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Storage Capacity Drop in Nickel  Metal Hydride and Other Rechargeable  Batteries

The rule of thumb on capacity reduction over storage time is dependent on capacity of cell. The higher the capacity of

 battery, the more capacity drops over to period of time in storage.

The reason is that the N/P ratio is less in higher capacity and is more compacted together in electrode coining process.

Secondly, it is also dependent on the model/type of battery.

The bigger the size of the battery cell the higher the capacity drop.

Cylindrical type battereies always have a higher capacity drop than coin/button cell battery.

When not in use, if you keep on a charger that gives the battery a constant steady trickle charge, there is less capacity drop.

For example: AA2100 will have about 5-6%

drop in capacity while AA2500 has 8-10% capacity drop in a year.

 If we trickle charge the battery for a year, the capacity drop is only with 1-2%.
 
The limiting factor is battery's natural aging.

It is best to keep the battery in full charge at all the times in order to keep the battery in the highest electrical energy

 potential. However, the active materials in a battery will normally crystallize or hydroxide with H2O inside the

 electrolyte will dry out over a period of a1-2 years from the heat generated in the charging process.

Electrolyte can also attack the separator PP material to make it less able to affiliate (mix) with OH- ions.
 
In Ni-MH, The most perfect case is about 7-10 years.

A  flooded type prismatic battery (SLA) where its N/P ratio is relatively high, contains lots of electrolyte, has thicker

 separator, has been trickle charge continuously that can sometimes last many years longer than a AA/AAA/C /D or 9

 volt Nickel Metal Hydride battery.

Thus, there is no cylindrical battery in the world which can last for 10-20 years.