Ahhh, now it makes sense. Let me see if I can explain why the NiMH battery is not a one-for-one substitution for a lead-acid battery.
This is the typical charge curve for lead-acid battery:
Source:
Charging the lead-acid battery
The lead-acid battery nicely develops a back-EMF, an anti-charging voltage, so it stops taking current. But that is not the case with the NiMH batteries.
This is what an NiMH battery does:
Souce:
Battery Chargers and Charging Methods
It just keeps taking a charge and transitions from charging to gas generation and heating mode ... until it does something bad.
The two battery chemistries have distinct charging characteristics and are not a one-for-one substitute. The charging circuit has to be matched to the battery chemistry. The lead-acid battery having a longer history is really pretty easy to deal with. NiMH dates from the early 1990s and takes an entirely different approach.
We can design a circuit to let the NiMH battery replace the lead-acid original. However, it won't be trivial. The question is what advantage does the NiMH battery and specialized charging circuit bring over the easier to deal with lead-acid battery. This is not a question I can answer.
Bob Wilson