The memory effect of rechargeable batteries

Have you noticed your rechargeable batteries don't seem to last as long as they did when they were new? This memory effect occurs in some rechargeable batteries when you don't sufficiently discharge them before recharging. The batteries then ‘remember’ where they were up to in earlier discharge cycles and won’t recharge fully.

In some battery cells, the memory effect is caused by how the metal and electrolyte react to form a salt (and the way that salt then dissolves again and metal is replaced on the electrodes when you recharge it). In our ideal battery cell, we would want a uniform coating of small crystals of salt on a perfect metal surface, but that’s not what we get in the real world. Crystal formation is very complex, as is the way metals deposit during recharge. These imperfections mainly depend on the charge state of the battery to start with, the temperature, charge voltage and charging current. Over time, the imperfections in one charge cycle can cause the same in the next charge cycle, and so on, and our battery picks up some bad memories.

The memory effect is strong for some types of cells, such as nickel-based batteries. Other types, like lithium-ion, don’t suffer from this problem.