I got the same problem : failed to turn on after charging and found out the issue: it's the supply barrel jack!
Let me explain:
- I tested the NiMH battery and it was good (voltage & discharge charge current)
- I tested the motor and it was good (close to 0 ohm between poles), it also worked fine when supplied around 1.3V DC
- I tested the switch and it was also good
- I came to suspect the diode between the supply jack and the positive supply but by rebuilding the schematic, it could not be suspected
Then I tried to connect directly the battery to the motor bypassing the PCB and found out it worked fine. So the only remaining component was the supply jack!! It's not a dual pole jack, it's a 3 pole and I think the issues lies here: the 3rd pole is switched open (or close) when inserting the supply jack and this prevent the trimmer to start when it's charging.
As I didn't want to buy another supply jack, I just soldered 2 small cables between the battery and the motor and exited the device through a small hole: when connected together, it allowed to start the trimmer, but to charge it, I need to untie the 2 cables. Yes, it's unaesthetic but it's fine for me!
I hope this help, honestly it's very simple to solve and it's not the usual suspect (the battery)!