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Low Capacity Cells
Even with modern manufacturing techniques, the
capacity of a cell cannot be accurately predicted. As part
of the manufacturing process, each cell is measured and segregated
into categories according to their inherent capacity levels.
The high capacity A cells are commonly sold for special applications
at premium prices; the large mid-range B cells are used for
commercial and industrial applications such as mobile communications;
and the low-end C cells are mostly sold in supermarkets at
bargain prices. Cycling will not significantly improve the
capacity of the low-end cell. When purchasing rechargeable
batteries at a reduced price, the buyer should be aware of
the different capacity and quality levels offered.
As part of quality control, the battery assembler
should spot-check each batch of cells to examine cell uniformity
in terms of voltage, capacity and internal resistance. Failing
to observe these simple rules will often result in premature
battery failures. When buying quality cells from a well-known
manufacturer, battery assemblers are able to relax the matching
requirements somewhat.
Cell Mismatch
Cell mismatch can be found in brand-new as well
as aged battery packs. Poor quality control at the cell manufacturing
level and inadequate cell matching when assembling the batteries
cause unevenly matched cells. If only slightly off, the cells
in a new pack adapt to each other after a few charge/discharge
cycles, like players in a winning sports team.
A weak cell holds less capacity and is discharged
more quickly than the strong one. This imbalance causes cell
reversal on the weak cell if the battery is discharged below
1V/cell. The weak cell reaches full charge first and goes
into heat-generating overcharge while the stronger cell still
accepts charge and remains cool. In both situations, the weak
cell is at a disadvantage, making it weaker and contributing
to a more acute cell mismatch condition. An analogy can be
made with a high school bully who picks on the weaker kid.
High quality cells are more consistent in capacity
than lower quality counterparts. During their life span, high
quality cells degrade at about the same rate, helping to maintain
the matching. Manufacturers of power tools choose high quality
cells because of their durability under heavy load conditions
and temperature extremes. Lower-cost cells have been tried,
but early failure and consequent replacement is costlier than
the initial investment.
The capacity matching between the cells in a
battery pack should be within +/- 2.5 percent.
Tighter tolerances are required on batteries with high cell
counts that also must generate high load currents and are
operating under adverse temperatures. There is a strong correlation
between well-balanced cells and the longevity of a battery.
Lithium-based cells have tighter matching tolerances
than their nickel-based cousins. Tight matching of all cells
in a pack is especially important on lithium-based chemistries.
All cells must reach the end-of-discharge voltage threshold
at the same time. The full-charge point must be attained in
unison by all cells. If the cells are allowed to get out of
match, the weaker cell will be discharged to a lower voltage
point before the cut-off occurs. On charge, this weak cell
will attain the full-charge status before the others, causing
the voltage to go higher than on the stronger cells. This
larger voltage swing will put undue strain on the weak cell.
Each cell in a lithium-based pack is electronically
monitored to assure proper cell matching during the battery’s
life. An electronic circuit is added to some packs that compensate
the differences in cell voltages. This is done by connecting
a shunt across each cell string to consume the excess energy
of the cells which are more energetic. The low-voltage cut-off
occurs when the weakest cell reaches the end-of-discharge
point.
The Li-ion battery is controlled down to
the cell level to assure safety at all times. Because this
chemistry is still relatively new and unpredictable under
extreme conditions, manufacturers do not want to take undue
risks. There have been a few failures but such irregularities
are often kept a secret. This chemistry is considered very
safe, considering the large number of Li-ion batteries
that are in use.
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