Continuous Overcharge
The
overcharge capability of Energizer cylindrical nickel-cadmium cells is
outstanding. The next chart illustrates initial and subsequent discharge curves
after 2 years continuous overcharge without periodic discharges. The first
discharge after the 2 year charge period yields a slightly reduced voltage
curve and 65% capacity. The second cycle after 2 years continuous overcharge
provides essentially the same discharge curve as the initial one.
The chart above illustrates maintenance vs. months of continuous
overcharge at the 20 hour rate with periodic discharges every 3 months at the 1 hour rate. The cells
maintain 90% of their initial capacity after 2 years of this overcharge regimen. This pattern of use would occur
if batteries are left on charge continuously and used one cycle only on an
occasional basis.
Memory Effect
Memory effect is that characteristic attributed to
nickel-cadmium cells wherein the cell retains the characteristics of the
previous cycling. That is, after repeated shallow depth discharges the cell
will fail to provide a satisfactory full depth discharge. Energizer cylindrical
nickel-cadmium cells are particularly excellent with regard to lack of memory
effect. The chart below depicts initial and subsequent cycles after repeated
shallow discharges. The graphs show the initial discharge curve and the first
and second discharge curves after 100 cycles @ 40% depth of discharge. You will
note that the subsequent full depth discharges yield nearly equal capacity to
the initial curve at slightly reduced voltage levels.
Self-Discharge
Self-discharge characteristics of Energizer nickel-cadmium cells are
shown in the chart below. The characteristics are shown as a decline in percent of rated capacity
available. Self-discharge is increased by elevated temperatures. Batteries are
not harmed even if not used for long periods of time.
High Current Pulse Discharge
High rate nickel-cadmium cells will deliver
exceedingly high currents. If they are discharge continuously under short
circuit conditions, self-heating may do irreparable damage. The heat problems
vary somewhat from one cell type to another, but in most cases internal metal
strip tab connectors overheat or the electrolyte boils. In some instances both
events occur. General overheating is normally easy to prevent because the
outside temperature of the battery can be used to indicate when rest, for
cooling, is required. In terms of cutoff temperature during discharge, it is acceptable
practice to keep the battery always below 45oC (113oF). The overheated internal connectors are
difficult to detect. This form of overheating takes place in a few seconds or
less, and overall cell temperature may hardly be affected. It is thus advisable
to withdraw no more ampere seconds per pulse, and to withdraw it at no greater
average current per complete discharge, than recommended on the data sheet for
the "Eveready" cell in question. In special cases, where cooling of
the cell or battery is likely to be poor, or unusually good, special tests
should be run to check the important temperatures before any duty cycle
adjustment is made. Output capacity is any discharge composed of pulses is
difficult to predict accurately because there are infinite combinations of
current, "on" time, rest time, and end point voltage. Testing on a
specific cycle is the simplest way to get a positive answer.
Recommended Charging
Constant current charging is recommended for sealed
nickel-cadmium cells. The 10 hour rate should not be exceeded unless overcharge
is specifically to be prevented. The recharge efficiency of sealed
nickelcadmium cell is dependent on a number of things, but it is most important
to remember that charging becomes more difficult as temperature increases and
charge rate decreases. It is possible, under certain conditions, to charge at
rates much higher than the 10 hour rate, but control devices which prevent high
rate over-charge are sometimes required. The nickel-cadmium battery can be
trickle charged but floating and constant voltage charging are not recommended.
For maximum performance in situations of long term trickle charge current
required to keep the battery fully charged is approximately the 30-50 hour rate
plus whatever is necessary to compensate for any major withdrawals.
Paralleling of Cells
Sealed nickel-cadmium cells should not be charged in parallel unless
each cell or series string of the parallel circuit has its own current limiting
resistor. Minor differences in internal resistance of the cells may result,
after cycling, in extreme variation in their states of charge. This may lead to
overcharge at excessive currents in some cells and undercharge in other cells.
Polarity Reversal:
When cells are connected in series and discharged
completely, small cell capacity differences will cause one cell to reach
complete discharge sooner than the remainder. The cell which reaches full
discharge first will be driven into reverse by the others. When this happens in
an ordinary nickel-cadmium sealed cell, oxygen will be evolved at the cadmium
electrode and hydrogen at the nickel electrode. Gas pressure will increase as
long as current is driven through the cell and eventually it will either vent
or burst. This condition is prevented in some sealed nickel-cadmium cells by
special construction features. These include the use of a reducible material in
the positive in addition to the nickel hydroxide, to suppress hydrogen
evolution when the positive expires. If cadmium oxide is used it is possible to
prevent hydrogen formation and to react the oxygen formed at the negative by
same basic process used to regulate pressure during overcharge. A cell is
considered electrochemically protected against reversal of polarity if, after
discharge at the 10 hour rate down to 1.1 volts, it may receive an additional 5
hour discharge with the same current without being damaged or otherwise
affected. "Eveready" cylindrical cells are protected against cell
rupture, caused by gassing generated during polarity reversal, by a pressure
relief vent
Electrical Characteristics
Energizer sealed nickel-cadmium cells exhibit
relatively constant discharge voltages. They can be recharged many times for
long lasting economical power. They are small convenient packages of high energy output, hermetically sealed in steel cases,
leak resistant and will operate in any position. The cells have very low
internal resistance and impedance, are rugged and highly resistant to shock and
vibration. The temperature range under which these cells may be operated is
wide. Use at high temperatures, however, or charging at higher than recommended
rates, or repeated discharge beyond the normal cut-offs may be harmful..
Voltage Characteristics
Except in the case of complete discharge, neither
cell condition nor state of charge can be determined by open circuit voltage.
Within a short while after charging it may be above 1.4 volts. It will fall shortly thereafter to 1.35V and continue to drop as the
cell loses charge. During discharge, the average voltage of a sealed
nickel-cadmium battery is approximately 1.2 volts per cell. At normal discharge
rates the characteristic is very nearly flat until the cell approaches complete
discharge. The battery provides most of its energy above 1.0 volt per cell. If
the cell is discharged with currents exceeding the rated value, however, the
voltage characteristic will have more of a slope, a lower endpoint voltage will
be necessary and the ampere hours per cycle will be reduced.
Temperature Characteristics
Sealed nickel-cadmium cells experience a relatively
small change of output capacity over a wide range of operating temperature.
Charging, however, must be done in a much narrower range. Temperature limits
applicable to operation of the cells are listed in the specification sheets for
each battery.
The capacity vs. temperature curves which are on some individual
specification sheets represent cells discharged at the temperatures shown after charging at room temperature
for 14 hours at the 10 hour rate. This characteristic is also generalized on
the following curve.
Charging nickel cadmium cells below the recommended temperature can
cause oxygen pressure build up and activation of the resealable safety vent.
Multiple vent activations will reduce cell capacity.
No comments:
Post a Comment