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Performance Depreciation of Power Batteries for Electric Vehicles

published: 2011-07-21 15:59

Performance depreciation of power batteries could be a result of natural ageing through time or caused by lack of maintenance, operation in rigid environment, and inappropriate charge/discharge.

High Self-discharge Rate

It is natural for batteries to self-discharge at various rates. However, inappropriate usage could make self-discharge rate remain constantly high. This unwanted result will drain the battery faster. Self-discharge rate of a battery is approximately invers linear in relation to the time elapsed after being unplugged, meaning the highest discharge-rate occurs right after the battery is unplugged and decreases thereafter.
NiMH battery has a higher self-discharge rate. Under room temperature, fully recharged NiCd battery reduces its highest remaining energy by 10% within the next 24 hours and maintains about a decreasing rate of 10% per month thereafter. NiMH battery’s self-discharge rate increases after hundreds of discharges and the battery plate will start to swell to

Nickel-ion battery’s self-discharge rate is about 5% within 24 hours after being unplugged and reduces at a rate of 1-2% per month afterwards. The number of deep discharge makes no difference to the LiMH battery’s self-discharge rate while the self-discharge rate of lead-acid batteries reduces by 5% per month or 50% per year and repeated deep discharges make the self-discharge rate of a lead-acid battery increase.

Battery Assembly

During manufacturing process, batteries are labeled and classified into groups according to their energy capacity. Even the most advanced testing technology cannot precisely predict the capacity outcome. Higher capacity batteries are sold at a premium while lower capacity batteries are sold at a lower price.

In a battery pack in which a number of smaller batteries are combined and connected together, matching of batteries should be limited to ±2.5% capacity variation. When the number of component batteries increases and larger power output is required under low-temp work environment, the need for stricter quality control is prominent. The matching of component batteries in a large battery system is vital to the lifespan of the system since the unbalanced self-discharge rate will compromise the overall battery performance.

Although batteries are sealed, a certain amount of electrolyte in the battery is lost during the shelf life especially when careless usage may lead to gas leakage. Once it happens, it is hard to reseal the battery pack and loss of electrolyte will ultimately reduce the battery capacity. Under correct usage, lithium-ion batteries in general have less problem with the loss of electrolyte resulted from gas leakage. However, lithium batteries do incur internal pressure under certain circumstances. Some batteries’ internal circuit switch may cut down the electric current when internal pressure reaches a limit while others are designed to release gas from safety separator.

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