Memory: myth or fact?

The word 'memory' was originally derived from 'cyclic memory'; meaning
vgp-bps9a/b a nickel-cadmium battery could remember how much energy was drawn
on preceding discharges. On a longer than scheduled discharge, the
voltage would rapidly drop and the battery would lose power. Improvements
in battery technology have virtually eliminated this phenomenon.



The problem with nickel-cadmium is not so much the cyclic memory but
the effects of crystalline formation. The active cadmium material
is present in finely divided crystals. In a good cell, these crystals
remain small,vgp-bps9/s obtaining maximum surface area. With memory, the crystals
grow and conceal the active material from the electrolyte. In advanced
stages, the sharp edges of the crystals penetrate the separator, causing
high self-discharge or electrical short.



When introduced in the early 1990s, nickel-metal-hydride was promoted
as being memory-free. Today, we know that this chemistry is also affected
but to a lesser degree than nickel-cadmium. The nickel plate, a metal
vgp-bps9/b is shared by both chemistries, is partly to blame. While nickel-metal-hydride
has only the nickel plate to worry about, nickel-cadmium also includes
the memory-prone cadmium plate. This is a non-scientific explanation
why nickel-cadmium is affected more than nickel-metal-hydride.



The stages of crystalline formation of a nickel-cadmium cell are illustrated
in Figure 1. The enlargements show the cadmium plate in a proper functioning
crystal structure vgp-bps10, crystalline formation after use (or abuse) and
restoration.

New nickel-cadmium cell. The anode is in fresh condition. Hexagonal cadmium hydroxide crystals are about 1 micron in cross section, exposing large surface area to the electrolyte for maximum performance. Cell with crystalline formation. Crystals have grown to 50 to 100 microns in cross section, concealing large portions of the active material from the electrolyte. Jagged edges and sharp corners may pierce the separator, leading to increased self-discharge or electrical short. Restored cell. After pulsed charge, the crystals are reduced to 3 to 5 microns, an almost 100% restoration. Exercise or recondition are needed if the pulse charge alone is not effective.

Figure 1: Crystalline formation on nickel-cadmium cell.
Illustration
courtesy of the US Army Electronics Command in Fort Monmouth, NJ, USA.


How to restore and prolong nickel-based batteries

Crystalline
formation is most pronounced if a nickel-based battery is left in the charger
for days, or vgp-bps12 if repeatedly recharged without a periodic full discharge. Since
most applications do not use all energy before recharge, a periodic discharge
to 1 volt per cell (known as exercise) is essential to prevent memory.


Nickel-cadmium in regular use and on standby vgp-bps13 mode (sitting in a charger for operational
readiness) should be exercised once per month. Between these monthly exercise
cycles, no further service is needed. No scientific research is available on the
optimal exercise requirements of nickel-metal-hydride. Based on the reduced crystalline
buildup, applying a full discharge once every three months appears right. Because
of the shorter cycle life compared to nickel-cadmium, over-exercising is not recommended.

Exercise and
Recondition - Research has shown pa3399u-2brs the crystals ingrain themselves if no
exercise is applied to nickel-cadmium for three months or more. A full restoration
with exercise becomes more difficult the longer service is withheld. In advanced
cases 'recondition' is required.


Recondition is a slow, secondary discharge
applied below the 1 volt/cell threshold. During this process, the current must
be kept low to minimize cell reversal. Nickel-cadmium can tolerate a small amount
of cell reversal but caution must be applied pa3534u-1brs to stay within the allowable current
limit.

Tests
performed by the US Army have shown that a nickel-cadmium cell needs to be discharged
to at least 0.6V to effectively break up the more resistant crystalline formation.
Figure 2 illustrates the battery voltage during a discharge to 1V/cell, followed
by the secondary discharge to 0.4V/cell.,pa3535u-1brs

Figure 2: Exercise and recondition features of a Cadex battery analyzer.If a nickel-cadmium battery has not been exercised for three months or longer, recondition is required to restore capacity. Recondition is a slow, deep discharge to 0.4V/cell. If service is denied for 6 to 12 months, recondition becomes ineffective.

Figure
3 illustrates the effects of exercise and recondition. Four nickel-cadmium batteries of inspiron 6400 battery afflicted with various degrees of memory are serviced. The batteries are first
fully charged, then discharged to 1V/cell. The resulting capacities latitude d630 battery are plotted
on a capacity scale of 0 to 120% in the first column. Additional discharge/charge
cycles are applied and the battery capacities are plotted in the subsequent columns.
The green line represents 'exercise', and the blue line 'recondition'. The exercise
and recondition cycles are applied manually at the discretion of the research
technician.

Figure 3: Effects of exercise and recondition.Four batteries afflicted with memory are serviced. Battery 'A' improved capacity on exercise alone; batteries 'B' and 'C' required recondition. The new battery improved further with recondition.

Battery
'A' responded well to exercise alone and no recondition was required.
This battery of xps m1530 battery may have been in service for only a few months or has
received periodic exercise cycles. Batteries 'B' and 'C' required
recondition to restore performance. Without recondition, these two
batteries would have been discarded.



After service, the restored batteries for inspiron 1525 battery were returned to full
use. When examined after six months of field service, no noticeable degradation
in the performance was visible. The regained capacity was permanent but periodic
service will be needed to maintain the performance.


Applying the recondition
cycle on a new battery (top line on chart) resulted in a slight capacity gain.
This increase is not fully understood, other than to assume that the battery improved
by additional forming. Another explanation is early presence of memory. Since
new XPS M1730 Battery are stored with some charge, the self-discharge that occurs during
storage produces some crystalline formation. Exercising and reconditioning reverse
this effect.


Recondition has its limitations. If no exercise had been
applied for 6 to 12 months, permanent damage may have been inflicted. The capacity
may not recover or the pack may suffer from high self-discharge caused by a marred
separator. Older batteries for Latitude D820 Battery may get worse with recondition. These packs can be
compared to an old man to whom strenuous activity is harmful. Such batteries must
be replaced.


Typically 50%-70% of discarded nickel-cadmium batteries
can be restored when using the exercise and recondition methods of a Cadex battery
analyzer or equivalent. The recovery rate of nickel-metal-hydride is about 40%.
This lower yield Sony battery is, in part, due to the battery's low cycle count.

Field
results on exercise and recondition


After the Balkan War, the Dutch
Army examined how many field batteries Acer Aspire One battery could be restored with a battery analyzer
(Cadex). The army was aware that the packs were used under less than ideal conditions.
They had been sitting in the chargers with only 2-3 hours use per day.


The capacity on some packs had dropped from 100% to 30%. With the analyzer's recondition
function, 9 of 10 batteries Acer Aspire 4315 battery were restored to 80% and higher. The nickel-cadmium
batteries were 2-3 years old.


The importance of exercising and reconditioning
is emphasized by another study carried out for the US Navy by GTE Government Systems Acer Aspire One 10.1 inch battery.
To determine the percentage of batteries needing replacement in the first year
of use, one group of batteries received charge only (no maintenance), another
group was periodically exercised and a third group received recondition. The batteries
studied were used for two-way radios on US aircraft carriers.


With charge
only (charge-and-use), the annual percentage of battery failure was 45% (Figure
4). With exercise, Aspire 4520G battery rate was reduced to 15%. By far the best results
were achieved with recondition. The failure rate dropped to 5%.

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