NiMH stands for Nickel-Metal Hydride, the materials used in some battery packs. Unlike NiCad batteries, NiMH batteries do not use heavy metals that may have toxic effects.
In addition, they can store up to 50% more power than NiCad batteries and do not suffer from memory effects. I use Rayovac rechargeable NiMH AA batteries for all my outdoor and household devices.
In fact, the Rayovac rechargeables started losing their strength only after eight years of continuous usage. And I still use a couple of them from the first set I bought many years ago. That is more than 1,000 cycles!
Over the last couple of years, NiMH batteries of higher capacity hit the market, especially in the popular AA size which is what most of the metal detectors use. The first cells were only 800-900mAh capacity but now we have a wide choice of cells ranging from 1500 to 2700mAh. Too bad that the currently made rechargeables do not last as long as the cells that were made in the past decade.
The higher capacity means the longer run time of your batteries. I run my detectors on eight 1800mAh NiMH batteries and get at least 36 hours of continuous detecting time from each machine. In case the rechargeables "die" on me during the search, I charge them from the car battery by plugging my charger into the cigarette lighter (make sure the car adapter comes along with your batterry charger).
NiMH battery technology is improving at a fairly rapid pace, and it is expected to upgrade and replace the cells every one to three years. The chargers, on the other hand, are a longer-term investment, and charger performance has a significant impact on battery performance. The 8-cell battery charger that I use is not a fast charger but it is a convenient charger and it does a great job.