This is what happened when someone left their solar array charging NHW20 traction battery on a 'cloudy day:'
The thing about NiMH chemistry is they have this relatively flat voltage vs charge curve until the very end of the charge. Then there is a bump up and they become 'wicked.'
I believe Radio Shack carries something like an LM317L and if you bias it for 7.8-8.0 V. max, you should be safe. But anything over 8.0 V. and my experience is the NHW11 modules (and I suspect the NHW20s) become 'gassy' and bad things can happen. You'll need ~1.2 to 1.5 V. head room above the output adjusted voltage. A 9-12 V. 'wall wart' would be sufficient.
Of course the other approach is to detect the temperature rise but that tends to lag gas generation.
Bob Wilson