Memory effect

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This article is about the 'memory' observed in batteries. For other uses, see Memory (disambiguation).

Memory effect, also known as lazy battery effect, is an effect observed in some rechargeable batteries that causes them to hold less charge. In its original meaning it describes one very specific situation in which certain NiCd batteries gradually lose their maximum energy capacity if they are repeatedly recharged after being only partially discharged. Today, the term is commonly applied to almost any case in which a battery appears to hold less charge than was expected.

[edit] Other problems described as memory effect

[edit] Voltage depression due to over-charging

A common process often ascribed to memory effect is voltage depression. In this case the peak voltage of the battery drops more quickly than normal as it is used, even though the total energy remains almost the same. In modern electronic equipment that monitors the voltage to indicate battery charge, the battery appears to be draining very quickly and therefore about to run out of energy. To the user it appears the battery is not holding its full charge, which seems similar to memory effect. This is a common problem with high-load devices such as digital cameras.

Voltage depression is caused by repeated over-charging of a battery, which causes the formation of small crystals of electrolyte on the plates. These can clog the plates, increasing resistance and lowering the voltage of some individual cells in the battery. This results in a seemingly rapid discharge as those individual cells discharge quickly and the voltage of the battery as a whole suddenly falls. This effect is very common, as consumer trickle chargers typically overcharge.

[edit] Deep discharge

Some rechargeable batteries can be damaged by repeated deep discharge. Batteries are composed of multiple similar, but not identical, cells. Each cell has its own charge capacity. As the battery as a whole is being deeply discharged, the cell with the smallest capacity may reach zero charge and will "reverse charge" as the other cells continue to force current through it. The resulting loss of capacity is often ascribed to memory effect.

[edit] Age and use

All rechargeable batteries have a finite lifespan and will slowly lose storage capacity as they age due to secondary chemical reactions within the battery whether it is used or not. Lithium ion batteries can lose 5%-20% of their storage capacity every year from the time of manufacture. ^[1]

All rechargeable batteries have a finite number of charge/discharge cycles and will lose a very small amount of storage capacity during each cycle. Typically, rechargeable batteries are rated for hundreds or thousands of cycles.

[edit] Possible fixes

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Please improve this article if you can (December 2006).

There are numerous urban legends suggesting how to fix batteries in which voltage depression has occurred.

There is some scientific basis behind one of the most common recommendations: completely drain the battery in another device in order to dissolve the crystals. In practice, however, this technique more often damages the other cells in the battery pack, considerably shortening battery life. Because some cells may discharge before others, they are charged in reverse by the remaining cells, severely damaging them. To avoid damage to the other cells, each cell must be individually discharged, not the entire pack as a whole. (Do not try discharging a cell by shorting it out with a wire, as this will cause it to overheat and possibly explode. A resistance, such as a flashlight bulb, should be used.) To protect the cell, the voltage should be monitored and the discharging stopped when it gets down to 1 volt. (See NiMH.)

A graph showing the memory effect is given in a document by Duracell (near the end). The graph also shows how the cell is restored by a couple of deeper discharge/recharge cycles.

If your device malfunctions due to memory depression it is a result of a design failure of the power management system in the device, not the battery cells or how the device is used. The definitive reference for electrical engineers wanting to use NiCd technology has been available since at least 1992 from Gates Energy Products in the Rechargeable Batteries Applications Handbook. There is very little you can conveniently do if your device was not designed to accommodate this effect.