I'm trying to build a set of 18650-based batterybank devices of various voltages (5-12 V). I assumed I would charge these devices via USB-C.

Looking into the USB standards, I see 5 A is the highest available amperage in any USB flavor. I've been using 2S and 3S BMS breakout boards to charge my batteries and they have max. current draws of 7 A and 10 A.

How do I charge multi-cell battery banks from a USB 5 A source when the BMS draws in excess of the USB standard? How are large devices such as laptops powered with this amperage limitation?

  • 4
    \$\begingroup\$ Simple: charge the batteries at a lower current than the maximum. \$\endgroup\$
    – Hearth
    Aug 6 at 1:27
  • 1
    \$\begingroup\$ A boost converter can step up the 5V USB voltage to whatever is needed for a battery pack, or a buck converter can lower the voltage for a single cell. But power will be limited to 25 watts. \$\endgroup\$
    – PStechPaul
    Aug 6 at 3:19
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    \$\begingroup\$ Some USB C supplies allow 65W. A boost converter is required from 5,V. At high wattage USB C outputs 19v \$\endgroup\$
    Aug 6 at 3:27

1 Answer 1


The way it is done is using a battery charger.

Batteries, or even a BMS, can't be connected to USB, or any other voltage source, not without a charger.

A typical laptop supply could be 65W and that's a bit over 3A at 20V. A charger with a step down circuit can convert that 65W to be suitable with the battery, e.g 5A for roughly 12V battery or 16A for roughly 4V battery.

  • \$\begingroup\$ Thanks, @JustMe. Something like a TP 5100 upstream from the BMS? \$\endgroup\$ Aug 7 at 0:34

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