Heat Pump vs. Resistive Heating
The first thing that can help explain range loss is the climate control system on-board the vehicle, also known as the heating, ventilation, and air conditioning (HVAC) system. Heating is simpler for internal combustion engine (ICE) vehicles, because they can take advantage of the waste heat from the engine. For an EV however, all of the heat must be created. For many vehicles, this is done with resistance heating, in which electricity is passed through an electrical resistive element, the element gets hot, and a blower or fan blows the hot air into the cabin. This is similar to baseboard heating in your home. It is an efficient way to make heat, because 100% of the electricity is converted to heat. However, there is a way to use even less electricity to reach the same temperature.
Some vehicles are starting to use heat pumps instead. You may have heard of heat pumps for your home, especially since they are gaining attention from incentives within the Inflation Reduction Act (IRA). An schematic showing how heat pumps work is shown below. The same concept applies for a heat pump in a vehicle. Instead of converting electric energy to heat directly, you power a compressor to transfer the heat from outside, to inside the vehicle. As a bonus, the heat pump can run in reverse to cool the vehicle, where the hot air in the vehicle is pumped out, similar to how your refrigerator keeps food cold.
It is expected that because heat pumps use less energy, that they will reduce the effects HVAC use has on range reduction. If we take a look at the data from the Recurrent study, we can see that verified data vehicles with heat pumps generally show an improvement in winter range.
Similarly, Michael Kim from EV Charger Reviews saw a difference between using resistance heating and heat pumps when comparing two Tesla models: a Model 3 with resistance heating and a Model Y with a heat pump. With both operating around 30°F, the Model 3 saw a 26% increase in energy consumption, while the Model Y only had a 8% increase.
On the other hand, a study by AAA did not see a considerable difference in energy usage between heat pump and resistance heating. However, this counterintuitive result could be due to the exact temperature during their experiment. Heat pumps lose efficiency once it gets really cold - 20°F and below. At this temperature, the efficiency savings are minimal. It’s possible the vehicles in the AAA study were tested at a colder temperature than in other studies, making the difference harder to distinguish.
Other Factors Leading to Range Reduction
The other factors that can lead to range reduction are unfortunately out of the driver’s control. Mostly, remaining range reduction is due to the chemistry of the battery at the colder temperature. When batteries are cold, the chemical reactions slow down, reducing the power available.
To understand how much of the range reduction is a result of the battery chemistry, let’s take a look again at data from the AAA study.
Data from AAA ELECTRIC VEHICLE RANGE TESTING
Let’s look at two of the vehicles from the study. The graphs show the vehicle range in mild temperatures, the range in cold temperatures with the HVAC off, and the range in cold temperatures with the HVAC on. We can look at the combined highway and city data to can get a sense of how much influence the HVAC use has on range.
For the BMW i3, the range decreased 20% in cold temperatures with the HVAC off, but then decreased 50% compared to mild temperatures when the HVAC was on.
In other words, 33 additional miles were lost from heating the vehicle.
For the Chevy Bolt, we see a similar result. With the HVAC off, the range decreased by 10% in cold temperatures, but then decreased 47% with the HVAC on.
In this case, 88 additional miles were lost to heat the vehicle.
It is difficult to discern the exact vehicle component that leads to the overall reduction, but it is at least clear that a significant portion is due to HVAC use. And luckily, that is within the driver’s control.
What can you do?
So, what exactly can you do? First, you can reduce the amount of work the HVAC needs to do while driving by preconditioning the vehicle. Preconditioning essentially allows you to heat your vehicle while it is plugged into the charger and before you leave for your trip. Next, review the HVAC system on your EV or potential EV. If you have a resistance heater or are driving in conditions below 20°F, consider putting on another layer before turning up the heat if you are unsure of the next charging opportunity.
Want to Know More?
The EV field is moving quickly, and the information available in the article is greatly improved through citizen science, or EV vehicle owners sharing vehicle use and performance data. To be a part of improving the field, join the Recurrent community.