Memory effect detected in Li-ion batteries, claims research team
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One of the problems with NiCd and NiMH batteries was the memory effect, a chemical process which progressively reduced the battery's ability to store and deliver energy. Until now, lithium ion batteries have been believed to be immune from this effect, but scientists at the Paul Scherrer Institute (PSI) and Toyota Central R&D Labs have discovered a memory effect in a lithium-ion battery.
According to the researchers, the memory effect has now been detected in lithium-iron phosphate – commonly used as the positive electrode in Li-ion batteries. With lithium-iron phosphate, the voltage remains practically unchanged over a large range of the state of charge. This means that even a small anomaly in the operating voltage could be misinterpreted as a major change in the state of charge. Putting it another way, says the team, when the state of charge is determined from the voltage, a large error can be caused by a small deviation in the voltage.
Professor Petr Novak, head of the PSI's electrochemical energy storage section, said the study disproves a long cherished misconception. "Ours is the first study that has specifically looked for a memory effect in lithium-ion batteries. It had simply been assumed that no such effect would arise."
The team says its finding is particularly relevant for the use of lithium-ion batteries in electric vehicles. Here, the battery is partially recharged during each braking operation by the engine running in a generator mode and partially discharged to assist the engine during acceleration phases. The numerous successive cycles of partial charging and discharging lead to individual small memory effects adding up to a large memory effect. This is said to lead to an error in the estimate of the current state of charge of the battery, when the state of charge is calculated on the basis of the value of the voltage.
