Memory effect
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Memory effect, also known as lazy battery effect or battery memory, is an effect observed in nickel cadmium rechargeable batteries that causes them to hold less charge. 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. The battery appears to "remember" the smaller capacity. [1] The source of the effect are changes of the characteristics of the underused active materials of the cell.[citation needed] The term is commonly misapplied to almost any case in which a battery appears to hold less charge than was expected. These cases are more likely due to battery age and use, leading to irreversible changes in the cells due to internal short-circuits, loss of electrolyte, or reversal of cells.
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[edit] Other problems perceived 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 the 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 based batteries have one of the longest idle lives of any construction, and examples abound that are nearly 20 years old which exhibit almost their as-new capacity. Unfortunately the number of operational cycles is still quite short and the best examples rarely last more than 500 or so complete charge/discharge cycles.
[edit] Possible fixes
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This section may require cleanup to meet Wikipedia's quality standards. Please improve this section 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.[citation needed] 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. It is not necessary to monitor the voltage as the cell can be discharged to zero volts without harm. (See NiMH.) Some manufacturers recommend storage with cells so discharged and short circuited.[citation needed]
If device malfunctions are 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. There is very little one can conveniently do if the device was not designed to accommodate this effect.
[edit] References
- ^ David Linden, Thomas B. Reddy (ed). Handbook Of Batteries 3rd Edition. McGraw-Hill, New York, 2002 ISBN 0-07-135978-8 page 28-18
- Rechargeable Batteries Applications Handbook from Gates Energy Products, published since 1992.
