3S1P Li-Ion pack charging with USB

Hello :slight_smile: Sorry to use this forum when not regarding ESK8 but I know there are a lot of electronic gurus over here so I gave it a shot :slight_smile:

I’m making a portable Bluetooth speaker and need some help with how to charge the battery pack.

I have made a 3 cell Li-ion pack with a BMS to get a 12V battery for the speaker. My thought was to charge the pack over USB with the help of a 5-12 Volt step-up converter. I’ve set the converter to output 12.6 Volts and then connected it to the charge points on the BMS. This all works and the battery is charging just fine. But…

The step-up converter gets really hot, even though I’m only using a 1 amp USB-port as the input power. What am I doing wrong? Or is that just how it is, that it gets really hot? I’ts so hot that it hurts when I touch it and I don’t feel comfortable putting it inside a sealed enclosure (the speaker box).

Please help me. Is it drawing too much current? How can I limit that? Or is my design just not possible?

Thanks! :slight_smile: