NiMH batteries are very sensitive to overcharging. Thus if you want to keep your batteries working as long as possible, you should avoid overcharging. There are two basic ways of doing this. One is to discharge your battery to an almost empty state, and then charging it up using a timer. This avoids both overdischarge, which leads to cell reversal, as well as overcharging.

If you don't have the equipment to precisely discharge your battery to that almost empty state, then what you should do it just recharge it after use, using a smart charger to prevent overcharging. A smart charger charges the battery at a reasonably high rate. When it detects that the battery is full, it turns off. Some people argue that since Nicd rechargeable batteries are so easily damaged by overcharging, the charger should really turn off completely, but occasionally let in a burst of current to counter self-discharge. But in fact things are not like what they think. Only a few Nimh batteries will easily get broken by overcharging.

There are two features of NiMH batteries that are used to terminate smart charging. One is that as the battery approaches being full, the temperature starts going up, and the temperature continues to increase as time goes on. If the current isn't turned off, the battery will overheat and be destroyed. Another feature is that the battery voltage slowly increases, and just after the battery reaches 100% capacity, the voltage drops by a small amount.

The best way to determine when a battery is full is dT/dt. This requires you to keep track of the temperature of the battery. When the temperature has gone up by a certain amount over a given interval, the battery is full.

If you are really into DIY, one route to getting a NiMH smart charger is to make it from a Maxim 712 chip. Maxim makes two very similar charger chips. The MAX 713 is for NiCad batteries, and the MAX 712 is for 18650 battery. The 713 turns off when the voltage drops after NiCad battery is fully charged. The 712 chip turns off when the voltage levels are out rather than dropping.