NiMh batteries provide much higher storage capacities than traditional NiCad. They also tend to hold voltage longer (under load) than NiCad. To simplify, a 800mAh NiCad will see the voltage fade as it discharges whereas a similar NiMh will hold full voltage longer…then fade rapidly at the end.

Another thing to consider is the batteries designed current draw. One AA may support high discharge rates where another won’t. Note that most ‘domestic’ grade batteries don’t support high discharge rates. Many hobby batteries are designed for higher discharge rates.

Fast charging is possible with most NiMh, but to gain maximum cycles/life try lower charges. Personally, I use no more than 800mAh charge rates for all NiMh AA sizes.

NiMh batteries also heat whilst charging, and you may even notice they drop back to ambient as they approach full charge. As long as they don’t get too toasty they should be fine. Warm is better than hot.

Another warning for NiMh is to be aware that some discharge whilst stored. Typically, higher discharge rate batteries will also discharge faster whilst stored. If your batteries fade quickly when stored they are at the end of their life.

The best way to charge batteries (if you depend on them) is to buy a specialised charger. Many R/C hobby stores sell models that will cater for everything from NiCad, NiMh, LiOn, LiPo, LiFe, and A123. These often run off 12VDC which makes hooking one up to your car (or car battery) a breeze. You can buy decent quality computerised chargers for as little as $50-$75. Some will even provide individual cell balancing.

In summary, if you depend on batteries for hunting/camping etc then it may be worth buying speciality batteries. They are generally better quality and capacity (see DSE, Jaycar or similar). Buy a good hobby grade charger, they are much more capable and relatively inexpensive. Finally, keep the charge rates sensible (I like 800mAh for NiMh).