Tyrone Siu/Reuters/File
A sign is painted on a parking space for electric cars inside a car park in Hong Kong. The 2014 Smart ForTwo is the smallest, shortest, and cheapest electric car sold in the US.

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.

Before the Chevy Volt (a plug-in hybrid) went on sale, Volt Chief Engineer Andrew Farah openly acknowledged that the extreme temperatures found in the Southwest have the potential to permanently reduce the battery pack’s capacity to store energy:

“The Volt may not be right for everyone. If you live in the Southwest, depending on how you use your car, the Volt might not be right for you.”

So what is a manufacturer to do if a given customer’s driving habits consistently exposes his or her battery pack to excessively high temperatures in a place like Tucson, or charges it five times a day, or maybe applies a blowtorch to it? 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.

As the chief engineer for the Volt warned above, Nissan’s owner’s manual also warned that excessive exposure to ambient temperatures above 120 degrees F would degrade battery capacity. However, when a handful of Leaf owners in the Southwest realized that their batteries were losing capacity faster than batteries not repeatedly exposed to temperatures over 120 degrees F, they filed a class action lawsuit. 

How Nissan responded:

 “So today, we are announcing that we are enhancing the warranty coverage of the battery system that powers the Nissan LEAF electric vehicle. With this action, Nissan becomes the first and only manufacturer in the automotive industry to provide limited warranty coverage for battery capacity loss for electric vehicles.

Under an expanded New Electric Vehicle Limited Warranty, Nissan will protect against capacity loss in LEAF batteries that fall below nine bars, of the available 12 bars displayed on the vehicle’s battery capacity gauge, for the first five years or 60,000 miles in the United States, whichever comes first. For LEAF vehicles whose batteries have fallen below nine bars during this period, Nissan will repair or replace the battery under warranty with a new or remanufactured battery to restore capacity at or above a minimum of nine bars.

A vehicle whose battery has nine remaining bars indicated on the gauge is retaining approximately 70 percent of its original battery capacity. This new limited warranty coverage remains subject to the other terms, conditions and exclusions of the Nissan New Electric Vehicle Limited Warranty, which otherwise remain unchanged.”

 How does the new Leaf warranty compare to the Volt warranty?

Nissan claimed that they were “..the first and only manufacturer in the automotive industry to provide limited warranty coverage for battery capacity loss for electric vehicles.” It’s possible the Volt was left off that list because it’s a plug-in hybrid as opposed to being a fully electric vehicle, or they left it off because it isn’t clear what the Volt battery warranty covers. At first glance, the Volt warranty also appears to guarantee 70 percent of battery capacity at the end of the warranty period except it makes that claim in the section of the warranty for component defects related to materials or workmanship:

This Voltec warranty covers repairs to correct any Voltec component defect related to materials or workmanship occurring during the 8 year or 100,000 miles (160 000 kilometers) term for the following:

The list of components following the statement “component defect related to materials or workmanship”includes: High Voltage Wiring, Voltec Control Modules, E-Compressor, Traction Power Inverter Module, Accessory Power Module, On-Board Charger …and, uh oh, the Propulsion Battery.

But all electric cars come with a warranty that covers component defects related to materials or workmanship. What those manufacturers didn’t cover (until Nissan’s announcement) is battery capacity loss if, as the Volt chief engineer said, ” …you live in the Southwest, depending on how you use your car… “

By placing the propulsion battery in that list of components covered for defects related to materials and workmanship, GM has muddled the answer to the question of whether or not a Volt battery degraded because ” …you live in the Southwest, depending on how you use your car… ”  is really covered for things like hot parking lots. Lawyers, where would we be without them?

The Volt warranty went into great detail about how they would fix or replace a battery …that is listed along with all of the other Voltec items covered for a defect related to materials or workmanship:

 “Like all batteries, the amount of energy that the high voltage “ propulsion ” battery can store will decrease with time and miles driven. Depending on us  e, the battery may degrade as little as 10% to as much as 30% of capacity over the warranty period. A dealer service technician will determine if the battery energy capacity (kWh storage) is within the proper limit, given the age and mileage of the vehicle. Typical tests can take up to 24 hours.


 If possible, components will be repaired or replaced, and the original battery will be returned to the vehicle.

 Replace (If Necessary)

 Under warranty, the high voltage battery will be replaced with either a new or factory reconditioned high voltage battery with an energy capacity (kWh storage) level at or above that of the original battery prior to the failure. Your Volt battery warranty replacement may not return your vehicle as an “ as new ” condition, but it will make your Volt fully operational appropriate to its age and mileage.”

The Volt and  Electric Ford Focus  have an active battery thermal management system that cools the batteries in the same way that most car engines stay cool; by pumping antifreeze over the hot batteries to a radiator that is in turn cooled by a fan, however, the coolant pump and fan work only when the car is running. Not that it would do any good to leave the Volt running in a Tucson parking lot with a 140 degree surface temperature as some Volt enthusiasts discovered when investigating this issue:

“Since outside air temperature was hotter than battery coolant temperature this actually resulted in heating the battery …”

As if that were not an expected result. As mentioned by the Volt chief engineer, all lithium battery powered cars share this vulnerability to things like very hot parking lots.

From the Ford Focus Owner’s Manual:

 Battery: The best battery performance and life comes when the high voltage battery is at a moderate temperature.

 Indicator message: “It’s Hot Outside–Plug Vehicle in When Not in Use” displays when the ambient temperature is hot.

 Action: Plug in the vehicle to cool the high voltage battery for optimum performance.

 Indicator message: Severely Limited Performance Due to hot battery.

 Action: Displays when 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.

 The A/C system is used to cool both the interior of the vehicle and the high voltage battery. In hot outside temperatures, interior cooling may be temporarily reduced if the high voltage battery requires cooling. Keep the vehicle plugged in (when possible) to allow automatic battery cooling in high temperature conditions.

 If you drive in extremely hot climates: Note: It may be necessary to have an authorized Ford dealer decrease the coolant concentration to 40%. Note: A coolant concentration of 40% will provide improved overheat protection. Decreased coolant concentrations below 40% will decrease the corrosion/freeze protection characteristics of the coolant and may cause damage.

Bottom line, if your electric Ford Focus (or Volt for that matter) sits too long too often in hot Southwest parking lots, or if one of the dozens of the coolant system’s rubber hoses, or connectors, valves, or water pump fails or even if you have the wrong mix of water and coolant in your radiator your battery could get toasted and not be covered by the warranty.

 From the Ford Focus Warranty:

 The Lithium-ion battery (EV battery) will experience gradual capacity loss with time and use (similar to all lithium-ion batteries), which is considered normal wear and tear. Loss of battery capacity due to or resulting from gradual capacity loss is NOT covered under the New Vehicle Limited Warranty. See your Owner’s Manual for important tips on how to maximize the life and capacity of the Lithium-ion battery.

 From the Tesla Warranty:

 The Battery, like all lithium-ion batteries, will experience gradual energy or power loss with time and use. Loss of Battery energy or power over time or due to or resulting from Battery usage, is NOT covered under this Battery Limited Warranty. See your owner documentation for important information on how to maximize the life and capacity of the Battery.

 Why was the Nissan Leaf the first to test the warranty waters?

  1. Somebody had to be the first.
  2. There are more Leafs out there than any other electric car (the Volt is a plug-in hybrid).
  3. The Leaf has a rather obvious indicator gauge that actually warns the owner that the battery can’t hold as much charge as it did when brand new (12 indication bars is full capacity, 9 bars is 75 percent). My Leaf is approaching 20,000 miles and the battery capacity gauge still shows all 12 bars. Why Nissan thought it was important to provide that information, I have no idea but it probably contributed to their being the first to deal with battery degradation not caused by workmanship or defects. To get that information on an electric Ford Focus requires a 24 hour test at the dealership.
  4. Some concerned Leaf owners found that there was nothing wrong with their battery after all. The gauge was simply miscalibrated.
  5. Another factor may have been that the Leaf owner’s manual actually quotes a not to exceed value of 120 degrees while most other manufacturers are more vague, never actually quoting a number. After reading their manuals, many owners in the Southwest were concerned by the fact that their car may have been exposed to temperatures higher than that.
  6. Range degradation would be much less noticeable with a plug-in hybrid where the engine simply takes over as is the case with the Volt.

 Active liquid-cooled thermal management verses passive air-cooled thermal management

Electric Ford Focus cooling system

The word “active” implies that the system uses moving parts like pumps, flowing liquids, fans, and valves. “Passive,” of course, implies no moving parts. Ultimately, both systems are air cooled but in the active system the heat has to be moved via a hot liquid to a radiator before the air can cool it, so the title could be shortened to “Active thermal management verses passive thermal management.”

The Leaf’s competitors have jumped on the bandwagon to make hay while its cooling system weathers the onslaught of various bloggers, conspiracy theorists,  and wiki authors by portraying that Rube Goldberg collection of parts in the video above that is their cooling system as if it is a good thing. If complexity is a good thing, the Volt, with its five radiators, wins, hands down.

A good place to start seeking evidence that the Leaf’s method of controlling battery temperatures  is inferior to a liquid thermal cooling system would be via battery warranties. But, as discussed above, until recently with the Nissan warranty, none of the manufacturers’ warranties cover owners for thermal damage quite simply because they can’t control everything consumers are wont to do, like leave a car in a Tucson parking lot.

Will the Leaf become the Prius of electric cars?

When the Prius first arrived its competitors claimed it was a money losing, unreliable, engineering farce that actually consumed more lifetime energy than a Hummer. We all know how that story ended.

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.

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.

Simply put, the Nissan engineers have fine tuned their software, battery limits, and car performance in such a way as to not need a heavy, complex and expensive, active liquid cooling system. Leaf drivers rarely notice the performance changes happening to keep the battery pack cool. I’ve never noticed my battery temperature gauge moving at all.  It is in some ways analogous to the fine tuned design of the Prius which remains unmatched for gas mileage performance after all these years. The consistent high ambient temps in the Southwest punched through the Leaf’s design envelope for a small number of drivers. They’ll get it under control, possibly with this new battery:

From Green Car Reports:

 Nissan is testing a revised lithium-ion cell chemistry for its Leaf electric car that the company says appears to be as durable in sustained extreme heat as its current battery is under normal conditions.

 If tests confirm that the new cells degrade at no more than the standard rate, even at high temperatures, the company hopes to have battery packs using those cells available next April.

 Those packs would become the replacement for any current Leaf batteries replaced under warranty for loss of capacity.

 Hayes said Nissan has thus far replaced only 22 batteries for capacity loss in extremely hot climates–less than 0.1 percent of the 31,200 Leafs sold in the U.S. through July.

I suspect that the Leaf may very well prove to be the Prius of the electric cars. The Leaf solution to controlling battery temperatures is actually technologically superior in just about every way to the primitive, ham-fisted system of hoses, pumps, fans, valves and radiators used in cars since the Model T … and in WW II era fighter planes I might add.

Every aircraft I have ever flown, or have ever been a passenger in, uses passive (requiring no moving components) air cooling for the engines (no fans, pumps, liquid coolant, hoses, or radiators), except one, a WW II era P-51 mustang fighter I once hitched a ride in. I had an old professor who had helped to design the system of shut off valves for that plane to prevent loss of coolant from bullet holes. Unlike the air cooled fighters and bombers, a single hole in the cooling system could bring a Mustang down.

So, obviously, if  aircraft use passive air cooling, it must not only be adequate for cooling, it must also have some advantages over active liquid cooling systems in some applications. For commercial aircraft the two overarching reasons are:

  1. Less weight which translates into greater efficiency (the compact four passenger Volt weighs over 400 pounds more than a mid-sized five passenger Leaf, although not entirely due to its cooling system).
  2. Greater reliability via less complexity (fewer components like hoses, connectors, pumps, valves, and radiators to fail). The Volt has five radiators.

For the Leaf, you can add to that list, less cost.

In my life I have experienced dozens of instances in my cars or in the cars of people I know, of failures in the coolant system. One failure like that with a battery that relies on a pumped coolant is likely to permanently damage it, or at least to cause the computers to shut it down.

Source: Testing the Electric Car Battery Warranty Waters

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