2 Li-Polymer in series charging ... while still delivering voltage

Thread Starter


Joined Apr 5, 2023
Hello all,

I have a circuit I'd like to power with batteries. I'm thinking about using Li-Polymer 3.6V.
The circuit voltage need to be supplied by 5V. I have the idea of using two batteries in series as power source (7.2V), this also fits quite well the small space I have in my housing.
I'd like to load my batteries thanks to standard USB loader (meaning 5V source). And would like to keep the 5V source operational during my batteries are loading.
My question is.
How can I organize my power circuit to manage to have at one time the two batteries being in series in normal circuit operation (a step down circuit will generate the 5V from the 7.2V) and during the USB charger being connected, to have the batteries being in parallels to be loaded by the 5V source?

Any good recommendations?

NB: I'm talking about small system with batteries lower than 300 mAh...


Joined Jul 10, 2017
It would be much more efficient to connect the two cells in parallel and use a step-up converter to get the 5 V power for the circuit. You could then use a simple 5V single cell charger module to charge the cells. These are available from AliExpress and cost next to nothing, and "yes", you can charge while powering your device.


Joined Jun 4, 2014
I have just done something similar. I have two 18650 cells in series. Each cell is connected to a TP4056 module for charging. These need a 5V supply.
The supply to the TP4056 connected to the upper cell MUST be isolated or bad things will happen so a 5V-5V isolated DC-DC converter is needed.