The battery started draining quickly.
After disassembling it, I discovered that the motor gets very hot when brush is off, which is apparently causing the battery to lose charge.
The motor is connected to contacts M1 and M2.
When I turn on the brush, 0.4V appears on these contacts, and the motor starts running as expected.
But when I turn off the brush, the voltage changes to 3.8V—exactly the same as the battery.
The motor gets so hot you can't touch it. And after 5 minutes, the battery drains to 0V.
(To...