I have 3x 18650s wired in a series, connected to the BMS seen here. The two outputs of the BMS are wired to a MOSFET that drops the voltage from 12 to 5, in order to power an Arduino Trinket Pro.

During prototyping I was using your basic DC jack to connect to the Arduino and charge the battery pack. However with the 3D printed part I have this all in I am using a different battery contact to connect the battery pack and Arduino.

Now I am needing to find a different way to charge the battery pack, preferably through a micro USB or USB C charging port, like you see on portable power packs.

I've picked up the TP4056, seen here, and the USB C version. My question is can I / would it be a good idea to splice off of the BMS outputs, and insert a diode into the connection between the battery contact that connects to the Arduino and a diode into where the TP4056 runs.

The first time I had tried it with a standard micro USB port, no diodes, which heated up very quickly.

Or, would I be better off just running the TP4056 straight to the positive and negative terminals on the pack itself, bypassing the USB. enter image description here

  • \$\begingroup\$ [MOSFET drops the voltage from 12 to 5, in order to power an Arduino] = very lossy. Have you considered a switcher? \$\endgroup\$
    – rdtsc
    Jul 23, 2019 at 22:40
  • \$\begingroup\$ No I have not - I will have to look into that. \$\endgroup\$
    – Brian
    Jul 24, 2019 at 16:44

1 Answer 1


Three 18650 lithium battery cells will be 12.6V when fully charged. The TP4056 is made to charge one cell or a few paralleled cells to 4.2V so it will not charge your cells that are in series. A Mosfet by itself will not regulate the 12.6V down to 5V.

  • \$\begingroup\$ The Mosfet is the L7805CV, while not the most efficient, it works for this prototype. Any ideas on what to use instead of the TP4056, potentially wire 3 TP4056s together to a single micro usb port? \$\endgroup\$
    – Brian
    Jul 24, 2019 at 16:46
  • \$\begingroup\$ The L7805 is a linear voltage regulator IC, not a MOSFET. \$\endgroup\$ Jun 23, 2020 at 0:40
  • \$\begingroup\$ If you are reducing 12 volts from the battery to 5 volts using a 7805, you are wasting 60% of the battery power heating up the7805. \$\endgroup\$ Jun 23, 2020 at 1:09

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