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( January 2004) Non-Correctable Battery Problems
Some rechargeable batteries can be restored through external means, such as applying a full discharge. There are, however, many defects that cannot be corrected. These include high internal resistance, elevated self-discharge, electrical short, dry-out, plate corrosion and general chemical breakdown.
The performance loss of a battery occurs naturally as part of usage and aging; some is hastened by lack of maintenance, harsh field conditions and poor charging practices. This paper examines the cause of non-correctable battery problems and explores ways to minimize these breakdowns.
All batteries are affected by self-discharge. This is not a defect per se, although improper use enhances the condition. Self-discharge is asymptotical; the highest loss occurs right after charge, and then tapers off.
Nickel-based batteries exhibit a relatively high self-discharge. At ambient temperature, a new nickel-cadmium loses about 10% of its capacity in the first 24 hours after charge. The self-discharge settles to about 10% per month afterwards. Higher temperature increases the self-discharge substantially. As a general guideline, the rate of self-discharge doubles with every 10°C (18°F) increase in temperature. The self-discharge of nickel-metal-hydride is about 30% higher than that of nickel-cadmium.
The self-discharge increases after a nickel-based battery has been cycled for a few hundred times. The battery plates begin to swell and press more firmly against the separator. Metallic dendrites, which are the result of crystalline formation (memory), also increase the self-discharge by marring the separator. Discard a nickel-based battery if the self-discharge reaches 30% in 24 hours
The self-discharge of the lithium-ion battery is 5% in the first 24 hours after charge, and then reduces to 1% to 2% per month thereafter. The safety circuit adds about 3%. High cycle count and aging have little effect on the self-discharge of lithium-based batteries.
A lead-acid battery self-discharges at only 5% per month or 50% per year. Repeated deep cycling increases self-discharge.
The percentage of self-discharge can be measured with a battery analyzer but the procedure takes several hours. Elevated internal battery resistance often reflects in higher internal battery resistance, a parameter that can be measured with an impedance meter or the OhmTest program of the Cadex battery analyzers.
Even with modern manufacturing techniques, the cell capacities cannot be accurately predicted, especially with nickel-based cells. As part of manufacturing, 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 mid-range `B' cells are used for commercial and industrial applications; and the low-end `C' cells are sold at bargain prices. Cycling will not significantly improve the capacity of the low-end cells. When purchasing rechargeable batteries at a reduced price, the buyer should be prepared to accept lower capacity levels.
The cells in a pack should be matched within +/