Chances are if you’ve used rechargeable batteries for a long period of time, you’ve noticed that they don’t hold a charge as long when they get older. Eventually, they get to a point where when you charge them, they only last a few minutes before they are flat again. This is due to a phenomenon called voltage depression, but it is commonly referred to as “the memory effect.”
Voltage depression plagues every type of rechargeable battery, from Nickel-Cadmium (Ni-Cad) batteries to Lithium Ion (Li-Ion) batteries. Inside of a Ni-Cad battery, there is a cadmium plate that when charging occurs, creates tiny crystals that store energy along their boundaries. If the battery is overcharged (that is, if it is left on the charger for too long), these crystals grow to be too large, making the crystals harder to dissolve during energy consumption. This issue also occurs if the battery is recharged when it still has a relatively high amount of energy still in the battery or if the charger is too slow.
Often times, the effects of voltage depression can be reversed by fully depleting your battery’s charge before recharging it. Once the battery reaches about 10% of its total capacity, the large crystals have been dissolved and the cadmium plate has reformed. You can now recharge the battery fully (remembering to take it off the charger once it is fully charged) and it will retain most of its total capacity again. The true “memory effect” however, is from damaging the battery cells from over charging or frequent discharge/recharge cycles. This damage to the individual cells prevents the formation of the microcrystal cadmium, thus permanently reducing the total capacity of the battery.
Eventually, all rechargeable batteries will stop charging altogether due to this damage, but reducing overcharging and constant charging cycles will prolong their life.
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