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How to cool an electric car battery

What is a manufacturer to do about electric car batteries susceptibility to heat? As it turns out, the answer depends on what the warranty says, not so much on what the owner’s manual warns you not to do, Finley writes.

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At this point you may be wondering …”How does the Leaf keep its batteries from getting too hot without resorting to the pumps, coolant, radiators, hoses, valves, and fans (see  above photo)? As with the Prius and its unprecedented mileage, their solution is pure elegance. Instead of cooling batteries down, they don’t let them get hot. According to an interview in the Nissan Technology Magazine with Takeshi Miyamoto, (taking a deep breath …Engineering Director of EV Technology Development Division, EV Energy Development Department, Battery Engineering Group) the temperature is controlled by adjustment of the battery’s internal resistance. Of course, you can’t physically go inside a battery and alter its internal resistance but the internal resistance expressed is different depending on whether it is being discharged, or charged, and how fast that is happening, and at what temperature.

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Translation: The computer controls battery internal resistance by varying performance parameters while letting battery temperature float up or down in a safe range. For example, I’ve noticed that my Leaf does not allow much, if any regenerative braking right after a fresh recharge. Regenerative braking does not return in full force until the batteries cool and/or discharge to a lower level. From the Leaf owners manual:

 Power Limitation Mode: This mode protects the health and operation of the vehicle’s Li-ion battery. This mode operates in certain extreme conditions (heat, cold, low state of charge). Power available to vehicle systems, including its traction motor, is limited resulting in limited performance, acceleration and top speed. Charging may be automatically terminated, especially with repeated quick charging in extreme hot weather.

Suppose you’re driving 80 mph up a mountain on a 120 degree day and the battery temperatures approach a level that could cause damage. The car’s software slows the car down (reduces power draw from the battery pack) as much as needed to keep the temperatures in a healthy range (and the car at a safe speed). If you are in stop and go traffic on a 120 degree day, if necessary, it will limit how fast your car can accelerate, and how much it can regenerate, etc, etc, to the point of shutting the car down in steps to prevent battery damage if necessary or if acceleration or top speed is too limited for further safe driving.

The car wouldn’t suddenly shut down. As with a low battery charge condition, it would begin by giving you warnings to get the car to a safe parking spot before finally going into Turtle mode, which will only allow the car to move maybe five miles per hour long enough to get to a safe spot. How often does a Leaf have to go into Turtle mode to protect the batteries? It must be a very rare event because I have yet to find with Google searches anyone who went into turtle mode because of high battery temperatures. On the other hand, as mentioned earlier, my Leaf’s computer often limits the car’s ability to use regenerative braking, in which range is traded off for speed and acceleration.

After reading parts of the Ford Focus owner’s manual, its obvious that they also have a limited ability to vary performance to control temperature. It has a warning indicator message that reads “Severely Limited Performance Due to hot battery” which means ” vehicle performance is severely affected by hot battery temperatures. Drive with caution. Keep vehicle plugged in when not in use to maintain proper battery temperature.” But that sounds more like an emergency measure as opposed to a design feature.

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