My cheapo 18V Black and Decker cordless drill recently died, and thinking I could maybe fix it, I took everything apart and then couldn't figure out how it was ever working in the first place! Here's a picture of the internals:
![Black and Decker 18V cordless drill internals](https://i.stack.imgur.com/BOqOr.jpg)

The wall wart is 3.6 VAC, 200 mA output, which runs straight to the terminals of the 18 VDC battery at the bottom of the picture (and I've taken the battery apart to make sure and there is no circuitry inside, just battery cells). This seems completely odd, and I can't figure out how the battery ever charged while it was working.

The only thing I can possibly figure is that the switch housing also contains boosting/rectifying in addition to the PWM etc... to run the motor at various speeds according to the depression level of the switch. There does appear to be a transistor attached to silver colored heat sink, and I'd imagine they could be implementing both the PWM and a switching power supply with that, but that seems ... advanced for a cheap B&D drill, or maybe not? It also seems strange that the boosting circuitry would be connected to the same leads as those coming off the VAC power supply.

Measuring with my multimeter, I get 4 VAC from the wall wart open circuit, and connected to everything I measured 1.7VAC at the battery terminals, and 1.8V DC. (but again, this system is broken in that the charger no longer seems to work.)

Can someone shed light on how they think this setup is supposed to work? (and bonus points if you can think of what broke?). Thanks!