Many of our everyday devices that get their energy supply from a battery, while not always being as smart as they are described in the adverts, often come equipped with a kind of memory. For example, a battery powered shaver or electric toothbrush that is recharged before the battery runs out, can later take revenge on the prudent user. The NiMh battery appears to remember that you have only taken part of its storage capacity and eventually no longer supplies its full energy. This means that despite the battery still being discharged, the voltage it supplies is sometimes too low to drive the device in question.
The memory effect has two negative consequences. Firstly, the usable capacity of the battery is reduced, and secondly the correlation between the voltage and the charge status is shifted, so the latter cannot be determined reliably on the basis of voltage. The memory effect has long been known to exist in NiCd and NiMh batteries.
The memory effect and its associated abnormal working voltage deviation have now been confirmed for one of the most common materials used as the positive electrode in lithium-ion batteries, LiFePO4 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.
The existence of a memory effect is particularly relevant in the context of the anticipated steps towards using Polymer Lithium ion Battery in the electric mobility sector. In hybrid cars in particular, the effect can arise during the many cycles of charging/discharging that occur during their normal operation. The battery is partially recharged during each braking operation by the engine running in a generator mode. It is in turn discharged, and usually only partially, 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, as this new study demonstrates. This leads to an error in the estimate of the current state of charge of the battery, in cases where the state of charge is calculated by software on the basis of the current value of the voltage.