Last month, parts of the US and Canada braced for record-breaking low temperatures as a blast of arctic air blew across North America. Here in Ottawa, the thermometer stayed below -20C for three days in a row, plunging to as low as -28C.
A temperature of -25C is an important threshold for the LEAF batteries because the chemical process that produces electricity will basically freeze at that temperature. To prevent that from happening, the battery pack is surrounded by thermal blankets and electrical heaters to keep it warm. According to the Owner’s Manual, the heaters kick in at -17C, heat up the batteries to -10C, and then turn themselves off until the battery temperature hits -17C again.
The heaters consume about 300 Watts when running, which is not a lot energy compared to what the motor uses (up to 80 kW). If the car is plugged in, the heating energy comes from the grid, but if the car is unplugged, the heaters will use electricity from the battery itself, creating yet another impact on total range if you park outside for long periods of time.
Another important point to keep in mind is that the heaters will only turn themselves on if the batteries are at least 30% charged. What happens if there’s not enough charge? Well, if the battery temperature drops below -25C, a safety mechanism will prevent the car from operating. You will either have to wait for the air temperature to rise again or plug the car in so that it can warm itself up using energy from the grid.
That’s all the theory but how often do the battery heaters run in practice, and how much energy do they use?
I ran a little experiment during the cold snap by leaving the car unplugged during the night and also during the day, and used the data logger from FleetCarma to verify how much energy was effectively used. I also measured the impact on my daily range, simulating the case where I would not able to trickle charge at work.
Parking overnight outside unplugged
With the weather forecast calling for very cold temperatures overnight, I charged the LEAF to 80% (10 bars) and left it parked outside for the night, to see how the battery heaters would work. I also wanted to measure the effect of driving to work in the morning without any pre-heating from the grid, basically the opposite of what I would normally do — a worst case scenario we all try to avoid. The result was quite interesting.
Early in the morning, with the temperature just a notch from -28C, I actually caught the LEAF in the act. From inside the house, I could see the bright blue charging status indicator lights blink in a specific pattern, showing that the battery warmer was busy doing its work.
CarWings indicated that the battery charge had dropped from 10 to 8 bars overnight. More precise data from Fleetcarma showed that the battery warmer ran for 2 hours and 42 minutes, bringing the battery charge from 79.6% down to 67.4% (a 12.2% loss).
Once inside the car, the dashboard confirmed the 8 bars of charge. It also showed a single bar of battery temperature. For some reason, the car thermometer showed -23C, almost 5 degrees warmer than the outside temperature. Notice the estimated range of 64 km. That is very optimistic.
It usually takes me 2 to 3 bars to get to work without any climate control, but at this temperature, with no pre-heating and with the cabin heater on, it took me 5 bars instead. As you can see on the left, I arrived at work with only 3 bars left and estimated range of 21 km. Fleetcarma data showed that the 37 min, 17.15 km trip reduced the battery charge from 67.3% to 39.8%, arriving at half of the original 80% charge I had in the morning. As for that optimistic range of 64 km, in reality it was more like 38 km (with an 80% charge).
With -25C on the dashboard thermometer, the remaining 21 km of range would be enough to take me back home. However, if I had to spend 8 to 9 hours at work with the car unplugged, the battery warmer would probably kick in during the day again causing the estimated range to drop. Fortunately, I have access to a 120 V outlet at work, which makes a big big difference in winter, so that wasn’t a concern.
So, parking outside unplugged overnight: definitely not a good idea!
Parking outside at work, unplugged
On a different day, I did the exact opposite: parked overnight plugged-in as usual inside the garage, and parked outside the whole day at work unplugged. The battery warmer did not kick in at any time during the night, even though my garage is not heated. Temperatures during the day were not as harsh as during the night, going from -27C at 8am to a high of -22.5C at 4pm.
I left to work with a full charge (88.7% SOC), drove the same 17 km, arrived with 67% left, or 77 km of estimated range. I parked outside as planned. The battery warmer kicked in 4 hours later and ran for 3 hours and 7 minutes, causing the battery charge to drop from 66.3% to 57.5%, an 8.8% loss.
Before I left work, I pre-heated the cabin remotely, while still unplugged, which took another 1.7% of the charge, down to 55.8% SOC. I arrived home with 37.7% charge, or 26 km of estimated range left.
Total range for the day (with a full charge, without charging at work): 34km driven + 26 km left = 60km.
Conclusion: Parking outside at work is not as bad as parking outside overnight, mainly because hours are shorter and temperatures are usually higher so impact on range is not as critical.
So how much energy does the battery warmer consume?
During the experiment, I was able to measure three battery warming events in total. While that is not enough data to extrapolate a pattern or a formula, this is the summary of what I was able to observe:
- For temperatures varying between -22C and -28C.
- Time it took for the battery warmer to kick in: 3.5h to 7.5h
- Warming time: 2.7h to 5.2h
- SOC% drop: 8.8% to 16.4%
- Power consumption: 287W to 421W