Memory effect (Glossary)

Terms such as memory or lazy-battery effect are used to describe phenomena that make accumulators become 'lazy' before the end of their expected service life on account of incorrect handling during charging and discharging and which have a very unfavourable effect on their performance. Different types of accumulators react in different ways to the memory effect.

The most common memory effect occurs with nickel-cadmium accumulators. If these are constantly charged or overcharged after only a partial discharge over a longer period of time (particularly if the partial discharge rate is always the same), larger cadmium crystals form on the anode (negative pole) which cause a significant drop in the accumulator capacity. Another memory effect occurs through the formation of a nickel-cadmium alloy (Ni5Cd21) on the anode. These effects occur, for example, if cordless phones are left in the charging station every night without them being significantly discharged during the day.

The memory effect is partly reversible and above all avoidable. It often helps to recharge these 'mistreated' accumulators repeatedly in succession when the accumulator is fully spent (though without a total discharge!) to eliminate or at least moderate the unwanted loss of battery capacity. The memory effect is also often greatly reduced through fast charging. It can be avoided through the use of intelligent chargers and by avoiding constant overcharging after only partial use.

Nickel-metal-hydride accumulators are much less susceptible to memory effects, lithium-ion accumulators practically not at all.

The memory effect also plays a role with lead accumulators if the spontaneous discharge is always of the same strength before the next charging. It has been known for over 100 years. In this connection the terms 'good charge' and 'bad charge' were coined, whereby as few as 10 'bad charges' (low charging current, high overcharge) can reduce the accumulator capacity by 20%. The following are defined as 'good charges': high charging currents, the battery is only slightly charged, but low currents at the end of the charging process as well as little overcharging.

Modern lead accumulators in private cars and modern chargers should be able to prevent such effects with no intervention by the user.

© Marc Stenzel

	See also:
		Glossary overview Nickel-cadmium accumulators (Glossary)

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