Reasons and remedies for performance degradation of electric vehicle rechargeable batteries

The degradation of battery performance is a natural result of use and aging, but it is also accelerated by lack of maintenance, harsh operating environments, and poor charging practices. The following will explore the various difficult problems of rechargeable batteries, their causes, and ways to remedy these problems.

High self-discharge rate

All batteries have self-discharge, but improper use will promote the development of this state. The self-discharge rate follows an asymptotic law, with the highest discharge rate occurring just after charging, and then gradually decreasing.

Nickel-based batteries exhibit a high self-discharge rate. At normal ambient temperature, a new nickel-cadmium battery loses about 10% of its charge during the first 24 hours. Thereafter, the self-discharge rate stabilizes to about 10% per month. Generally, the higher the temperature, the greater the discharge rate. A general rule is that the self-discharge rate doubles 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.

After hundreds of cycles, the self-discharge rate of nickel-based batteries also increases. The battery plates begin to expand, squeezing the diaphragm between the electrodes more tightly, forming metal dendrites, which is the result of crystal growth (memory effect), thus damaging the battery diaphragm and increasing the self-discharge rate. If the self-discharge of nickel-based batteries reaches 30% in 24 hours, they should be discarded.

The self-discharge rate of nickel-ion batteries is 5% in the first 24 hours after charging. After that, it drops to 1%-2% per month, and the battery's safety protection circuit increases by about 3%. High cycle times 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 repeated deep cycle charging and discharging increases the self-discharge.

The percentage of battery self-discharge can be determined using 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 using an impedance meter or the ohm test procedure of a battery analyzer.

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 battery is tested and sorted by its capacity. High-capacity "A" batteries are usually sold as special-purpose batteries at premium prices; medium-capacity "B" batteries are used in industrial and commercial products; and low-end "C" batteries are sold at low prices. The capacity of low-end batteries cannot be improved by cycling. Buying a low-priced rechargeable battery will result in a low battery capacity.

In a battery pack composed of multiple cells, the matching of the cells should be controlled within ±2.5%. In battery packs with a large number of cells, as well as battery packs that need to output large load currents and operate at low temperatures, stricter cell tolerance control is required. If the individual cells in a new battery pack have a small mismatch, they will be able to balance and adapt to each other after several charging cycles. Whether the cells can be well balanced and adapted is related to whether the battery pack has a long service life.

Why is battery matching so important? This 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" battery to reverse polarity when it is discharged through a low voltage. When charging, the "weak" battery will first enter a hot overcharge state during the charging process, while the stronger battery can still accept the charge normally without heating. In both cases, the "weak" battery is in a disadvantageous state, making it even "weaker" and causing a serious mismatch.

High-quality batteries have a more consistent and balanced capacity than low-quality batteries. High-quality batteries should be used for high-end, high-power tools because they have high durability under heavy loads and extreme temperature environments. Although the cost is high, the reward is a longer life of the battery pack.

Lithium-based batteries are inherently well matched when they come off the production line. It is very important that each individual cell in the battery pack meets strict tolerances. All cells in the battery pack must reach full charge within the same time and reach the same threshold voltage at the end of discharge. The protection circuit built into the battery pack should play a safety protection role when the battery is not in normal working state.

Short-circuited battery

Battery manufacturers often cannot explain why some batteries show high leakage rates or electrical short circuits when they are still in a relatively new state. The suspected reason is that foreign particles may be mixed into the battery during the manufacturing process. Another is that rough spots on the electrodes cause damage to the diaphragm. Therefore, the manufacturing process of the battery should be improved, which can greatly reduce the battery's "infant mortality".

A deep discharge that causes a reversal of the battery's polarity can also cause a short circuit in a battery. This condition can also occur if a nickel-based battery is discharged at a high current until it is completely discharged. The high reverse current can cause a permanent electrical short circuit. Another cause is damage to the diaphragm due to the formation of uncontrolled crystals, which is the so-called memory effect.

Attempting to repair a shorted cell by applying a momentary high current pulse has very limited success. The short may be temporarily vaporized, but damage to the separator material remains. Such repaired cells often exhibit high discharge rates and the short will reoccur. Replacing a shorted cell in an aged battery pack is not advisable unless the new cell is matched in cell voltage and capacity to the other cells in the pack.

Loss of electrolyte

Although the battery is sealed, it will lose some electrolyte during its service life, especially if excessive gas pressure is generated due to careless and improper charging, resulting in gassing. Once gassing occurs, the spring-loaded vent seal on nickel-based batteries may be difficult to reseal properly, resulting in the deposition of white powder around the seal. The loss of electrolyte will eventually reduce the battery capacity.

Permeation, or loss of electrolyte in valve-regulated lead-acid batteries (VRCA), is an old problem. It is caused by overcharging and operating at high temperatures. Replenishing electrolyte loss by adding water has limited success and although it can partially restore battery capacity, the battery performance will be less reliable.

If charged correctly, lithium-ion batteries should not produce gas to cause venting problems. However, lithium-ion batteries can also generate internal pressure under certain conditions. Some batteries have internal circuit switches that cut off the current when the battery pressure reaches a certain critical value. Other batteries are designed to release gas in a controlled manner or open a safety diaphragm.