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Both NiCad and NiMH batteries are rechargeable. The main difference between the two is that NiMH batteries offer higher energy densities than NiCads. NiMH delivers approximately 30% more capacity than NiCad. NiCad batteries tend to suffer from what is called a “memory effect.” When a NiCad battery is only partially discharged before charging, the battery “forgets” that it has the capacity to further discharge all the way down.
To illustrate: If you, on a regular basis, fully charge your battery and then use only 40% of its capacity before the next recharge, eventually the battery will become unaware of its extra 60% capacity which had remained unused. Your battery will remain functional, but only at 40% of its original capacity.
The way to avoid the dreaded “memory effect” is to fully cycle your NiCad battery at least once per month. In other words, fully discharge your battery and then fully charge it.
Batteries can be discharged by allowing the device to run on the battery until it ceases to function. This will ensure your battery remains healthy. NiMH batteries are “memory free” so they do not suffer from this affliction. Thus, if you have a NiMH battery, the only time it is necessary to cycle it is during its initial use and after a long storage period. This is done to “exercise” the battery and bring it up to full capacity.