MEMORY EFFECT – TRUTH OR FICTION?

In the early 1970s, nickel-cadmium (NiCd) batteries gained a reputation for developing this memory effect. According to early observations, these cells behaved as if they “learned” how much energy had been removed previously and artificially limited their output during the next cycle. Many engineers compared this to a symbolic protest — a NiCd cell simply refusing to “work overtime”.

Modern NiCd cells no longer exhibit this classic form of cycle memory. Their real issue lies elsewhere: cadmium crystals tend to grow inside the electrode material. As these crystals expand, the active material available for storing energy decreases, which reduces capacity. In advanced stages, the separator between the electrodes may also be damaged, leading to rapid self-discharge. When we talk about the memory effect today in NiCd cells, we actually refer to this unwanted crystallization rather than the old concept of cycle memory.

To reduce the risk of crystallization, NiCd batteries should never stay in a charger longer than necessary. It is also advisable to avoid aggressive fast chargers claiming to replenish, for example, 1500 mAh in half an hour. Additionally, fully discharging and recharging a NiCd pack two or three times a month helps keep the active materials stimulated and slows down crystal growth.

If a NiCd pack already shows symptoms of the memory effect, one solution is to use a charger equipped with a regeneration or reset function. Another method — for more advanced users — involves manually discharging each cell to around 0.6 V, which breaks down most crystalline formations. However, such a deeply discharged cell must be recharged immediately to avoid permanent damage.

When nickel–metal hydride (NiMH) cells entered the market in the 1990s, they were advertised as immune to memory problems. In practice, NiMH batteries can also develop a mild form of memory effect, though significantly weaker than in NiCd chemistry because they do not contain cadmium — a material that has since become heavily restricted in the European Union due to its environmental impact.

To conclude: the memory effect is a complex chemical behavior inherent to NiCd cells and, to a lesser extent, NiMH cells. Should this phenomenon determine your battery choice? Not necessarily. With proper handling, its negative impact can be minimized. As an alternative, you can choose lithium-ion cells, which are free from memory effect — although they do come with a higher price tag. More on that in the next article.