EV Winter Problems and Solutions

Driving an electric vehicle, you have to be aware of the weather conditions.  While I can comfortably drive the 40 mile round trip to work and back when it is warm outside, in the winter time, my batteries are not as cooperative. Cold batteries can reduce the normal range of an electric vehicle by 30% or more. I have gotten around this for the most part by insulating the batteries and insulating my garage (what a mess I made but it was worth it in the end).  

Even with well below freezing temperatures outside, the garage stays a comfortable 43 degrees F.  This also makes it extra pleasant when you have to go somewhere and the vehicles are already partially warmed up.

At the beginning of the week, after my truck has sat all weekend in the garage without any regular charging, the batteries are approaching 45 degrees. This doesn't cause too much problem on the way to work but after the truck has sat in the parking lot at work for 10 hours, the batteries have dropped even lower in temperature.  The drive home on the first work day of the week is an anxious one. I watch the volt meter very closely. 120, 115, 110, 105 volts. Once the under-load voltage drops to 100 Volts, I know I have less than 2 miles of range before it's game over. At that point, I have to pull over and let the batteries rest for 10-15 minutes before I can slowly drive the rest of the way home.

Every other day of the week isn't a problem because charging the batteries up the night before also heats them to 75 degrees or so. That is sufficient so my truck can sit in the cold, exposed parking lot all day and the batteries only drop to 58 degrees or so. I am still able to make it home without any problems.  But for me, that's not good enough. 

Battery Warmer:

I bought some ice melting cable, (the kind you string on your roof to prevent ice dams and roof damage) and wrapped it around my batteries. If my batteries are not warm enough, I can plug in the warming cable. This allows me to warm up the batteries overnight without having to over-charge them to create heat. Eventually, I want to connect this cable up to a thermostat and wire it to the battery pack itself. At the cost of a couple miles in range, the batteries will maintain temperature all day at work and still have sufficient range that I can make it home.

Truck Bed Cover: 

I originally wanted to build a super fancy, aerodynamic, teardrop shaped truck bed cover, but that would not be very practical.  I want one that won't get in the way of me swinging by Home Depot on a whim for a couple sheets of OSB.  I compromised for a quick and dirty flat cover.  Using the last scraps of plastic, (from the sheet used for the air dam), I hastily built a cover that could withstand an 80 mph head wind (freeway driving conditions).  

As luck would have it, I drove to work the next day during some of the worst winds that Davis County has seen in 20 years.  As I drove southbound through Centerville, through the high wind corridor, the 90-103 mph east wind gusts made quick work of my truck bed cover, tearing half of it off.  Watching electrical transformers light up the early morning sky while trying to avoid flying debris and rubberneckers looking at semi trucks that had flipped over on I-15, I somehow made it in to work.  

Here is my sort-of-repaired bed cover.  It does help reduce the wind drag quite a bit.  Wind is not partial to ugliness, only form. 

Reducing Cold Induced Friction:

I noticed in the winter time that on the way home from work, my current draw is higher than on the way in to work. I have dismissed it in the past assuming it is caused by a headwind or something.

I read that cold motor oil is thicker and will rob horsepower until the oil can heat up and reach its desired viscosity. 

Another EV driver in the Salt Lake area suggested to me that transmission/gear oil is the same way. In a normal engine, the transmission heats up as the engine heats up.  This is because the bell housing of a transmission is mechanically and thermal coupled to the engine. A hot engine will yield a hot transmission. Thick gear oil is specified so that when the engine warms up, the oil thins to its correct viscosity.

But in my truck, the electric motor doesn't get hot like in a gas engine.  Thick oil will remain thick.  I decided to replace the transmission and differential oil with thinner oil.  It is anyone's guess what the negative long term effects will be of using thin motor oil as gear oil in a cool running electric vehicle.  I am willing to give it a try. 

Initial Results:

I started out by only replacing the differential oil before I drove my electric truck to work the next morning. To my surprise, it reduced my 55mph current draw by 5 amps (from 100 amps to 95 amps).  For a large portion of my to-work commute, I was only pulling 75 amps at 55mph. I suspect I had a tailwind and some traffic corridor wind in my favor as well. But usually under these conditions the truck is pulling about 80 amps.

On my way home, I had so much battery capacity remaining, I began to question the gauges.  After a recharge overnight, I was surprised that it only took 12.3KWh to charge back up. I usually need 13KWH or more to drive 40 miles. Replacing the 75W90 differential oil with 0W30 synthetic reduced my energy use from 325KWh/mile to 308KWh/mile. That's a 5% reduction in energy use. Wow!  That is really surprising.  I didn't expect it would make that much difference. 

The thin oil in the rear differential made a huge improvement.  What about the transmission oil?
That evening, I changed out the 75W90 oil in the manual transmission case with some 5W30 that I had on hand. I didn't have any more 0W30 synthetic. The next day, I drove to work without any problems.  Keep in mind the weather inversion in the Salt Lake valley (ironically caused by engine exhaust) maintains a depressing cloud cover and keeps the air temperatures in the high 20's and low 30's F all day long.  On the way home, there was a 10-14mph headwind that caused my truck to pull about 115 Amps (at 55mph) from the batteries.  I was pleasantly surprised that I didn't have any range issues and made it all the way home comfortably. 

I recharged my batteries that night, accidentally leaving them on for an extra couple hours.  It only took 12.7KWh. Not bad for driving in a headwind half the time.  

These are some really preliminary results so I must collect several more days worth of data.