High Self-discharge
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.

Cell matching
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 +/- 2.5%. Tighter tolerances are required on batteries with high cell count, those delivering high load currents and packs operating at cold temperatures. If only slightly off, the cells in a new pack will adapt to each other after a few charge/discharge cycles. There is a correlation between well-balanced cells and battery longevity.

Why is cell matching so important? A weak cell holds less capacity and is discharged more quickly than the strong one. This imbalance may cause cell reversal on the weak cell if discharged too low. On charge, the weak cell is ready first and goes into heat-generating overcharge while the stronger cell still accepts charge and remains cool. In both cases, the weak cell is at a disadvantage, making it even weaker and contributing to a more acute cell mismatch.

Quality cells are more consistent in capacity and age more evenly than the lower quality counterparts. Manufacturers of high-end power tools choose high quality cells because of durability under heavy load and temperature extremes. The extra cost pays back on longer lasting packs.

lithium-based cells are by nature closely matched when they come off the manufacturing line. Tight tolerances are important because all cells in a pack must reach the full-charge and end-of-discharge voltage thresholds at a unified time. A built-in protection circuit safeguards against cells that do not follow a normal voltage pattern.

Shorted Cells
Manufacturers are often unable to explain why some cells develop high electrical leakage or an electrical short while still relatively new. The suspected culprit is foreign particles that contaminate the cells during manufacturing. Another possible cause is rough spots on the plates that damage the separator. Better manufacturing processes have reduced the 'infant mortality' rate significantly.

Cell reversal caused by deep discharging also contributes to shorted cells. This may occur if a nickel-based battery is being fully depleted under a heavy load. nickel-cadmium is designed with some reverse voltage protection. A high reverse current, however, will produce a permanent electrical short. Another contributor is marring of the separator through uncontrolled crystalline formation, also known as memory.

Applying momentary high-current bursts in an attempt to repair shorted cells offers 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.

Loss of Electrolyte
Although sealed, the cells may lose some electrolyte during their life, especially if venting occurs due to excessive pressure during careless charging. Once venting has occurred, the spring-loaded vent seal on nickel-based cells may never properly close again, resulting in a build-up of white powder around the seal opening. The loss of electrolyte will eventually lower the battery capacity.

Permeation, or loss of electrolyte in valve regulated lead-acid batteries (VRLA) is a recurring problem. Overcharging and operating at high temperatures are the causes. Replenishing lost liquid by adding water offers limited success. Although some capacity may be regained, the performance becomes unreliable.

If correctly charged, lithium-ion cell should never generate gases and cause venting. But in spite of what is said, the lithium-based cells can build up internal pressure under certain conditions. Some cells include an electrical switch that disconnects the current flow if the cell pressure reaches a critical level. Other cells rupture a membrane to release the gases in a controlled way. lithium-ion-polymer in a pouch cell sometime grows to the shape of a small balloon because these cells do not include venting. Ballooning cell are known to damage the housing of the portable device.

Figure 1: lithium-ion-polymer cell in a pouch pack. Made ultra-slim, some cells generate hydrogen gas during charge and puff up. The force can damage the housing of the portable device.