||This article needs additional citations for verification. (January 2007)|
Memory effect, also known as battery effect, 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. The source of the effect is changes in the characteristics of the underused active materials of the cell. 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.
True memory effect 
Specifically, the term 'memory' came from an aerospace nickel-cadmium application in which the cells were repeatedly discharged to 25% of available capacity (plus or minus 1%) by exacting computer control, then recharged to 100% capacity without overcharge. This long-term, repetitive cycle regime, with no provision for overcharge, resulted in a loss of capacity beyond the 25% discharge point. Hence the birth of a "memory" phenomenon, whereby nickel-cadmium batteries purportedly lose capacity if repeatedly discharged to a specific level of capacity. True memory cannot exist if any one of the following conditions holds:
- Batteries achieve full overcharge.
- Discharge is not exactly the same each cycle, within plus or minus 3%
- Discharge is to less than 1.0 volt per cell.
True memory effect is specific to sintered-plate nickel-cadmium cells, and is exceedingly difficult to reproduce, especially in lower ampere-hour cells. In one particular test program—especially designed to induce memory—no effect was found after more than 700 precisely-controlled charge/discharge cycles. In the program, spirally-wound one-ampere-hour cells were used. In a follow-up program, 20-ampere-hour aerospace-type cells were used on a similar test regime. Memory effects showed up after a few hundred cycles.
Other problems perceived as memory effect 
Phenomena which are not true memory effects may also occur in other battery types than sintered-plate nickel-cadmium cells.
Temporary effects 
Voltage depression due to long-term 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. 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 causes the battery as a whole to seem to discharge rapidly 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.
The effect can be overcome by subjecting each cell of the battery to one or more deep charge/discharge cycles. This must be done to the individual cells, not a multi-cell battery; in a battery some cells may discharge before others, they are then subjected to a reverse charging current by the remaining cells, causing irreversible damage.
High temperatures 
High temperatures reduce the charge accepted by the cells and the voltage charged to.
Other causes 
- Operation below 0 °C
- High discharge rates (above 5C) in a battery not specifically designed for such use
- Inadequate charging time
- Defective charger
Permanent loss of capacity 
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.
Battery users may attempt to avoid the memory effect proper by fully discharging their battery packs. This practice is likely to cause more damage as one of the cells will be deep discharged. The damage is focused on the weakest cell, so that each additional full discharge will cause more and more damage to that cell.
Age and use—normal end-of-life 
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. Some cells may fail sooner than others, but the effect is to reduce the voltage of the battery. 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 at approximately 400-1200 complete charge/discharge cycles. The lifetime of lithium batteries decreases at higher temperature and states of charge, whether used or not; maximum life of lithium cells not in use is achieved by refrigerating (without freezing) charged to 40%.
- David Linden, Thomas B. Reddy (ed). Handbook Of Batteries 3rd Edition. McGraw-Hill, New York, 2002 ISBN 0-07-135978-8 page 28-18
- Repair FAQ, quoting GE tech note
- Repair FAQ, quoted above, but not directly quoting GE tech note
- Batteries as sources of electrical power
- Battery Types and Characteristics for HEV ThermoAnalytics, Inc., 2007. Retrieved 2010-06-11.
- Rechargeable Batteries Applications Handbook from Gates Energy Products, published since 1992.