NiMH Pack won’t stay charged
The chances are that one or more of the cells is failing and self-discharging.

You would have to:


Charge the pack and then wait for the voltage to drop.
Un-package the battery pack so you can get at the individual cells.
Use a voltmeter to find which cells have a lower voltage than the others.
Source new cells of the same amp-hour rating.
Disconnect the faulty cells and replace with new ones.  With any luck they will be soldered together.
Package it all back up again.


Don't mix cells of different amp-hour ratings, as it will damage the lower rated cells.

Or just throw the whole thing in the (recycling) bin, and buy a new battery pack.