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LW1DSE > TECH 31.08.08 18:05l 269 Lines 14571 Bytes #999 (0) @ WW
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Subj: Nickel Cadmium Battery (2/2)
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Comes from part 1 of 2
7) Characteristics:
The maximum discharge rate for a NiCd battery varies by size. For a
common AA-size cell, the maximum discharge rate is approximately 18 amps; for
a D size battery the discharge rate can be as high as 35 amps.
Model aircraft or boat builders often take much larger currents of up
to a hundred amps or so from specially constructed small batteries, which are
used to drive main motors. 5-6 minutes of model operation is easily achievable
from quite small batteries, so a reasonably high power-to-weight figure is
achieved, comparable to internal combustion motors, though of lesser duration.
7.1) Charging:
NiCd batteries can charge at several different rates, depending on
how the cell was manufactured. The charge rate is measured based on the
percentage of the amp-hour capacity the battery is fed as a steady current
over the duration of the charge. Regardless of the charge speed, more energy
must be supplied to the battery than its actual capacity, to account for
energy loss during charging, with faster charges being more efficient. For
example, the typical "overnight" charge, called a C/10 charge, is accomplished
by applying 10% of the batteries total capacity for a period of 14 hours;
that is, a 100 mAh battery takes 140 mAh of energy to charge at this rate.
At the "fast charge" rate, done at 100% of the rated capacity, the
battery holds roughly 80% of the charge, so a 100 mAh battery takes 120 mAh
of energy to charge (that is, approximately 1 hour and fifteen minutes) The
downside to faster charging is the higher risk of overcharging, which can
damage the battery.
The safe temperature range for a NiCd battery in use is between -20C
and 45C. During charging, the battery temperature typically stays low, around
0C (the charging reaction absorbs heat), but as the battery nears full charge
the temperature will rise to 45-50C. Some battery chargers detect this
temperature increase to cut off charging and prevent over-charging.
When not under load or charge, a NiCd battery will self-discharge
approximately 10% per month at 20C, ranging up to 20% per month at higher
temperatures. It is possible to perform a "trickle charge" at current levels
just high enough to offset this discharge rate; to keep a battery fully
charged. However, if the battery is going to be stored unused for a long
period of time, it should be discharged down to at most 40% of capacity
(some manufacturers recommend fully discharging, or even short-circuiting),
and stored in a cool, dry environment.
7.2) Inspecting:
The battery should have no external damage and depending on the number
of cells it should have 1.2V per cell when fully charged and about 0.8-1V when
discharged.
7.3) Charge condition:
High quality NiCd's have a thermal cut-off so if the battery gets too
hot the charger stops. If a NiCd is still warm from discharging and been put
on charge, it will not get the full charge possible. In that case, let the
battery cool to room temperature then charge. Watch for the correct polarity.
Leave charger in a cool place or room temperature when charging to get best
results.
7.4) Charging method:
A NiCd battery requires a charger with a slightly different voltage
charge level than a lead-acid battery, especially if the NiCd has 11 or 12
cells. In addition, the charger requires a more intelligent charge termination
method if a fast charger is used. Often NiCd battery packs have a thermal
cut-off inside that feeds back to the charger telling it to stop the charging
once the battery has heated up and/or a voltage peaking sensing circuit. At
room temperature during normal charge conditions the cell voltage increases
from an initial 1.2 V to an end-point of about 1.45 V. The rate of rise
increases markedly as the cell approaches full charge. The end-point voltage
decreases slightly with increasing temperature.
8) Electrochemistry:
A fully charged NiCd cell contains:
A) a nickel hydroxide positive electrode plate;
B) a cadmium negative electrode plate;
C) a separator;
D) an alkaline electrolyte (potassium hydroxide).
NiCd batteries usually have a metal case with a sealing plate
equipped with a self-sealing safety valve. The positive and negative electrode
plates, isolated from each other by the separator, are rolled in a spiral
shape inside the case.
The chemical reactions in a NiCd battery during discharge are:
At the cadmium electrode, and
at the nickel electrode.
The net reaction during discharge is
During recharge, the reactions go from right to left. The alkaline
electrolyte (commonly KOH) is not consumed in this reaction and therefore its
Specific Gravity, unlike in Lead- Acid batteries, is not a guide to its state
of charge.
When Jungner built the first nickel-cadmium batteries, he used nickel
oxide in the cathode and iron and cadmium materials in the anode. It wasn't
until later that pure cadmium metal and nickel hydroxide were used. Until
about 1960, the reaction in nickel-cadmium batteries was not completely
understood. There were several speculations as to the reaction products. The
debate was finally resolved by spectrometry, which revealed cadmium hydroxide
and nickel hydroxide.
Another historically important variation on the basic nickel-cadmium
cell is the addition of lithium hydroxide to the potassium hydroxide electro-
lyte. This was believed to prolong the service life by making the cell more
resistant to electrical abuse. The nickel-cadmium battery in its modern form
is extremely resistant to electrical abuse anyway, so this practice has been
discontinued.
9) Problems with NiCd:
9.1) Overcharging:
Overcharging must be considered in the design of most rechargeable
batteries. In the case of NiCds, there are two possible results of overchar-
ging: If the anode is overcharged, hydrogen gas is produced If the cathode is
overcharged, oxygen gas is produced. For this reason, the anode is always
designed for a higher capacity than the cathode, to avoid releasing hydrogen
gas. There is still the problem of eliminating oxygen gas, to avoid rupture
of the cell casing. NiCd cells are vented, with seals that fail at high
internal gas pressures. The sealing mechanism must allow gas to escape from
inside the cell, and seal again properly when the gas is expelled. This
complex mechanism, unnecessary in alkaline batteries, contributes to their
higher cost.
NiCd cells dealt with in this article are of the sealed type. Cells
of this type consist of a pressure vessel that is supposed to contain any
generation of oxygen and hydrogen gasses until they can recombine back to
water. Such generation typically occurs during rapid charge and discharge and
exceedingly at overcharge condition. If the pressure exceeds the limit of the
safety valve, water in the form of gas is lost. Since the vessel is designed
to contain an exact amount of electrolyte this loss will rapidly affect the
capacity of the cell and its ability to receive and deliver current. To
detect all conditions of overcharge demands great sophistication from the
charging circuit and a cheap charger will eventually damage even the best
quality cells.
9.2) Cell reversal:
Another potential problem is reverse charging. This can occur due to
an error by the user, or more commonly, when a battery of several cells is
fully discharged. Because there is a slight variation in the capacity of
cells in a battery, one of the cells will usually be fully discharged before
the others, at which point reverse charging begins seriously damaging that
cell, reducing battery life. The by-product of reverse charging is hydrogen
gas, which can in some circumstances be dangerous. Some commentators advise
that one should never discharge multi-cell nickel-cadmium batteries to zero
voltage; for example, torches/flashlights should be turned off when they are
yellow; before they go out completely.
Individual cells may be fully discharged to zero volts and some of
the battery manufacturers recommend this if the cells are to be stored for
lengthy intervals. At least one manufacturer even recommends short-circuiting
each cell for storage. However, it is normally recommended that NiCd Batteries
be charged to around 40% capacity for long-term storage.
A common form of this deprication occurs when cells connected in
series develop unequal voltages and discharge near zero voltage. The first
cell that reaches zero is pushed beyond to negative voltage and gasses
generated open the seal and dry the cell. In modern cells an excess of
anti-polar material (basically active material ballast at positive electrode)
is inserted to allow for moderate negative charge without damage to the cell.
This excess material slows down the start of oxygen generation at the negative
plate. This means a cell can survive a negative voltage of about -0.2 to -0.4
volts. However if discharge is continued even further, this excess ballast is
used up and both electrodes change polarity, causing destructive gassing.
Battery packs with multiple cells in series should be operated well
above 1 volt per cell to avoid placing the lowest capacity cell in danger of
going negative. Battery packs that can be disassembled into cells should be
periodically zeroed and charged individually to equalize the voltages.
However, this doesn't help if old and new cells are mixed, since their
different capacities will result in different discharge times and voltages.
9.3) Memory and lazy battery effects:
It is sometimes claimed that NiCd batteries suffer from a "memory
effect" if they are recharged before they have been fully discharged. The
apparent symptom is that the battery "remembers" the point in its charge
cycle where recharging began and during subsequent use suffers a sudden drop
in voltage at that point, as if the battery had been discharged. The capacity
of the battery isn't actually reduced substantially. Some electronics designed
to be powered by NiCds are able to withstand this reduced voltage long enough
for the voltage to return to normal. However, if the device is unable to
operate through this period of decreased voltage, the device will be unable
to get as much energy out of the battery, and for all practical purposes, the
battery has a reduced capacity.
There is controversy about whether the memory effect actually exists,
or whether it is as serious a problem as is sometimes believed. Some critics
claim it is used to promote competing NiMH batteries, which apparently suffer
this effect to a lesser extent. Many nickel-cadmium battery manufacturers
either deny the effect exists or are silent on the matter.
The memory effect story originated from orbiting satellites, where
they were typically charging for twelve hours out of twenty-four for several
years. After this time, it was found that the capacities of the batteries had
declined significantly, but were still perfectly fit for use. It is thought
unlikely that this precise repetitive charging (e.g. 1000 charges / discharges
with less than 2% variability) would ever be reproduced by consumers using
electrical goods.
An effect with similar symptoms to the memory effect is the so-called
voltage depression or lazy battery effect. (Some people use this term as a
synonym for "memory effect"). This results from repeated overcharging; the
symptom is that the battery appears to be fully charged but discharges quickly
after only a brief period of operation. Sometimes, much of the lost capacity
can be recovered by a few deep discharge cycles, a function often provided by
automatic NiCd battery chargers. However, this process may reduce the shelf
life of the battery. If treated well, a NiCd battery can last for 1000 cycles
or more before its capacity drops below half its original capacity.
9.4) Dendritic shorting:
NiCd batteries, when not used regularly, tend to develop dendrites
which are thin, conductive crystals which may penetrate the separator membrane
between electrodes. This leads to internal short circuits and premature
failure, long before the 800-1000 charge/discharge cycle life claimed by most
vendors. Sometimes, applying a brief, high-current charging pulse to
individual cells can clear these dendrites, but they will typically reform
within a few days or even hours. Cells in this state have reached the end of
their useful life and should be replaced. Many battery guides, circulating on
the Internet and online auctions, promise to restore dead cells using the
above principle, but achieve very short-term results at best.
9.5) Environmental consequences of Cadmium:
NiCd batteries contain cadmium, which is a toxic heavy metal and
therefore requires special care during battery disposal. In the United States
part of the price of a NiCd battery is a fee for its proper disposal at the
end of its service lifetime. In the European Union, the Restriction of
Hazardous Substances Directive (RoHS) bans the use of cadmium in electrical
and electronic equipment products after July 2006, though NiCd batteries are
not restricted.
Cadmium, being a heavy metal, can cause substantial pollution when
landfilled or incinerated. Because of this, many countries now operate
recycling programs to capture and reprocess old NiCd batteries.
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บ Compilled from Wikipedia.com . Translatted to ASCII by LW1DSE Osvaldo บ
บ F. Zappacosta. Barrio Garay, Almirante Brown, Buenos Aires, Argentina. บ
บ Made with MSDOS 7.10's Text Editor (edit.com) in my AMD's 80486. บ
บ September 01, 2008 บ
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