Battery heating for EV efficiency
 

Most hypermilers, ecomodders, and fuel-economy fans are familiar with using a block heater to improve fuel economy in a gasoline vehicle. However, drivers of electric vehicles should also know that warming your batteries in the winter can give greatly improved EV performance.

In general, my cheap-o electric car (running on used batteries) has had a range of about 20 miles in the summer, but could be as low at TEN miles in the winter!

While there is snow, increased rolling resistance, thicker transmission fluid, etc in the winter, the MAIN difference is battery temperature.

Many battery types just can't give out as much power when they are cold. A simple example of that is trying to start a gas car on a cold winter day. The starter (powered by the battery) just doesn't crank as fast and hard.

So, to keep our cold-climate EVs in top notch performance for the winter, here's a few things to try:

USE YOUR GARAGE:
If you have a garage, use it! Even though my garage is not attached to the house, isn't heated or insulated, it still keeps the wind, rain, and snow off the car. What little heat is in the car gets retained a bit better. If you have a heated garage, that's probably the ultimate way to get better winter EV performance. (Small EV's such as scooters or electric bicycles could simply be brought inside.)

OTHER SHELTER:
If you don't have a garage, at least try to park out of wind, perhaps next to a tree, which in real-world testing have been shown to act as heat storage and help prevent frost formation on the car's windows. (But watch out for pine sap!)

INSULATE YOUR BATTERIES:
Batteries will be much happier if they are wrapped in a cozy blanket. If batteries are exposed to the outside world (such as under the hood) heat can also be lost to wind. Any insulation should be water-resistant and non-conductive - using foil-faced foam is a bad idea - but using pink builder's foam works great.

WARM YOUR BATTERIES:
Find some way to get a little heat into your batteries. The best way is with something in direct contact with the batteries, either under them, or on the sides. It will need to have some sort of automatic temperature control, to prevent overheating.

Waterbed mattress heaters, electric blankets, and water-pipe freeze prevention tape all have automatic temperature controls, and can easily be repurposed to warm batteries. Make sure to not set batteries directly on heating elements. Many heaters can be easily wrecked that way.

IF YOU ARE HAPPY, YOUR BATTERIES ARE HAPPY:
While I don't mind wearing a heavy coat, hat, and gloves in the winter, it is nice to be just a bit cozier than that in my car. Last winter, I experimented with an oil-filled electric radiator space heater. I simply put it in the car (behind the passenger seat) and ran an extension cord out the window. That was plugged into a timer going to the wall. The heater came on automatically in the morning, and heated the inside of the car for about 45 minutes before I left for work. I would unplug the heater, and drive off. The heater would stay warm for about 10 minutes after that. (In my gas car, it takes 10 minutes for the engine to warm up in the winter!)
The unexpected side effect of warming the inside of the car, was that it also warmed the batteries! By trying to make myself more comfortable, I also improved the range of my battery pack!

CHARGING = HEAT:
Another trick is based on the fact that running electricity through the batteries (either discharging OR charging) warms the batteries.
Set the car's charger up on a timer so that the charge is just finishing up when you will next use the car. The batteries will be a little warmer than they would be if they simply sat charged all night. Also, opportunity charge any chance you get. Even short charges can increase your range more from the heat than from the electricity to the batteries.

I hope these tips help keep your car happy and healthy this winter! If you have any other winter electric car tips, please post them below!

Ben: this is a great list.

Another option for people with smaller EV's: bring the batteries inside. This would apply to the millions of owners of electric bicycles in the world (hey: electric bicycles are EV's too!). Many have packs that are designed to be easily removed, and even have handles. Charge indoors, store indoors until ready to use.

Noted and edited!

It seems like if you have any batteries inside the car then heating them would heat the inside of the car as well, 500 pounds of 60 F degree batteries sitting in the back hatch seems like a nice idea.
Unlike the rest of you, my batteries are all outside of the passenger compartment and the boxes are hard to insulate, but they do have silver mylar lining them, this looks to be factory original, I do however now have a brand new set of batteries in my car, now I just need to get a defrost heater and install it and the batteries in my Winter Beater EV.

Isn't it more about battery capacity than efficiency for cold LAs?

Ben,

Very interesting reading, thanks.

I see the idea of battery temp maintenance in the same light as grill
blocking and block heaters; a driver selectable FE/MPG aid to be used as
particular conditions warrant.

Don't have an EV, but my Prius' HV battery seems to have lowered initial
power output after an overnight cold soak. I wondered about the possible
benefits of pre-warming the battery in this thread at PRIUSchat:

HV Battery Pre-Heat... Anybody tried it? - PriusChat Forums

At any rate, I still feel that pre-warming the HV battery during/after an
overnight cold soak has merit. After an awful lot if 'net searching, I've
come to the conclusion that some sort of heating pad or element under the
battery is the best way to go. (On the Prius, this wouldn't directly heat the
battery as it is inside a metal battery box that is an integral part of the HV
isolation scheme and is part of the battery cooling ducting.) Something like
this:

ClearMirror : The original fog free bathroom mirror defogger

[EDIT: Closer reading of this product's installation guide indicates it is not
effective under 50 degF or when against an exterior wall, which I suspect
would include the floor of the car:

NewHome Bath and Mirror, Inc. and its affiliates will not warranty the labor
or material costs for installation, replacement or use of a ClearMirror®
product, mirror or other affected items when installed on the inside of
exterior walls or when room temperatures on either side of the
application wall is or becomes lower than 50 degrees Fahrenheit.]

Just how is it that the heater element gets wrecked?

Would you care to hazard a recommendation for a specific product?

Many electric heating elements are VERY thin wires embedded inside plastic or rubber. Setting something pointed or or sharp, or even just plain heavy with any kind of protrusion from it, can sever the heating wire and break the circuit.

Many batteries have some sort of ridges on the bottom that could dig into the heating material. Think about a bed or desk on carpeting. When you move the item, you can see right where it was, because of the "dent" in the carpet.
As long as the weight is spread out, it's ok, but if you want to run heaters UNDER the batteries, the heater should be recessed or in some other way protected.

I'm no expert of Hybrid Car batteries, but it seems worth experimenting with. My car uses lead-acid batteries, which respond poorly to cold temps. I am not as familiar with how NiMH and other battery types respond to cold.

Ben,
At what temperature do you start noticing a drop in range? I'm not as concerned obviously as someone in a more northern climate, but we DO get a few days of single digit weather in Oklahoma, and maybe some stretches in the 20's. Given my modest range requirements (~2 miles), should I simply not worry about it? Looking at this on a purely ecological level, the energy I might use to keep my batteries warm might be more than the energy lost from cold weather; what do you think?

Anything freezing or below is pretty noticeable.

I guess I would say that I notice a difference in the fall (now) when it gets into the 40's.

Last year summer, I would get 20 mile range in the summer, but only 10 in the winter with unheated batteries. My area usually has temperatures in the winter anywhere from 0 to 20 - below zero is really cold, and anything 20 degrees or more and sunny is rather nice.

If you have far more range available in your batteries than you actually need, I don't think battery warming is an issue at all, although it may effect your top speed.

Ah nuts! Does that mean I won't be able to go 25 mph any more?:p Good thing I have a timer, I'll set it to charge a couple hours before work when it starts getting cold. Charging has taken a couple hours at 12 amps. Which brings another question: does the amount of charge time change at all during the winter?