Real-world range ramifications: heating and air conditioning

 In Inside FleetCarma

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Last month we released real-world data showing how the temperature affects the range of the Nissan Leaf and Chevrolet Volt. We saw ranges for each model slowly drop outside a sweet spot around 60-75 °F (15-24°C).

The most discussed result was the big difference between the average range and maximum range, showing that some drivers were getting up to 60% more range than the average.  This difference is due to a number of factors; some that a driver can control (ie. cabin heating), and some they can’t control (ie. extra aerodynamic losses due to denser air when cold).

Many readers asked for us to show the heater load across the temperature range. While we can’t isolate individual heaters, we can show you the total auxiliary loads. Auxiliary loads include:

Auxiliary load is something our loggers can measure and something we track closely.  In both our Electric Vehicles in Hot Weather and Electric Vehicles in Cold Weather webinars (both are free to watch), we found that one of the most important factors when looking at how sensitive an electric vehicle will be to temperature swings is the drivers preference for heating and air conditioning.   By measuring the average auxiliary power used throughout each trip, and comparing that with driver feedback the impact of heater and A/C use becomes clear.

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The Truth About Electric Vehicles in Hot Weather Webinar

The webinars go into detail and show how most drivers fit into one of three patterns when it comes to heating and A/C use.  In this post, we’ve included summary plots for both the Volt and the Leaf below.  Similar to the range plots, we again found a lot of variation.  We plotted the trips with the highest and lowest auxiliary power loads to demonstrate the impact of driver preference.  As you can see, some drivers have much higher auxiliary loads than others.

Chevrolet Volt Auxiliary Power Usage In Cold WeatherNissan Leaf Auxiliary Power Usage In Cold Weather

The data shows how the electric range of these vehicles drops as temperatures deviate from a comfortable 60-75 °F (15-24°C).  It’s no coincidence that this sweet spot coincides with the temperatures we often set our thermostats to. Maintaining a not-too-warm/not-too-cool temperature is definitely a comfort we have grown to expect while driving.  When we look at the average driver (in blue) we can see their auxiliary power load bottoms out in a similar sweet spot.

When we look at data like this, we see a similar pattern emerge time and time again, the bow-tie.  The bow-tie shows that with increased power used to heat and cool the cabin, combined with a drop in component efficiency, the available range takes a dive.  This bow-tie can be affected by factors like where the vehicle is stored, and driver behavior details we explain in the Hot Weather Webinar.

We also looked at at how each vehicle model uses auxiliary power.  What we can see from the graph below is that as temperatures change, on average the Volt uses more auxiliary power.  As with our range comparison plot, trips below 25 °F (-4 °C) are not shown for the Volt since the engine is intermittently turned on below that threshold.

Plot of how average auxiliary power used over trips in the Nissan Leaf and Chevrolet Volt  changes according to ambient temperature

Are Volt drivers heat-hungry people demanding a permanent state of toastiness? Do LEAF drivers instinctively love being a little bit too cold or little bit too hot? No.

Throughout the EV Champion Challenge we saw posts from both Volt and LEAF drivers alike showing their efforts to reduce the effect heating and air conditioning have on their power consumption.  With drivers in both camps donning layers upon layers of insulating clothes, I think we can lump them together as more chill tolerant than the average commuter. These were of course the Champions though, and the average vehicle owner (both of the plug-in and conventional flavor) are more likely to move the heater knob.

There is a technical difference between the Volt and Leaf when it comes to their heating systems.  Both cars have a cabin heater and a component (battery) heater, but the Volt has a slightly larger cabin heater (5kW) and a significantly larger battery heater (1800W compared to 300W).  So there is a technical reason that we are seeing steeper curves with the Volt.

But here’s our big question to readers: do you think the reason the Volt auxiliary load is steeper is a) because the heaters are larger (technical), or b) because Volt drivers are more likely to use more cabin heat since it simply means the engine will kick on sooner unlike the Leaf where there isn’t an engine back-up (psychological)?

Matt, the CEO of FleetCarma and a Volt owner, is guessing that psychology (b) is the bigger factor.   What do you think?

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  • John Tamplin

    Assuming the total heating required to achieve the desired temperature is the same (and the heaters can achieve the desired temperature), the size of the heater won’t have an impact on how much energy is needed to offset heat loss in the cabin – it will just take longer to get there, and if it is cold enough it won’t be able to stay there.

    The differences would have to be one or more of: more efficient heater (newer LEAFs have both heat pumps and resistive heaters), better insulation to prevent cabin heat loss, or different conditions (either external temperature, moisture content of the air, etc or user temperature settings).

    • zeroping

      I totally agree. A quick search got me a random forum post where someone calculated a COP of about 2.1 for their Leaf’s heat pump heater. That beats the 1.0 COP of the Volt’s electric-only heater. I understand that GM was probably wary of putting more complexity in a complex vehicle, and was relying on the gasoline engine for heat in extreme cold, but it explains a lot of the difference.