Battery lifetime
Even if never taken out of the original package,
disposable (or "primary") batteries can lose two to
twenty-five percent of their original charge every year. This
rate depends significantly on temperature, since typically chemical
reactions proceed more rapidly as the temperature is raised.
This is known as the "self discharge" rate and is
due to non-faradaic (non-current-producing) chemical reactions,
which occur within the cell even if no load is applied to it.
Batteries should be stored at cool or low temperatures to reduce
the rate of the side reactions. For instance, some people make
a practice of storing unused batteries in their refrigerators
to extend battery lifetime, although care should be taken to
ensure the batteries do not freeze. Extremely high or low temperatures
will reduce battery performance.
Rechargeable batteries self-discharge more
rapidly than disposable alkaline batteries; up to three percent
a day (depending on temperature). Due to their poor shelf life,
they shouldn't be left in a drawer and then relied upon to power
a flashlight or a small radio in an emergency. For this reason,
it's a good idea to keep a few alkaline batteries on hand. Ni-Cd
Batteries are almost always "dead" when you get them,
and must be charged before first use.
Most NiMH and NiCd batteries can be charged several
hundred times. Also, they both can be completely discharged
and then recharged without their capacity being damaged or shortened.
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 many amps as possible in a reasonably
small package, and are only drained a small amount before being
immediately recharged. Care should be taken to avoid deep discharging
a starter battery, as the recharging process melts a small amount
of the lead from the plates. When holes form in the plates it
results in less surface area for the chemical reaction, which
results in less measured voltage. Leaving a lead-acid battery
in a deeply discharged state for any length of time allows the
sulfate to become more deeply adhered to the plate, making sulfate
removal during the charging process difficult. This can result
in less available plate surface and the resulting lower voltage,
shortening the batterys life. "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 lead-acid
is its low cost, the main drawbacks are their large size and
weight per a given capacity and voltage. Lead-acid batteries
should never be discharged to below 20% of their full capacity
as internal resistance will cause heat and damage when attempting
to recharge them. 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 batterys 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.
