|
Shorted Cells
Manufacturers are often unable to explain why
some cells develop high electrical leakage or an electrical
short while the batteries are still relatively new. There
are a number of possible reasons that contribute to this irreversible
form of cell failure.
The suspected culprit is foreign particles that
contaminate the cells during manufacture. Another possible
cause is rough spots on the plates that damage the separator.
Better quality control at the raw material level and minimal
human interface during the manufacturing process has greatly
reduced the ‘infant mortality’ rate of the modern rechargeable
cells.
Cell reversal caused by deep discharging also
contributes to shorted cells. This commonly occurs if a nickel-based
battery is being fully depleted under a heavy load. A NiCd
battery is designed with some reverse voltage protection and
a small reverse current in the magnitude of milliamperes can
be tolerated. A high current, however, causes the reversed-polarized
cell to develop a permanent electrical short. Another cause
of a short circuit is marring the separator through uncontrolled
crystalline formation.
Applying momentary high-current bursts in an
attempt to repair shorted cells has had limited success. The
short may temporarily evaporate but the damage to the separator
material remains. The repaired cell often exhibits a high
self-discharge and the short frequently returns.
Replacing a shorted cell in an aging pack is
not recommended unless the new cell is matched with the others
in terms of voltage and capacity. Otherwise, an imbalance
may occur. One may remember the biblical verse “No one puts
a patch of unshrunken cloth on an old garment. . .” or “No
man would put new wine into old wineskins. . .” (Mt 9.16-17).
Attempts to replace faulty cells have commonly lead to battery
failures after about six months of use. It is best not to
disturb the cells in a battery pack but allow them to age
naturally. Maintaining the batteries while they are still
in good working condition will help to prevent premature failure.
Shorts in a Li-ion cell are uncommon. Protection
circuits monitor an ailing Li-ion cell and render the
pack unusable if serious voltage irregularities are detected.
Charging such a pack would (protection circuit permitting)
generate excess heat. The battery’s temperature control circuits
are designed to terminate the charge.
Loss of Electrolyte
Although sealed, battery cells may lose some
electrolyte during their life. Typical loss of moisture occurs
if the seal opens due to excessive pressure. This occurs if
the battery is charged at very low or very high temperatures.
Once vented, the spring-loaded seal of nickel-based cells
may never properly close again, resulting in a deposit of
white powder around the seal opening. Losses may also occur
if the cell cap is not correctly sealed in the manufacturing
process. The loss of electrolyte results in a decrease of
capacity, a defect that cannot be corrected.
Permeation, or loss of electrolyte in sealed
lead acid batteries, is a recurring problem. Overcharge is
the main cause. Careful adjustment of charging and float voltages
reduces loss of electrolyte. In addition, the battery should
operate at moderate temperatures. Air-conditioning is a prerequisite
for VRLA batteries, especially in warmer climates.
Replenishing lost liquid in VRLA batteries by
adding water has had limited success. Although lost capacity
can often be regained with a catalyst, the performance of
the stack is short-lived. After tampering with the cells,
it was observed that the battery stack turned into high maintenance
mode and needed to be closely supervised.
A properly designed, correctly charged Li-ion
cell should never generate gases. As a result, the Li-ion
battery does not lose electrolyte through venting.
But in spite of what is being said, the lithium-based
cells can build up an internal pressure under certain conditions.
Provisions are made to maintain safety of the battery and
equipment should this occur. Some cells include an electrical
switch that opens if the cell pressure reaches a critical
level. Other cells feature a membrane that safely releases
the gases if need be. Controlled release of the pressure prevents
bulging of the cell during pressure buildup.
Most of the safety features of lithium-based
batteries are one-way; meaning that once activated, the cells
are inoperable thereafter. This is done for safety reasons.
|
