Dave covered most of what you need to know but there is something else to consider.

Those NiMH tend to self discharge just sitting which can cause them to get out of balance. You may have a couple cells that are totally dead while a couple others are nearly full which causes peak chargers to cut off. About the only things you can do about it are slowly trickle charge them overnight or discharge the cells individually to a common voltage before charging them.

Or just convert to LiFe or LiPo and eliminate all the headaches.