Letter Re: Shelf Life of NiMH Batteries?

No one seems to know about storing new NiMH batteries pre-charging for a long term, I wouldn't try it with testing it for several years first as any chemicals of this type age with time. Here is some specific info on the storage and longevity of NiMH batteries, from Wikipedia and other battery FAQs. They are your best bet for power density and economy for rechargeables, but for really long term storage 5-10 years the single use Lithium type batteries are the best (and are available in AAA, AA, C, D and 9 volt sizes).

Disposable alkaline batteries are designed to be used only once. Even if never taken out of the original package, disposable (or "primary") batteries can lose two to twenty-five percent of their original charge every year.

Many people believe that storing batteries at cool temperatures, such as in the refrigerator, reduces the rate of these side reactions and extends the storage life of the battery -- this may have been true in the past with older technology batteries. Modern batteries should be stored in a dry place and at normal room temperatures.

All rechargeable batteries self-discharge more rapidly than disposable alkaline batteries. In fact, they can self-discharge up to three percent a day (again, depending on temperature).

With the exception of lead-acid batteries, most NiMH batteries can be recharged 500-1000 times while NiCd batteries can only be recharged about approximately 400 times. Nickel metal hydride batteries have a high self-discharge rate of approximately 30% per month and more. This is higher than that of NiCd batteries, which is around 20% per month.

Like any rechargeable battery, even the one in your car, some capacity will be lost after a certain amount of time when rechargeable batteries are stored and not used for extended periods of time.. Normally all that is required with most NiMH batteries is to run them through 3 charge / discharge cycles and normally this will revive them to full capacity. Also keep in mind that even when used on a regular basis, some capacity is lost with each cycle, although very small.