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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.
14.1 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.