What is “fast enough” for electric car batteries?

Battery researchers typically talk about the speed that a battery is charging or discharging at in terms of its “C rate”. The C rate is just the inverse of the time, in hours, that the battery takes to charge or discharge, (i.e. C = 1/hr). So a battery discharged at 1 C takes 1 hour to empty, at 2 C it takes 30 minutes, and at 0.5 C it takes 2 hours.

In research papers you often see plots comparing how a battery performs at a variety of C rates, like 0.1 C, 1 C, or 10 C.  Operating at higher C rates is more challenging, while getting good performance at low C rates is considered easy. But it’s not often clarified what rates are realistic for an application like electric vehicles. So what C rate is realistic for an electric car?

I can think of two ways to ask this question for different situations.  In battery research, the C rate typically describes a situation where you empty the whole battery at a constant rate.  The closest real approximation for a car is taking a trip where you drive the whole range of the battery at about the same speed. For example, the 2018 Chevy Bolt has a rated range of 240 miles.  This is just barely longer than the distance from my home in Ithaca, NY to Philadelphia, which Google Maps tells me takes 4 – 5 hours, depending on route, mostly on 55-60 mph highways. So the average C rate for this kind of steady driving would come out to 0.2 – 0.25 C.  This is a fairly low C rate, and safely on the “easy” side of the C rates reported in the research literature.

The more difficult situation is when the battery has to deliver the highest power possible, that is when you are flooring it to get onto the highway. What is the maximum C rate that the battery needs to be able to operate at?  The Bolt has a 150 kW (200 horsepower) electric motor and a 60 kWh battery.  That means that the battery discharges at a maximum of 2.5 C.  So if you kept flooring it you’d have about 24 minutes before the battery was empty, provided it didn’t overheat first.

Research papers often report good performance at C rates quite a bit higher than this, so it’s good to keep in mind that 2-3 C is the maximum realistic C rate needed.  Modern Li-ion batteries easily cross the bar of just working at these C rates, but keeping the battery from overheating when doing so can be more of a challenge. Engineering good cooling systems for vehicles and battery packs is a big part of this, but researchers working on battery chemistry and materials should also focus on making sure that the battery produces as little excess heat at high rates as possible.

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