The memory effect in batteries

Memory effect battery: What is it?

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, and soon the battery will need to be charged more frequently. In a worst-case scenario, batteries can become unusable well before the end of their service life. But not all batteries are actually affected by this loss of capacity, as the memory effect is not an issue in modern lithium-ion batteries. For users, this has the major advantage that a Li-ion battery can be charged at any time, regardless of its existing charge level.

Memory effect battery: discovered in the 1960s 

Superstitious behavior guided by the memory effect in batteries is rooted in fact, as the memory effect once represented a serious challenge. It was discovered in the 1960s by engineers at NASA, who noticed that batteries built into a satellite lost their capacity over time. Specifically, 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.

In other words, the memory effect has a negative impact on storage capacity and occurs after repeated partial discharge. The battery “remembers” the partial discharge level, and only supplies this “remembered” quantity of energy for further use. To explain further, 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.

Memory effect battery: Which batteries experience 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. Voltage decreases as crystals grow in size, because the crystals impair the conductivity of the material. Similar behavior can be seen in nickel metal hydride batteries (NiMH) to a lesser extent, though here it is 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 electrical devices, but nowadays NiMh and NiCd batteries are seldom used elsewhere.

How does the memory effect happen in batteries?

According to current research, the memory effect is based on two processes.

What does the battery memory effect mean?

Nickel cadmium (NiCd) and nickel metal hydride batteries (NiMH), which were frequently used in the past, suffered from loss of capacity if they were repeatedly partially discharged. This phenomenon is known as the memory effect, because with frequent partial discharge the battery “remembers” the energy requirement only of the most recent charges, and charges the battery with this “remembered” energy quantity instead of the original full capacity. This leads to a premature loss of available voltage which eventually makes the battery unusable.

Summary: the 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
• Affected batteries are overcharged if not empty before charging
• The memory effect can reduce or even destroy energy capacity in batteries