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14.4 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.
14.5 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.