HPS40 internal battery charging circuit

[tzok]

I've recently scored an old Velleman HPS40 Handheld Scope. It was described as in working condition, but works only on external adapter. When the scope arrived, I discovered that the only reason for not working on batteries was the fact that batteries leaked and the contacts corroded. This was an easy fix, but later I discovered, that the internal charging circuit overcharges batteries. Originally it was designed for 5 Ni-Cd rechargeable AA size batteries, but the charging circuit heavily depends on the external adapter. Manual says that it should be 9VDC non-stabilized or 12VDC stabilized power supply of at least 300mA.

Pleas look at the charging circuit schematic:

...and its simulation:
http://tnij.org/hps40

Any ideas how to improve it to make it more safe to the NiMh batteries, but not radically complicating it?

P.S.
The scope is exactly the same as modern HPS50, the only differences in the hardware of HPS50 are USB out circuit and battery charging circuit (as it uses Li-Ion battery).