Reviving a new but dead tool battery.


Newly Enlightened
Jul 6, 2022
I got a new Chinese no-brand power pruner. The batteries look like they're a knock off of some big brand slide in type that normally has several contacts. These have two, and charge via a barrel jack on top.

One battery would charge, the other would not. So I opened up the defekt battery and found five 18650 cells connected to a BMS board with monitoring connections to all five cells. That's decent, not just linking them in series with a bare minimum of charge control.

Measuring the cells, the center one was over three volts, the other four were well under three. Obviously the BMS was setup to do nothing in such an imbalanced state of charge.

I happened to have a universal NiCd, NiMH, Li-Ion 1.2~3.7V single cell, magnetic, polarity agnostic, USB charger from Olight. (They very briefly had it available when a batch of the discontinued item was found in one of their warehouses.)

I clipped it onto one of the low cells and plugged it in for a few seconds then took it off and checked the voltage. It went up. Yay! ;) So I put it back on for about a minute and checked again, over 3 volts. I repeated that with the other three low cells then reassembled the battery and plugged in the (extremely light weight) wall wart that came with the pruner then left it until the red LED on the 'wart turned green.

Popped the battery into the pruner and it worked.

Some of the plastic edges needed deburring to remove sharp flash but the blades are robust and it nips small branches off without a struggle.