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I am working on a battery build project to drive a small 12V electric motor (7A max current draw), and so far I have gotten this far:

  • I will use Li-ion 18650 cells in a 4S6P configuration
  • I will wire up the cells in parallel first, series second
  • I will use a balancing 4S BMS with balance wires connected between each of the units in the series

So the motor is rated for 11-15V, so at max voltage, the battery would be out of range. Because of this, I want to use a buck converter to output a steady 12V.

Where I am a bit lost then is, how can I charge the battery? Do I just wire an input path and output path in parallel, with a buck converter set to 16.8V on the input and 12V on the output, or do I need some other components there?

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  • \$\begingroup\$ The idea of using a buck to get down to 12 V is probably good, as the Li-ion batteries should always be well enough higher that it will work okay. The part about charging the battery is entirely different with a 4S6P. Technically, you should want to charge each one independently as only then can you follow the correct charging procedure. Of course, chargers that are sold over the counter will do anything you want. And no, you do NOT want to charge a 4S6P to 12 V! \$\endgroup\$ – jonk Apr 10 at 18:15
  • \$\begingroup\$ That's why laptops have 19V power supplies: to charge their 16.8V batteries that supply 12V to the computer. \$\endgroup\$ – Simon Richter Apr 14 at 9:39
  • \$\begingroup\$ I'd look at drone parts. There is an entire subculture that does nothing all day except solve this particular problem. \$\endgroup\$ – Simon Richter Apr 14 at 9:39
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"Where I am a bit lost then is, how can I charge the battery? Do I just wire an input path and output path in parallel"

First off, I'm new to this part as well. But this is the idea yes.

Your li-ion charger will do it's constant current (CC) and constant voltage (CV) charging. But it is not just charging the battery it is also powering a "Parasitic load". So especially during the CC phase it will charge the battery a bit slower depending on how much current the motor is pulling. Turning it off during charging is ideal.

The charger should be a lithium charger configured for your 4S pack. And the charger is wired to the main battery positive and negative terminals. Use a BMS with both balancing and over and under voltage protection.

The other parts of your circuit don't have to change.

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