Life of rechargeable batteries
Rechargeable batteries traditionally self-discharge more rapidly than disposable alkaline batteries; up to three percent a day (depending on temperature). However, modern Lithium designs have reduced the self-discharge rate to a relatively low level (but still poorer than for primary batteries). Due to their poor shelf life, rechargeable batteries should not be stored and then relied upon to power flashlights or radios in an emergency. For this reason, it is a good idea to keep alkaline batteries on hand. NiCd Batteries are almost always "dead" when purchased, and must be charged before first use.
Although rechargeable batteries may be refreshed by charging, they still suffer degradation through usage. Low-capacity Nickel Metal Hydride (NiMH) batteries (1700-2000 mA·h) can be charged for about 1000 cycles, whereas high capacity NiMH batteries (above 2500 mA·h) can be charged for about 500 cycles. Nickel Cadmium (NiCd) batteries tend to be rated for 1,000 cycles before their internal resistance increases beyond usable values. Normally a fast charge, rather than a slow overnight charge, will result in a shorter battery lifespan. However, if the overnight charger is not "smart" (i.e. it cannot detect when the battery is fully charged), then overcharging is likely, which will damage the battery. Degradation usually occurs because electrolyte migrates away from the electrodes or because active material falls off the electrodes. NiCd batteries suffer the drawback that they should be fully discharged before recharge. Without full discharge, crystals may build up on the electrodes, thus decreasing the active surface area and increasing internal resistance. This decreases battery capacity and causes the dreaded "memory effect". These electrode crystals can also penetrate the electrolyte separator, thereby causing shorts. NiMH, although similar in chemistry, does not suffer from "memory effect" to quite this extent.
Automotive lead-acid rechargeable batteries have a much harder life. Because of vibration, shock, heat, cold, and sulfation of their lead plates, few automotive batteries last beyond six years of regular use. Automotive starting batteries have many thin plates to provide as much current as possible in a reasonably small package. Typically they are only drained a small amount before recharge. Care should be taken to avoid deep discharging a starting battery, since each charge and discharge cycle causes active material to be shed from the plates. Hole formation in the plates leads to less surface area for the current-producing chemical reactions, resulting in less available current when under load. Leaving a lead-acid battery in a deeply discharged state for any significant length of time allows the lead sulfate to crystallize, making it difficult or impossible to remove during the charging process. This can result in a permanent reduction in the available plate surface, and therefore reduced current output and energy capacity.
"Deep-Cycle" lead-acid batteries such as those used in electric golf carts have much thicker plates to aid their longevity. The main benefit of the lead-acid battery is its low cost; the main drawbacks are its large size and weight for a given capacity and voltage. Lead-acid batteries should never be discharged to below 20% of their full capacity, because internal resistance will cause heat and damage when they are recharged. Deep-cycle lead-acid systems often use a low-charge warning light or a low-charge power cut-off switch to prevent the type of damage that will shorten the battery's life.
Special "reserve" batteries intended for long storage in emergency equipment or munitions keep the electrolyte of the battery separate from the plates until the battery is activated, allowing the cells to be filled with the electrolyte. Shelf times for such batteries can be years or decades. However, their construction is more expensive than more common forms.
Extending battery life
Battery life can be extended by storing the batteries at a low temperature, as in a refrigerator or freezer, because the chemical reactions in the batteries are slower. Such storage can extend the life of alkaline batteries by ~5%; while the charge of rechargeable batteries can be extended from a few days up to several months. In order to reach their maximum voltage, batteries must be returned to room temperature; therefore, alkaline battery manufacturers like Duracell do not recommend refrigerating or freezing batteries.
