NiMH (Nickel Metal Hydride) batteries are the results of recent technology advance. NiMH batteries last much longer than NiCd (Nickel Cadmium) batteries per charge. NiMH batteries also are environmental friendly because of elimination of cadmium toxicity concerns. Today, more and more high-drain devices consume battery power quickly. Consumers will greatly benefit from the value proposition delivered by NiMH. Not only do they deliver more power, lasting longer in high drain devices, but they also can be charged up to 1000 times. Fits 9V TENS and Muscle Stim units.