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G4WYW > TECH 09.04.08 13:00l 157 Lines 6420 Bytes #999 (0) @ WW
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Subj: Charging Nicads part 2 of 4
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Part 2 of 4
OVERCHARGING.
Eventually the continued application of the current is no longer converted
to stored energy in the cell. Instead, the oxygen overpotential at the
positive plates is surpassed, and the oxygen gas is produced due to the
electrolysis of the electrolyte, not by the reduction of cadmium hydroxide
to cadmium. The electrolyte, composed of potassium hydroxide and water,
changes hydroxide ions into oxygen, water and free electrons.
The oxygen produced by electrolysis is quickly recombined in the
electrolyte
at the negative plates. However a marked increase in cell temperature and
pressure follows as the current input shifts from raising the cell's state
of
charge to generating oxygen gas.
The switch from generating oxygen at the plates to generating oxygen in
the
electrolyte also causes a sharp drop in the cell's impedance, since it is
easier to strip oxygen from the abundant hydroxide ions than from the
scarcer
cadmium hydroxide. This drop in impedance causes a corresponding drop in
voltage, creating a peak in the cell voltage curve.
Generating and recombining oxygen in the electrolyte are exothermic
reactions.
Overcharging a battery continues to generate oxygen gas, building
temperature
and pressures. Forcing a battery to vent causes a loss of electrolyte,
reducing battery capacity and damaging the cell. If the gas cannot vent
quickly
enough, then the battery can explode.
CHARGING CONCERNS.
The gas build-up in nickel-cadmium cells is a problem when contemplating
methods to charge batteries. Gas bubbles accumulate on the surface of
plates,
increasing impedance and reducing the plates surface area. Overcharging
produces gas which, if not recombined quickly enough, can cause damaging
pressures to build up inside a cell. Excess pressure causes a sealed cell
to
vent, resulting in a loss of electrolyte. As electrolyte escapes through
repeated venting, the capicity decreases and impedance increases as it
becomes
tougher to transfer ions between plates.
Exothermic heating contributes to shortening battery life by increasing
the
possibility of venting, the principle source of low battery life.
Nickel-cadmium batteries also have a negative temperature coefficient,
meaning an increase in the ambient temperature by a hot cell reduces the
no-load voltage of the surrounding cells.
Under normal use (ie., complete cell discharge) the crystal size on the
cell
plates remains small. If the nickel oxyhydroxide is not completely
converted
back into nickel hydroxide during a partial battery discharge, the
nickel oxyhydroxide crystals will clump together, forming larger crystal
structures. This crystal size change is the cause of the memory effect in
nickel-cadmium batteries.
Using low rate constant current trickle charges, crystalline fingers, or
dendrites, can propogate through the plate separators and across the cell
plates. In severe circumstances these crystal dendrites can partially or
completely short circuit a cell internally.
CONVENTIONAL CHARGERS.
The majority of chargers try to avoid the problems associated with
overcharging by applying a low current charge. Nickel-cadmium cells have a
natural charge decay rate of about C/30 to C/50 if left undisturbed. A
nickel-cadmium cell can accept a low rate current of C/10 or less for
extended
periods (18 to 22 hours) without excessive gassing or heating. A constant
current charger could apply a low trickle charge current for as long as
the
battery remained connected to the charger. As the cell slowly charges the
cell
can radiate heat to the surroundings, so long as ambient temperatures are
relatively cool.
The problem with trickle chargers is that they are fairly slow. The
capacity
of the cell determines exactly how slow: a one ampere/hour cell on a C/10
charge takes 10 hours or more. Continued recharging with slow rate
currents
also causes dentrite formations to occur. Most trickle chargers even apply
a
charge without any feedback control to monitor temperature of voltage in
case
of an emergency shutdown.
VOLTAGE TERMINATION.
Charges employing a feedback control mechanism are much safer to operate.
Some feedback systems monitor the battery voltage, some watch the cell
temperature, and others are timer controlled.
The most distinctive parameter to monitor is the peak in the cell voltage
that occurs as the cell transitions from charge to overcharge. Many
chargers
look for a negative change in voltage (-^V, negative delta V) and
terminate
charge at that point. Others look for a certain voltage threshold and stop
charging at that point.
Problems occur if batteries are composed of unmatched cells, each cell
with its
own characteristics. The voltage monitoring methods are inappropriate for
unmatched cells, since the voltages of different cells in a battery pack
are
often inconsistent with each other. Battery voltages can vary 5% with a
25 deg.C variation in ambient temperature, and the internal imbalances,
impedances, and levels of precharge will vary from cell to cell.
TEMPERATURE TERMINATION.
Another way to recognise full charge involves sensing cell temperature.
Batteries typically reach about 45 deg.C to 50 deg.C at peak charge.
Another
method is to monitor the temperature gradient between the inside and
outside of
the battery pack and terminate at some predetermined threshold.
With either case, though, the battery will be undercharged at high ambient
temperatures or damaged at low temperatures due to the negative
temperature
coefficient of nickel-cadmium cells. Variations in unmatched cell charge
acceptance cannot be monitored with any accuracy since there is no way of
to
tell if a particular cell out of several cells has reached its full
charge.
Also, the battery pack manufacturer must place the thermal sensors in the
appropriate positions inside the pack.
The goal of the ICS1700 Rapid Charge Controller is to quickly and safely
charge
a nickel-cadmium cell without stressing the cell. Solving some of the
problems
associated with rapidly charging a nickel-cadmium cell requires a
controlled
application of the charging current to the cell, as well as careful
monitoring
of the cell's condition. These two requirements are met by using a pulsed
current to charge the cell and a mathematically derived first derivative
of
the battery voltage to watch the cell's state of charge.
73 - Mel, G4WYW
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