Nissan and Ford Respond to Chevy Volt Fire Issue

@Evil_Attorney (not verified), "Wait, I thought Nissan switched over to a thermal management system for 2012 Leafs." The LEAF pack now includes a resistance heater for very cold weather situations, it's not a liquid active thermal management system like the Volt, Tesla, and Focus EV use.

@Londo Bell,

We'll have to see how well this works in real world. A 300 watt heater isn't going to do much when you car has been sitting a whole day at work in sub freezing temperatures. Also since it only comes on at low temperatures (< 14 F) it will be interesting to see what the range would be reduced to before then.

@Londo Bell, "In short - use the battery power to warm up the battery, pretty much to compensate the loss of battery power (naturally) due to cold temp."

The purpose of the LEAF battery warmer is not to increase range in cold weather but to keep the battery from from freezing in extreme cold weather. The battery warmer can run while the car is plugged in and the LEAF manual recommends leaving the car plugged in when stored in temperatures below -4 degrees F so as to not deplete the battery. The battery heater won't work below 30% SOC.

A cold LEAF battery will have reduced capacity and reduced ability to accept power during charging, including regenerative braking. However, using the battery warms it up and it has considerable thermal mass so it takes awhile to cool off after use. Nevertheless, reduced range in cold weather is a fact of life, even for thermally managed battery packs, due to cold air (higher density, increased drag), cold tires (increased rolling resistance), and wet, snowy, or icy roads (reduced traction efficiency).

The big hit to cold weather range for the LEAF is the use of the heater, which is why the cold weather versions have heated seats, steering wheel, and mirrors. The idea is that one can pre-heat the car while plugged in and then not have to use the cabin heater much to stay comfortable. But reduced range in cold weather is a fact of life.

@dgp: You beat me to it! :-)

@Brian Schwerdt,

First, let me say that I am a former sailplane and airplane pilot and learning about air density is, not surprisingly, critical to pilots. Cold dry air is the most dense and warm humid air is the least dense. At the old airport in Denver a fully loaded 747 couldn't take off on hot summer days because the air was not dense enough to provide enough lift over the length of the runway/takeoff run.

With regard to cars, I became aware of air density when I moved to high altitude in Colorado and found that my gas mileage was much better than at sea level, due primarily to reduced air density.

That said, the numbers for the reduction in air density versus temperature can be found in many places with a search, for example:
http://en.wikipedia.org/wiki/Density_of_air
http://voices.yahoo.com/how-does-air-density-vary-temperature-5677324.html
http://www.engineeringtoolbox.com/air-properties-d_156.html

As you can see from those references, air density is inversely proportional to temperature. For example, at 35ºC and 1 atm the air density is 1.1455 kg/m³, at 20ºC the density is 1.2041 kg/m³, at 0ºC the density is 1.2920 kg/m³, at -5ºC the density is 1.3163 kg/m³, and so forth. Note that there is a linear relationship between density and temperature. The power required to overcome drag is also a linear function:
P[drag] = F[drag] v = ½ ρv³AC[drag]
where:
P = power
F = force
v = velocity
ρ = density
A = reference area
C[drag] = coefficient of drag

Note that power to overcome aerodynamic drag is proportional to the air density.

@ HotinPHX - It is this little thing called thermodynamics. Heat flows "downhill", which means from high temperatures to low temperatures. It is possible to push heat uphill, but to do that you have to expend energy. You cannot do it passively.

Unless your wife was just holding an ice cold drink or something, your wife's fingers are actually warmer than room temperature. The reason her fingertips feel colder than your face is that both you and presumably your wife :) are mammals and so constantly generate internal heat that you both dump into the enironment. But air is a poor conductor of heat. This is why liquid cooled systems can dump more heat energy over a given period of time, by the way. Your head has a big mass compared to its surface area compared with your wife's fingers which have a relatively large surface area compared to their mass. This means that your wife's fingers dump heat to the enironment faster than your head does. Since both you and your wife are constantly generating heat, being mammals, your head balances out at a higher temperature at steady state than your wife's fingertips do. When your wife's fingertips touch your head heat flows "downhill" from your head to her fingertips due to the temperature difference, or gradient, between the two. Since your wife's fingertips are warmer than the room it seems counter intuitive that her fingertips would feel colder than the air in the room. But it turns out that the thermal conductivity of her solid fingertips allows heat energy to flow much faster into her than the heat transfers into the air despite the lower temperature difference. The air has both a lower thermal conductivity and a lower themal mass. It thus has to rely on passive thermal convection to cause the air that is warmed by your head to lower in density as it becomes warm and drift upwards, making the way for more cool air to flow up and remove more heat energy from your head. But the higher themal mass and higher thermal conductivity of your wife's fingertips sucks the heat right in and thus feels cooler to the heat sensors in your head.

So it is just an illusion that your wife's fingertips are cooler than the environment.

And if your car is sitting in the garage nothing can cool it below room temperature without expending energy unless that thing is lower in temperature than the room, like a big slab of ice.

New light is cast on that question, now that the Chevy Volt’s liquid coolant has been identified as a possible cause of fires that might occur weeks after a severe accident. The response from Nissan—which doesn’t use liquid cooling in the LEAF—has been to distance itself from the issue. For Ford, which does utilize liquid thermal management in its upcoming Focus Electric, the response is to reassure the public about its safety.

Thanks,
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