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Nice text on the pro's and con's of Nickel- cad bat
USE OF NICAD BATTERIES
Cell Grading
(adapted from "Cell grading improves communications batteries", Mobile
Radio Technology (an Intertec Publication), June 1987.
Nickel-cadmium battery packs have become a popular choice as a power source
for portable communications equipment. Advances in construction methods have
made the packaging smaller with more energy-storage capability.
One efficient technique for producing a longer-lasting battery lies not in
the construction, but in "cell grading," a measure of the staying power or
capacity of each cell for the battery pack is assembled. Grading ensures that
cells are used in the most efficient manner possible, making packs last
longer. Through grading, cells with similar capacity that may be needed for
different types of applications are grouped together.
**Voltage criterion**
A battery pack consists of two or more cells. The most common configuration
for communications equipment is a series connection of 6-12 cells, providing
7.2-14.4Vdc. Because nicad cells produce approximately 1.2Vdc each, rated
voltage is the number of cells multiplied by 1.2V. In reality the voltage
rises above 1.2 during and after charging. The voltage falls below 1.2V
during discharge, but is not considered depleted until it drops to 1V.
An observation of voltage alone cannot indicate a cell's integrity
accurately if the cell's voltage is less than 1.2V after charging. This
indicates internal damage to the cells. Because the pack voltage cannot be
used as a gauge of capacity, each cell must be cycled, which is the reasoning
behind cell grading.
Battery capacity, in milliamp-hours (mAh), quantifies the amount of energy
the battery can deliver over a specific period. Usually the period is one
hour. A battery rated at a 12V, 750mAh should, when fully charged, maintain a
0.75A discharge rate for one hour. The unit is considered depleted when its
terminal voltage under load reaches 1V/cell (10V in a 10-cell battery).
Over a period of several years, it has been found that some cells can vary
as much as 20% above the cell rated capacity. For example, within a group of
450mAh rated cells, a few actually run close to 540mAh. Cells with 12% to 17%
(504-530mAh) over rated capacity are common and 30-40% of the 450mAh cells run
500mAh or more.
**Pack quality**
Without cell grading, random groups of cells would typically be assembled
into a battery pack. Each group would probably have at least one cell in the
450mAh range and perhaps one or more in the 500mAh range. A battery has only
as much capacity as its weakest cell. Therefore, the entire pack, even with
several high capacity cells, can only perform at the 450mAh level.
For the same of comparison, a pack that contains 500mAh cells offers a 10%
increase in use time over one that contains all 450mAh cells. However, both
are physically the same size.
**Causes for variation**
A number of changes have taken place in the manner in which nicad batteries
are constructed since the technology began in the 1920s. Pocket-plate
construction was used to make reasonable quantities of small, sealed nicad
packs. Although they had high rates of discharge, they did not have any
overcharge protection and only could be charged slowly.
Technological improvements resulted in the sintered plate construction,
which gave thinner electrodes with large, active surface areas. This method
allows high charging and discharging rates and gives the batteries better low
temperature and overcharging properties. The size of the holes or the porous
construction of the electrode determines the amount of active material the
electrode can absorb.
Electrode porosity varies, which in turn causes cell capacity to vary. The
effort to produce high-quality, consistent electrodes is never ending.
Capacity is determined by separator material, the amount of electrolyte (KOH),
the density of the KOH and the space between positive and negative electrodes.
All of these elements have their own little problems. The interaction of all
the factors in individual cells make the process of cell grading more
valuable.
**The grading process**
Nicad cell grading is carried out with a computer-monitored system. The
system slowly charges the cells for 24 hours, ensuring a full charge for the
test. The cells are then placed into a fixture that is electrically connected
to a precise load. The computer monitors the voltage of each cell and
calculates its capacity as it discharges. When all cells under test are
depleted, the computer provides status data on each cell.
Grading is a quality control tool for the manufacturer because it allows
defective and low-capacity cells that do not reach specifications to be pulled
from the assembly process. Not only does it allow grouping by cell capacity,
it avoids the expense of having to rework a final assembled battery that does
not operate properly.
**No cell forming**
When a battery pack is constructed of unknown or mixed charged states, a
process called cell forming is necessary to bring the cells all to the same
state of charge. To form a battery, a slower trickle charge (C/10+) is used.
As an extreme example, a battery could have one cell fully charged and 11
cells not charged at all. If this battery is slow-charged the first time, the
low cells simply catch up to the full one with no damage. However, if the
pack is put into a rapid charger, the lone full cell will be severely
overcharged, while the others are charging. The entire assembly can be
damaged in the process.
By using a grading process prior to assembly, the mixed bag of individual
cells is avoided and cell forming is not required.
**Battery care**
In the assembly process, graded cells are welded into packs. The battery
pack maintains its rated capacity only if properly cared for. Battery
analyzers can be used to achieve the maximum performance and cycle life from a
battery.
When the battery is inserted, the analyzer begins a complete, current-
regulated rapid charge. During the charge period, a microprocessor-controlled
system makes multiple readings of the actual battery voltage. Each reading is
compared with the following reading to determine when the battery has reached
100% of its rated capacity.
Next the analyzer switches to a controlled discharge cycle, taking the unit
to a 1V/cell potential. At this point, the analyzer can calculate the battery
capacity in mAh and provide a printed documentation of the information.
Finally, the analyzer begins another charge cycle, returning the battery to
a 100% charge.
Because nicad batteries of today are constructed to last longer then ever
before, consideration must be given to ways to maintain their specifications
throughout the rated lifetime. Cell grading and sorting of cells prior to
assembly into packs plays a role in providing a battery that will have a good
long life. At the same time it allows the construction of battery packs with
higher capacity in the same packaging volume.
Getting Charged Up Right
Selecting the charger with the right combination of capabilities helps to
get the maximum use from an inventory of nickel-cadmium batteries.
How can you be sure nickel-cadmium [nicad] batteries deliver their
rated performance, time after time? The secret is in proper charging.
Many battery chargers are available, but each is not as effective as
another in charging and maximizing battery life. Several factors should
for the basis for selecting a particular mode.
QUESTIONS
1 Can the charger detect a level of discharge ? Can the charger
system discharge a partially charged battery to 1V/cell potential
before starting the charge cycle?
2 Is a proper current level used in charging the battery?
3 Is a trickle current applied to the battery after the charge cycle
is complete?
4 Is the technique to determine when a full charge has been obtained
effective and safe?
5 Is positive contact maintained between battery and charger terminals?
**Discharge cycles**
Discharging a battery before applying a charging current cures the
battery of at least three potential problems, including long-term
storage, long-term over-charging and memory effects.
A battery subjected to shallow discharges followed by full charges
is likely to exhibit a phenomenon known as memory effect. Memory is
not a loss of capacity, it is a voltage depression. Each cell in the
battery rapidly loses voltage during subsequent use, suffering a
reduction from 1.2V/cell to about 1V/cell. The reduced voltage mimics
the appearance of a loss of capacity. The memory effort is avoided
when the battery is fully discharged before each charge cycle.
Long-term over-charging causes the loss of 30-40% of the battery's
rated capacity the first time it is used after being subjected to the
over-charging. One discharge-charge cycle can restore the abused
battery to 75-90% of its rated capacity for its next use.
Long-term storage causes the battery electrolyte to become unevenly
distributed within the cells, reducing capacity. During long-term
storage a passivation layer may form on the battery's anode. The
layer acts as an insulator, diminishing the capacity as much as 55%.
After repeated discharge-charge cycles, the long-term over-charging
and storage effects are overcome and the battery will deliver its
rated capacity.
A battery is considered fully discharged if the terminal voltage
is 1V/cell or less when the battery is under a 1C load. A 1C load
draws a constant current while discharging a battery and reduces a
fully charged battery to a 1V/cell level in one hour. The load
normally is specified by the battery manufacturer.
The charger should not discharge the battery below 1V/cell. If it
does, weak cells in the stack may undergo a polarity reversal that
would shorten the battery's useful life.
**Charging currents**
A charger should deliver a constant current at a level that charges
the battery fastest. For rapid-charge batteries, a 1C charging rate
is preferred. On standard batteries, a C/4 charge rate should be
used, provided the charger can monitor the state of the charge and
reduce the current to a trickle or remove current from the battery
completely when a full charge is attained.
**Trickle charges**
A trickle current prevents the battery from discharging itself it
is left in the charger for an extended period.
A battery charged with the maximum safe current accepts the charge
more efficiently. The battery will deliver as much as 10% greater
capacity for a given amount of charge - comparing a 1C charging rate
to a 0.02C rate, for example.
A charger with 1C discharge and 1C and C/4 rates theoretically can
discharge and charge a rapid-charge battery fully in less than 2.5
hours. A standard-charge battery would need less than seven hours.
**Determining full charge**
Three methods are common in determining when a battery has reached
a state of full charge. One involves using the battery specifications
and applying a controlled charge; the other two involve measurements.
The controlled-charge method applies a constant current to a battery
that has been discharged to 1V/cell. The current is applied for a
specified period that satisfies the battery's specification.
Measurement methods monitor the battery voltage during charging to
detect a negative slope. As a battery reaches full charge, its
terminal voltage first slightly increases, then decreases. The
transition from the higher to lower voltage is known as the negative
slope and indicates a full charge.
Both methods are acceptable. But when the negative slope method is
used, a discharge cycle should still be included prior to charging.
Another method measures battery temperature. A thermal detection
device placed against the cells reveals an increase in temperature.
A nicad battery's temperature rises only after it reaches full charge.
Therefore, to some extent, the battery is overcharged with each charge
cycle. The thermal method may be acceptable if it is engineered
properly, but it is not the preferred technique.
**Positive contacts**
The charger terminals must make positive contact with the battery
terminals to ensure a low resistance connection for an efficient
discharge-charge cycle.
Positive contact also is a safety consideration with chargers that
use thermal sensing of the full charge. If proper contact is not
made, the battery could overcharge. A rapid-charge battery subjected
to an overcharge could reach temperatures high enough to melt the
battery case.
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