the trace between the via and the VUSB pad, giving me a larger pad to solder to. I would have preferred a via for VIN, but using 2 pads seems to make a pretty solid connection.
    – j0h
    Commented Feb 27, 2020 at 22:06

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