Some stories are timeless. In winter, there is a ton of media coverage about how cold weather temporarily reduces electric range. Whether or not your vehicle’s on-board range estimates reflect this range loss, it is an inevitable fact that lithium ion batteries lose range in the cold. 

Conventional wisdom also tells us that cold weather can lead to longer charge times. There has been some research into this effect, such as this discussion of how DC fast charge times increase in Nissan LEAFs when the weather is cold. 

The mechanism that slows charging in cold weather is actually a protective measure. The way a battery is charged is by using an electric current to push lithium ions from the cathode to the anode, where they enter into the anode structure in an orderly fashion. It’s a lot like how passengers enter an airplane and find their seats – there is a place for each person. 

When it’s cold, the chemical reactions that make batteries work are slowed, including the process by which ions enter the anode. So if you try to charge a cold battery at regular speed, the lithium ions will pile up outside of the anode, much like if the first airplane passengers can’t find their seat and take an extra long time. Unlike airline passengers, though, lithium ions don’t just wait their turn. They smash into each other and can form inert metallic compounds on the outside of the anode called lithium plating. This can cause long term degradation of the battery, since some of the ions get trapped in this metallic plating and can’t be used to power the battery anymore. The metal also builds resistance that impedes the flow of ions. 

In an EV, a complex computer system called a battery management system (BMS; #10 in the image above) helps control the charge speed so that in extreme temperatures, things like lithium plating don’t happen. The BMS will measure the battery’s temperature and restrict charging until it is warm enough to handle a normal current. This is why many EV manufacturers suggest keeping your car plugged in during the winter - the charger can supply energy to keep the battery warm so that you don’t have to wait to charge at full speed. Teslas are known to keep their batteries warm at all times, whether or not they are plugged in, but most other vehicles only kick in the battery heater when charging. 

Controlled Test: Cold Weather Charging with Onboard Devices

Recurrent was awarded funds from a prestigious National Science Foundation grant in order to conduct ground truth experiments to verify our range data with onboard devices. We sent Geotab OBD II devices to 50 Tesla Model 3 drivers and 50 Nissan LEAF drivers to get accurate, real time data on:

• State of charge
• energy from regenerative braking
• Energy used from the HV battery
• outside temperature
• Energy into the HV battery during charging

And more. 

We are still poring through the data to figure out what conclusions will be meaningful to the larger EV community, but preliminary exploration suggests that gross charging consumption per minute (inclusive of heat dissipation and transmission loss), may be up to 28% higher in warm weather than in cold weather. To get this value, we defined warm as above 50 F and cold as under 30 F. The exact breakdown of charging efficiency for warm vs. cold weather is as follows:

Tesla - percent change in charging efficiency warm vs. cold weather

LEAF -  percent change in charging efficiency warm vs. cold weather

It is still unclear why the LEAF is more efficient in cold weather for Level 1 and Level 3 charging, but it may have to do with early models’ thermal management systems. It is also possible that when uncertainty is factored in, the LEAF results will look different.

We are also still examining how much of this gross electricity makes it to the battery, and we will report back with more findings as we have them.

Uncontrolled Test: Cold Weather Charging with Citizen Scientists

Recurrent wanted to test the real world charge times that our drivers experience during the winter. We asked people to record some simple data to help us understand how cold weather affected charging. However, this citizen science pilot became more of a lesson learned in experimental design for us, as you’ll see below.

Temperature

Most of the findings that show decreased charging speed in cold weather take place around freezing: 32 F or 0 C. However, the average temperature for our citizen scientists when they took their data was 50 F - too warm for any pronounced effect. The range of temperatures was from 10 F to 69 F. 

Our first takeaway: ensure adequate representation in the temperature range we wanted to study! 

Vehicle Make and Model

We were pleasantly surprised to find many vehicle makes and models in our list. A few respondents entered multiple data points for their car, which was great data to have. However, we did not see a lot of temperature dependent variation in charge times - the Nissan LEAF reliably added 14 - 20 miles of range in an hour of level 2 charging, and the Tesla Model 3s on the list got between 35 and 53 miles per hour. Note that the 35 miles per hour added was for one vehicle; the repeat Model 3 was always between 44 and 53 miles added per hour. 

Here are some of the other models we saw, with the average mileage added per hour:‍

     Tesla: 35 - 53 miles added per hour

     Hyundai Kona EV - 23.5 miles added an hour

     Chevy Bolt LT - 23.4 miles added an hour

     Chrysler Pacifica - 21.2 miles added an hour‍

     LEAF: 14-20 miles added per hour

     Volt - 2.55 miles added per hour (110 volt)

Takeaway: it was great that some drivers reported data multiple times - we should encourage this to understand the distribution of data points! 

We had many other models that reported data, but since we did not make any data fields mandatory, we don’t have the same data for all the cars. For instance, many drivers plugged in their cars overnight, so did not enter in start or stop charge times, making it impossible to derive any rate information. 

Takeaway: be more intentional about what data we need to draw conclusions

In short, we learned not much at all about cold weather charging, but a lot about the best ways to ask our community for participation.

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