The false belief

A lot has been written about the so-called ‘Memory Effect’ in rechargeable batteries, most commonly linked to NiCd batteries. The common belief is that if you don’t let the batteries fully discharge before re-charging, then they will not work below the voltage they were last discharged to.

For example;

• Say you discharge a battery to 25% of its capacity then you put it on charge. Then the next time it is used, it will not work below that 25% level it was last discharged to. Or something along those lines.

People now generally use the term ‘Memory Effect’ as the cause of any problem or failure that may occur with NiCd batteries. But in reality ‘Memory Effect’ does not really exist and is impossible to create under normal battery use.

So what is ‘Memory Effect’ and where did it come from?

The original concept of Memory Effect came from testing NiCd battery packs for use in space vehicles, and satellites in particular. Under perfectly consistent conditions, a NiCd battery was charged up and partially discharged over a number of cycles. Each time the charging and discharging was done to exactly the same points and, in this very particular case, the performance of the battery did suffer what became known as Memory Effect.

The important thing to note is that this Memory Effect was found only when the charging and discharging regime was totally consistent and to very particular points; varying the regime, charging and discharging to other points, does not reproduce Memory Effects. Only in a regime where you have absolute repeatability and accuracy of control over the recharging regime can the Memory Effect even be encountered, and while this is indeed a concern for such situations as were tested for, it can be, and is, overcome by adjusting the regime.

So what is the cause of apparent lowered capacity in rechargeable batteries?

Voltage Depression is the effect people generally mistake for Memory Effect. It is more severe in NiCd batteries than in NiMH batteries, and is most commonly caused by overcharging. Electrochemistry is a very complicated science and without getting deep into the science, overcharging changes the crystal structure of the Nickel Hydroxide reducing the electrochemical potential and therefore lowering the discharge voltage of the battery.

Solutions to the problem

• Don't overcharge your batteries,
• Nickel systems prefer fast-charge, avoid slow/trickle charging as it causes crystalline formation.
• If not used immediately, remove from the charger, store correctly and apply a topping charge before use. Don't leave nickel-based cell in the charger for more than a few days, even if on trickle charge.
• Don't use, store or charge batteries in High Temperature environments.