Chapter 14: Non-Correctable Battery Problems
Non-correctable battery problems are those that cannot be improved through external means such as giving the battery a full charge or by applying repeated charge/discharge cycles. Deficiencies that denote the non-correctable status are high internal resistance, elevated self-discharge, electrical short of one or several cells, lack of electrolyte, oxidation, corrosion and general chemical breakdown. These degenerative effects are not only caused by normal usage and aging, but they include less than ideal field conditions and an element of neglect. The user may have poor charging equipment, may operate and store the battery in adverse temperatures and, in the case of nickel-based batteries, may not maintain the battery properly.
New battery packs are not exempt from deficiency syndromes and early failure. Some batteries may be kept in storage too long and sustain age-related damage, others are returned by the customer because of incorrect user preparation.
In this section we examine the cause of non-correctable battery problems and explore why they occur. We also look at ways to minimize premature failure.High Self-discharge
Self-discharge is a natural phenomenon of any battery. It is not a manufacturing defect per se, although poor manufacturing practices and improper maintenance and storage by the consumer enhance the problem.
The level of self-discharge differs with each chemistry and cell design. High-performance nickel-based batteries with enhanced electrode surface area and super conductive electrolyte are subject to higher self-discharge than the standard version cell with lower energy densities. Self-discharge is non linear and is highest right after charge when the battery holds full capacity.
NiCd and NiMH battery chemistries exhibit a high level of self-discharge. If left on the shelf, a new NiCd loses about 10 percent of its capacity in the first 24 hours after being removed from the charger. The rate of self-discharge settles to about 10 percent per month afterwards. At a higher temperature, the self-discharge rate increases substantially. As a rule, the rate of self-discharge doubles with every 10°C (18°F) increase in temperature. The self-discharge of the NiMH is about 30 percent higher than that of the NiCd.
A major contributor to high self-discharge on nickel and lead-based batteries is a high cycle count and/or old age. With increased cycles, the battery plates tend to swell. Once enlarged, the plates press more firmly against the delicate separator, resulting in increased self-discharge. This is common in aging NiCd and NiMH batteries but can also be seen in lead acid systems.
Loading less active materials on the plates can reduce the plate swelling on nickel-based batteries. This improves expansion and contraction while charging and discharging. In addition, the load characteristic is enhanced and the cycle life prolonged. The downside is lower capacity.
Metallic dendrites penetrating into the separator are another cause of high self-discharge. The dendrites are the result of crystalline formation, also known as memory. Once marred, the damage is permanent. Poorly designed chargers that ‘cook’ the batteries also increase the self-discharge. High cell temperature causes irreversible damage to the separator.
While the nickel-based systems can withstand some abuse and tolerate innovative or crude charge methods, the Li-ion demands tight charging and discharging regimes. Keeping the voltage and current within firm boundaries prevents the growth of dendrites. The presence of dendrites in lithium-based batteries has more serious implications than just an increase in self-discharge — dendrites can cause an electrical short, which could lead to venting with flame.
The self-discharge of the Li-ion battery is five percent in the first 24 hours after charge and averages 1 to 2 percent per month thereafter. In addition to the natural self-discharge through the chemical cell, the safety circuit draws as much as 3 percent per month. High cycle count and aging has little effect on self-discharge on lithium-based batteries.
An SLA self-discharges at a rate of only five percent per month or 50 percent per year. Repeated deep cycling increases the self-discharge. When deep cycling, the electrolyte is drawn into the separator, resulting in a crystalline formation similar to that of a NiCd battery.
The self-discharge of a battery is best measured with a battery analyzer. The procedure starts by charging the battery. The capacity is read by applying a controlled discharge. The battery is then recharged and put on a shelf for 24 hours, after which the capacity is measured again. The discrepancy between the capacity readings reveals the level of self-discharge.
More accurate self-discharge measurements can be obtained by allowing the battery to rest for at least 72 hours before taking the reading. The longer rest period compensates for the relatively high self-discharge immediately after charge. At 72 hours, the self-discharge should be between 15 and 20 percent. The most uniform self-discharge readings are obtained after seven days. On some battery analyzers, the user may choose to adjust the desired rest periods in which the self-discharge is measured.
Research is being conducted to find a way to measure the self-discharge of a battery in minutes, if not seconds. The accuracy and repeatability of such technology is still unknown. The challenge is finding a formula that applies to all major batteries and includes the common chemistries.
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 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.
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.