Nickel metal hydride (NiMH) rechargeable battery technology is a great step up from the more familiar nickel cadmium system. For the first time we are able to buy a battery that has similar capacity to alkalines. Unfortunately both types of rechargeable do not as yet approach the charge retainment of alkalines, which can be stored with full charge retention for several years. Additionally, many NiMH rechargeables tend to have reasonable retention times only when new, and often lose it after a few months of use. I own two cameras, both of which use AA cells. The only alkalines which can give the same longevity as a near-new set of NiMH rechargeables are Energizer lithiums, which are extremely costly, whilst regular Energizer cells typically only last for a few shots. I’ve found from experience, however, that even NiMH cells which have been charged and discharged a mere dozen times lose significant capacity and retention within a year, and must be replaced completely to be of any use. There have, unsurprisingly, been several major events in which I used my cameras extensively, where the NiMHs have lasted only a handful of shots and where lithiums or ordinary alkalines that I always carry a set or two of have saved the day.
Sanyo of Japan is a major manufacturer of NiMH cells, including numerous OEM rebrands, and has now developed newer NiMH technology which boasts a great improvement in charge retention. Their own brand of these cells is "Eneloop", and various other suppliers are now advertising similar products, probably under OEM arrangements. I have purchased several sets of Eneloop and Varta "Ready2Use" cells. How well do these work? The Eneloops are in use in my bigger camera, which requires four, and are presently giving excellent service, both in terms of longer cycle life, and retaining charge for long periods of disuse. The Vartas are in my smaller camera used in pairs. They are showing similar capabilities. So far, both have been exceptional in service. I would like to wait another six months to see if they are still making the grade and keeping my cameras running without the expense of annual battery replacements.