Here is a question for the team, I'm trying to repair my sons electric toothbrush which will only run if its just it's just been taken off charge. Ah ha, me thinks, duff battery, I open it up and measure the voltage across the Ni-MH battery when it is off charge, which was 1.44V but the brush will not run. Connect the brush upto a bench supply and give it 1.76V for a few seconds and the brush will switch on when the button is pressed and will continue running, even if I drop the voltage down to .7V.
![Confused :-//](https://www.eevblog.com/forum/Smileys/default/confused0024.gif.pagespeed.ce.5xOqKkq0Co.gif)
Pop the brush back onto its wireless charger base and the voltage across the battery terminals rises upto around 1.78V and the brush will start after a few seconds or so of being on the base. If I remove the battery and pop it back onto the charger base once again, the voltage across the battery terminals instantly rises upto 4.2V but the brush will not run.
I'm assuming that this is because the wireless charger is incapable of providing sufficient power to run the ultrasonic motor
So my question is, what is the maximum theoretical voltage that Ni-MH battery could be charged upto?