The degradation of battery performance is a natural result of use and aging on the one hand, and accelerated by lack of maintenance, harsh operating conditions, and poor charging practices on the other. The following will discuss the various insurmountable problems of rechargeable batteries, their causes, and ways to remedy these problems.
High self-discharge rate
All kinds of batteries have self-discharge, but improper use will contribute to the development of this state. The self-discharge rate shows an asymptotic pattern, with the highest rate appearing just after charging and then gradually decreasing.
Nickel-based batteries exhibit a high self-discharge rate. After charging a new NiCd battery at normal ambient temperature, its electrical high decreases by about 10% during the first 24h. Thereafter, the self-discharge rate stabilizes to about 10% per month. Usually the discharge rate increases at higher temperatures. The general guideline is that the self-discharge rate increases by a factor of 1 for every 10°C increase in temperature. The self-discharge rate of nickel metal hydride batteries is about 30% greater than that of nickel cadmium batteries.
The self-discharge rate of nickel-based batteries also increases after hundreds of cycles, and the electrode plates of the battery start to expand and thus squeeze the diaphragm between the electrodes more tightly, forming metal dendritic crystals as a result of crystal growth (memory effect), which damages the battery diaphragm and increases the self-discharge rate. If the self-discharge of nickel-based batteries in 24h up to 30%, should be discarded.
Nickel-ion batteries have a self-discharge rate of 5% for the first 24h after charging. Thereafter it decreases to 1%-2% per month and the safety protection circuit of the battery increases by about 3%. The high number of cycles and aging have no effect on the self-discharge rate of lithium-based batteries. The self-discharge rate of lead-acid batteries is about 5% per month or 50% per year, and the self-discharge increases with repeated deep cycle charging and discharging.
The percentage of battery self-discharge can be determined by a battery analyzer, but this procedure takes several hours. The measured internal resistance of the battery can often reflect whether the internal resistance of the battery is too high. This parameter can be measured with an impedance meter or with the battery analyzer's ohmic test procedure.
Battery Matching
Even with modern manufacturing techniques, it is impossible to accurately predict the capacity of a battery, especially for nickel-based batteries. During the manufacturing process, each cell is tested and classified by its capacity. High-capacity "A" cells are usually sold as special-purpose cells at premium prices; medium-capacity "B" cells are used in industrial and commercial products; and low-end "C" cells are sold at low prices. Cyclic charging and discharging does not improve the capacity of batteries in the lower end of the range. What you get when you buy low-cost rechargeable batteries is low battery capacity.
In a battery pack consisting of multiple cells, cell matching should be controlled to within ±2.5%. In packs with a large number of cells, and packs that require high load current output and operate at low temperatures, more stringent cell tolerance control is required. A small mismatch between cells in a new battery pack will balance itself out after a few charge cycles. The ability of the cells to adapt well to each other is critical to the long life of the battery pack.
Why is battery matching so important? It is because a "weak" battery contains less capacity and it discharges faster than a "strong" battery. This imbalance in the discharge process causes the "weak" cells to reverse their polarity when discharged past a low voltage. During charging, the "weak" battery is first overcharged by heat, while the stronger battery is still charged normally and does not heat up. In both cases, the "weak" battery is at a disadvantage, making it even "weaker" and leading to a serious mismatch.
High quality batteries have a more consistent and balanced capacity than lower quality batteries. High-quality batteries should be used for high-end high-power tools because of their high durability under heavy loads and extreme temperature conditions. The cost is high, but the payoff is a longer battery pack life.
Lithium-based batteries are well matched by nature when they come off the production line. It is very important that the individual cells within the pack meet strict tolerances. All cells in the pack must reach full charge in a uniform amount of time and reach the same threshold voltage at the end of discharge. The protection circuitry built into the battery pack should provide safety protection in the event of abnormal battery operation.
Short-circuited Batteries
Battery manufacturers are often unable to explain why some batteries show high leakage rates or electrical shorts when the batteries are still relatively new. The suspected reason is that foreign particulate impurities may have been mixed into the battery during the manufacturing process. Another is the damage to the diaphragm caused by the rough spots on the electrode. Therefore, the manufacturing process of the battery should be improved, which can greatly reduce the "early failure rate" (infant mortality) of the battery.
The reversal of cell polarity caused by deep discharge can also lead to short circuit. This state can also occur when nickel-based batteries are discharged at high currents until they are completely discharged. High reverse currents can cause permanent electrical shorts. Another cause is diaphragm damage caused by uncontrollable crystal formation, which is known as the memory effect.
Attempts to repair shorted cells using transient high current pulses have extremely limited success. The short circuit may be temporarily evaporated, but the damage to the diaphragm material remains. The repaired cell often exhibits a high discharge rate and the short circuit will reappear. It is not advisable to replace a shorted cell in an aging battery pack. Unless the new cell matches the performance of the other cells in the pack in terms of cell voltage and capacity.
Loss of electrolyte
Cells, although sealed, will lose some electrolyte during their service life, especially if they are carelessly and improperly charged to produce excessive gas pressure resulting in gas emissions. Once the gas is vented, the spring-loaded venting gasket on the nickel-based cell may not re-seal properly, causing a white powder to accumulate around the gasket, and the loss of electrolyte will eventually reduce the cell capacity.
Loss of electrolyte is a long-standing problem in permeable or valve-regulated lead-acid batteries (VRCA). It is caused by overcharging and working at high temperatures. The effectiveness of replenishing electrolyte loss with water is limited, and while the battery capacity can be partially restored, the performance of the battery will be less reliable.
If charged correctly, lithium-ion batteries should not produce gas so that the problem of exhaust. However, lithium-ion batteries can generate internal pressure under certain conditions. Some batteries are equipped with an internal circuit switch that cuts off current when the battery pressure reaches a critical value. Others are designed to release gas in a controlled manner or by opening the safety diaphragm.
