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- This article is about the 'memory' observed in batteries. For other uses, see Memory (disambiguation).
The memory effect in electrical batteries, also known as lazy battery effect, is an effect observed in some rechargeable batteries that causes them to hold less charge. The term has become almost universal in describing any such effect, though in its original meaning it describes one very specific case. NiCd batteries gradually lose their maximum energy capacity if they are repeatedly recharged after being only partially discharged. This is termed the memory effect. A major advantage of NiMH battery technology, apart from lack of toxicity, is the absence of any memory effect — NiMH batteries are remarkably tolerant of frequent 'top-up' charging. Lithium ion batteries do not have the memory effect either.
Memory effect occurs when a sintered-plate nickel-cadmium (NiCd) battery is repeatedly discharged to a particular level above a full discharge, that is, only "partially used", and then recharged to an equally precise "full" level. This "real" memory effect is extremely rare and is found only on very expensive, rare, and older unattended electronics such as communications satellites (which repeatedly use exactly the same amount of power while in the Earth's shadow). Repeated attempts to duplicate it in the lab have proven difficult.
A much more 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 electronics 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.
There are numerous urban legends suggesting how to fix batteries in which voltage depression has occurred.
(This section needs updating. Information supporting this fix is available in the Gates manual. Also, voltage depression is caused by an alternate chemical reaction, not crystals. The part about cell reversal is true.) 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.
If your device malfunctions due to memory depression it is the fault of the power supply and charging circuits in the device, not the battery cells or how it's used. The definitive reference for EEs 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 designed cheaply or by an ignorant EE.
Wrapping the battery in a plastic bag (to avoid high humidity) and leaving it in a refrigerator overnight helps to 'reset' the battery sometimes.
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