Memory effect in Batteries

What is the memory effect?

You often hear about the “memory effect” in batteries, causing them to lose energy capacity with repeated partial charging. Once the battery’s power drops, the operating time of your cordless power tool is reduced, leading to the battery needing to be charged more frequently. In worst-case scenarios, batteries can become unusable well before the end of their service life. Fortunately, not all batteries are affected by this loss of capacity, as the memory effect is not an issue in modern lithium-ion batteries. This is a major advantage as Li-ion batteries can be charged at any time, regardless of its existing charge level.

The Memory Effect: discovered in the 1960s 

The memory effect was discovered in the 1960s by engineers at NASA, who noticed that batteries used in a satellite lost capacity over time. They found that after repeated partial discharge, the batteries would only supply the amount of energy added with the most recent charging process – the previously charged residual power was not available for use.

For example, imagine a suitcase; instead of unpacking the last items from it, you fit a permanent false bottom. The new space you have created is now all the storage your suitcase can offer – there is less space to fill and you can’t access what you put in earlier. In batteries the memory effect manifests as a voltage drop, and can eventually cause affected batteries to become unusable when the voltage falls below the minimum requirement of the power tool.

Which batteries are susceptible to the memory effect?

The memory effect mainly occurs in nickel-cadmium batteries (NiCd), which used to be widely used in cordless tools. In these batteries the effect is caused by the formation of crystals on the cadmium cathode, because if the battery is not fully discharged the crystals tend to form in the undischarged portion of the battery cell. The voltage decreases as crystals grow in size, because the crystals impair the conductivity of the material. Similar behaviour can be seen in nickel metal hydride batteries (NiMH) to a lesser extent, known as the “lazy battery effect”. Standard-sized AA or AAA batteries are often NiMH types, as are the batteries which are integrated into small electronic devices, but nowadays NiMh and NiCd batteries are rarely used.

SUMMARY: MEMORY EFFECT IN BATTERIES

• STIHL products use lithium-ion batteries that have no memory effect
• The memory effect only occurs in older technologies such as NiCd batteries
• The memory effect can reduce or even destroy energy capacity in batteries