Understanding memory effect in Lithium-ion batteries • EVreporter

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Understanding memory effect in Lithium-ion batteries • EVreporter


Memory effect is a term commonly used in the battery industry, and it dates back to battery technologies such as Nickel-cadmium and Nickel-metal hydride. The memory effect is the ability of the battery to remember its regular usage pattern. It is a common scenario when the battery frequently operates at partial state-of-charge (PSoC). The memory effect would mean that the battery tends to remember this regular usage pattern of low depth-of-discharge (DoD) and gradually loses its ability to deliver energy equal to its usable capacity due to a lower window of working voltage.

How does memory effect affect the end consumers?

The memory effect leads to a reduction in the usable capacity of the battery in the overall window of the working voltage. This results in lower capacity being discharged and a wrong estimation of State of Charge (SoC). Applications using such batteries would show a different estimation of energy left in the battery compared to the real energy it can deliver.

Do Lithium-ion batteries have memory effect?

The answer is no and yes. Most Lithium-ion cells, such as NMC, NCA and LCO do not have memory effect, except for LFP chemistry cells. The effect is more evident in lower-grade LFP cells or LFP cells stored in harsh conditions for a long time.

SoC measurement is done by measuring the voltage, a popular method adopted by the industry. It is called the direct measurement method for SoC estimation. LFP chemistry cells exhibit the memory effect. And as LFP chemistry cells have a flatter voltage curve, a slight change in voltage results in a large change in SoC measuring algorithm.

LFP working voltage is vast compared to older technologies such as Ni-Cd and NIMH. However, the memory effect shifts the working voltage. The memory effect does not have much impact on the capacity delivered by the individual LFP cell over its complete lifetime. But in the battery pack, their voltage shift due to the memory effect can create balancing issues and make the battery pack cut off earlier than required.

Since the BMS of the battery pack is supposed to cut off the battery pack even if one cell reaches overvoltage (during charging) or undervoltage (during discharging), the memory effect can create situations leading to an earlier cut-off during overvoltage and undervoltage situations. This translates to lower energy stored and lower energy delivered by the pack.

About the Author

Rahul Bollini is a Lithium-ion cell and battery pack R&D expert with an industrial experience of over 7 years. He can be reached at +91-7204957389 and bollinienergy@gmail.com.

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