EV Touring in Cold Weather
Updated: Feb 21, 2019
We made a recent EV trip from Minnesota down to New Orleans, LA - in January. At this time, Minnesota offered up temperatures of -20F (-29C) whereas NOLA was sitting at a balmy 50F (10C). The 1,200 mile road southward was essentially flat and level. We wanted to evaluate the effect of temperature upon EV Touring. We made two observations, related the effect of temperature on both energy consumption and it's effect on EV battery charging.
Energy Consumption. While in Minnesota, replugged in to a residential 110V outlet for about 2 weeks. Before our departure, we prepared to leave and decided to check the tire pressure, topping them up to the required pressure of 45-psi. We made a mental note of the low temp of -20F and also a note to check them again as we warmed up on the way down. As we made our way south over one week, we stopped at EVHotels (hotels that have car charging facilities) along the way and measured the tire pressure as they gradually increased. As we approached NOLA, we touched 55-psi and the tire pressure warning light went off. The energy consumption improved by perhaps 5~6 Watt-hr/mile, as a function of tire pressure. Although, the road surface quality was also improving as we got out of the icy, salty conditions of the northern roads. We concluded that keeping the tires topped up was important. The effect of speed was far more influential. Dropping our average speed over 100 miles by 5-mph improved the energy consumption by around 10 Watt-hr/mile. But most EV drivers know this already.
Charging Strategies in the Cold. A cold car with a cold battery will not charge quickly. The act of charging itself will heat the battery, but not nearly enough to offset the cold temperatures. At about 5°C and below, Lithium Ions are less able to diffuse (drift) through the battery's internal membrane, and the internal resistance of the battery increases. At low temperatures, the charge current must be reduced to protect the battery from various effects such as plating of metallic lithium on the anode which weakens the battery.
We learned to charge more efficiently after our Colorado experience where we got into town late, decided to check into the hotel directly without re-charging the car. In the morning as we left, we went directly to the EV charger which was less than a mile away, and the car and battery was essentially frozen. This is a situation to be avoided. We were only able to add about 1-KW, if that, for a full 20 minutes before slowly the effect of Joule heating allowed for a faster charge rate. The Supercharger was behaving perfectly, but the car's battery was just too cold.
Ideally, if we were in an EVHotel, we would have arrived in the evening with a warm battery and left the car charging over night, thus being able to drive away in the morning with a full charge. However, if you can't find an EV Hotel, our strategy for cold weather evolved such that we try to stay at a hotel that is 20 miles earlier than the Supercharger, thus ensuring that the car had enough time to be able to charge up at a faster rate. We would then not only get an early start, but also use this time to grab our morning coffee and read the news. Of course, we made sure we planned to have enough battery capacity remaining to make the 20-mile journey in the morning, keeping in mind that the range also decreases with temperature.
Twenty miles is enough distance to warm up the battery so that the super charger can deal out 75-KW at least.
Always, drive 20 miles before charging when you battery is cold
In summary, If you can't find an EV Hotel, either top up with a Super Charger before you check in or, space your super chargers out so that you drive at least 20 miles in the morning, before charging.
Avoid charging your car when the battery is cold.
Of course, a world of EVHotels everywhere would solve alleviate all of these concerns, but the world is not perfect yet.