Constant-Voltage Batteries And "Memory Effect":

Electronics that run on batteries, such as laptop computers, need special
kinds of batteries to satisfy their electrical needs. Typical household
batteries, used to run flashlights and the like, are not acceptable because
their voltage lowers somewhat as they slowly drain; Digital electronics need
a solid, steady voltage, and so special batteries were developed which do not
decrease their voltage, even when they have lost much of their charge. The
first of these batteries to be developed was the nickel-cadmium (NiCad)
battery. A more recent development was the Nickel-Metal Hydride (NiMH)
battery, and most high-tech of all (as of this writing) is the Lithium-Ion
(Li-Ion) battery.

Much has been said in the field of personal computing about "memory effect"
in laptop batteries. This is the tendency of these batteries to lose their
ability to hold a full charge if they are repeatedly discharged, then
recharged because they are completely drained. Although some might say that
this trait has been over-hyped, and that in fact memory effect is not as
severe as some would have you believe, there is no doubt that this effect
does exist, at least to some small extent, and it will tend to shorten the
useful life of a battery if it is ignored.

Generally speaking, the memory effect does not set in after a single
discharge/recharge cycle, but several of them. For this reason, you can
probably use a battery normally for a while, but it is a good practice to
regularly discharge it fully, perhaps once a week; Just leave the laptop
running until the battery level gets so low that the computer shuts off.
There may still be some trace of charge left in the battery at this point,
but it should be low enough for you to fully recharge the battery. Memory
effect usually only sets in when the battery has been only slightly
discharged. Fully discharging a battery before recharging it is known as
"deep discharging", and discharging it only slightly before recharging is
known as "shallow discharging". It is worth mentioning that many types of
battery can actually be damaged by deep discharging, but these batteries are
not usually used in laptop computers. For example, car batteries, which use a
"wet" type of battery cell, can be hopelessly destroyed if they are allowed
to completely discharge. Generally, NiCad and NiMH batteries will not be
damaged by deep discharging.

A device known as a "conditioning charger" may be helpful in preventing
memory effect. These devices will begin by discharging the battery very
quickly, then recharging it very quickly. It seems that memory effect creeps
in when a battery is charged slowly, and blasting it with recharge helps to
eliminate the memory effect altogether. Conditioning chargers have even been
known to revive old NiCad batteries that were thought to be dead.

NiCad batteries, the first kind of stable-voltage batteries, were the first
in which memory effect was observed. Of the various types of electronics
batteries available today, they are the most susceptible to it.

NiMH batteries are less susceptible to the problems of memory effect because
they have a different electrode composition, but they still may be affected
by it. NiMH batteries are also more environmentally-friendly than NiCads
(cadmium is toxic), but they are more expensive, of course. NiMHs are also
especially prone to "self-discharging", a normal condition in which a
rechargeable battery which is not used for a while discharges by itself.

Lithium-ion batteries are supposedly immune to the memory effect, as well as
having a generally larger charge capacity than both NiCad and NiMH batteries.
As an added bonus, they are also lighter. The tradeoff, obviously, is that
Li-Ion batteries are the most expensive.