Derating of Lithium-ion Cells | Relationship between SoC, C rate and temperature • EVreporter

Derating is a term that comes into play for applications where batteries are either fully charged or deeply discharged. Some examples of derating are:

• Ever seen an EV reduce your driving speed when its battery is low? That is low SoC (State of Charge) level derating of discharging current/power.

• Why does battery charging speed reduce significantly after 90%? That is because of high SoC-level derating of charging current/power.

• Why does a cell phone stop functioning when it experiences very high temperatures? That’s temperature related derating to cool down the battery faster.

What is Derating?

Derating is an intentional process of reducing the operating current of the battery to reduce the stress it faces during scenarios such as:

• Fast charging when nearing full charge condition

• High power output from a nearly drained battery

Reducing the operating current during such conditions helps reduce the stress on the battery and improve the battery life. Regularly experiencing stress can also lead to safety issues and battery failures during the warranty period.

Derating is a very important factor for battery operation in many other scenarios. After a battery system has been stored for a very long time, calendar ageing reduces the retention capacity of the battery, and it is recommended to use this battery at a lower C rate hereafter; hence, derating comes into play.

Below are many other reasons why battery needs derating with ageing:

• Corrosion of current collector

• Electrode particle cracking

The above conditions are caused by Higher current/C rate operation, High-temperature operation, Mechanical stress, Low-temperature use, Low cell voltage storage, and Charging to a higher-than-recommended voltage.

Below is a derating table of an LFP prismatic cell provided by XDLE Battery, an LFP Prismatic cell manufacturer. This is for their EV 280Ah cell, which gives 8,000 cycles at 1C charge and 1C discharge standard rating but allows for 2C maximum continuous charge and 3C maximum continuous discharge. It is suitable for large commercial vehicles, earth-moving equipment, and mining truck applications.

Derating Table during Charge (in terms of C rate)

Derating Table during Discharge (in terms of C rate)

From the above table, it is clear that derating happens with respect to temperature, SoC (state-of-charge), and, accordingly, power (current at that particular voltage level) is adjusted. The current is represented in terms of C rate, wherein 1C means 1*battery discharge rating in terms of Ah. It is expressed in terms of Ampere units. For example, the 3C rate for a 280Ah cell would be 3*280A (840A). Similarly, the 0.50C rate for 280Ah cell would be 0.50*280A (140A).

Derating table tells us about a whole lot of things. Comparing various products’ derating tables will tell a lot about their power characteristics, which can vary due to different construction and additives. Going higher than the maximum recommended values can result in permanent, irreversible cell capacity loss and lead to lower cycle life. Also, using the cell close to these high values can lead to decreased cycle life because these values are higher than the standard values declared and are to be used only for situations that need higher power for a few minutes as compared to pulse power, which allows only for a few seconds.

Observations about the operating temperature range during charging and discharging can be made from the table. It is a known fact that LFP cells cannot charge below 0°C; hence, the table starts at 0°C for charging. However, discharging can go up to -20°C temperatures. Similarly, some manufacturers allow higher temperatures during discharging; in this case, 65°C is allowed during discharging, compared to 60°C being allowed during charging.

Derating applies differently during charging and discharging.

One can see that as the battery is nearing fully charged conditon, its charging speed needs to be reduced (charge derating). Similarly, as the battery is nearing a fully discharged condition, discharge derating needs to be applied.

Extreme low temperatures increase the cell’s internal resistance and slow down the battery power rating; hence, derating is required during charging and discharging.

On the other hand, higher temperatures decrease the cell’s internal resistance, and slowing down discharging is not required, but it is done at extremely high temperatures, such as 65°C, to slow down the effect the high temperature has on the cell cycle life.

Similarly, charging speed is derated at high temperatures to preserve the cell cycle life.

About the author

Rahul Bollini is an R&D expert in Lithium-ion cells with 9 years of experience. He founded Bollini Energy to assist in deep understanding of the characteristics of Lithium-ion cells to EV, BESS, BMS and battery data analytics companies across the globe. Rahul can be reached at +91-7204957389 and bollinienergy@gmail.com.

Also read: High voltage battery design for large electric vehicles – Part 1