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?

Ben -

Solar heat. Park your car in the sun (like you want to in the winter, anyway) and make sure there is access for your batteries to either directly benefit from that heat, or at least get the residual heat from the passenger compartment.

Wouldn't the batteries degrade faster if they're too warm? I remember that unused batteries should preferably be refrigerated (but not frozen). Really low temperatures probably aren't too good for them either, though.

What about add a reversing valve to the A/C so it also works as a heater and add a control circuit so it can cool down to 90F or heat up to 40F as needed whenever the charger is plugged in? And maybe also heat or cool as needed on command by remote control.

Good question! I am not sure. I just plug in at night and it's charged in the morning.

Voltage tends to be lower with cold batteries. Voltage is the main thing that effects speed, so yes, you may have a lower speed in the winter, and maybe poorer acceleration as well.

Also, if your voltage is lower, you need more amperage to do the same work, so you pull more amps!

Good thread Bennelson.

You are thinking of alkaline primary batteries. Keeping batteries cool will keep them charged longer but they should be warmed up prior to use. Some batteries are more heat sensitive than others but any blanket warmer will probably not get floodies hot enough to damage them especially in the winter.

From: Deep Cycle Battery FAQ

From Battery University

This is the lead acid battery in the Prius which has a short life. I don't know why they went with a VRLA. I would be interested in seeing if it gets overheated in it's compartment.

Capacity VS temp...

http://www.windsun.com/pictures/Batt_temperature1.gif

In real life I do data center modifications and other fun,

The big wet and gel battery UPS systems need to be at 68 degrees to get full pull on the batteries we actually control the environment better for the UPS than the computers..

On a megawatt ups a couple degrees makes a huge difference on run time, its better to be just a little hot than cold..

Dave

PS: They cell adhesive heat packs for batteries. But you need to allow for letting them shed heat as well. I think a box with a thermostat and a small muffin fan would do the trick.

The following is for non-Prius drivers... :)

The only lead acid battery in the Prius is the little, 12V, motor cycle-sized
one that is the source for power for all electronic systems but the HV battery
and the electrically assisted friction brakes. The HV battery provides power
for the HSD (Hybris Synergy Drive, the heart of the Beast) and...

Via a DC to DC converter, keeps the 12V battery fully charged when the car
is in the "Ready" mode, that is ready to drive or actually being driven.

The 12V battery lives below the floor pieces in the hatch back area, at the
rear of the car, on the passenger side. It pretty much sees the same
ambient temps as the passenger cabin and the HV battery, which is right
behind/below the rear seat. In high temps such as after a hot-soak in a
uncovered parking lot in high summer, it endures those temps somewhat
longer than the HV battery. The HV battery is cooled by air drawn from the
passenger cabin and exhausted from the car behind one of the rear fenders.
The 12V battery is not so directly cooled, or heated in winter for that matter.

Typically, the 12V battery has a service life of 4 years. (However in both
very hot and cold conditions, say Arizona and Alaska, its life can be reduced
by half. Especially if it was not fully charged when it was delivered to the
original buyer... or is ever discharged due a light left on overnight, etc.) It's
most important function is to boot the computers when the car is started --
something like 7 computers and 14 ECUs (Electronic control units, very
simple, single function "computers").

If the 12V battery is dead, ain't no way the car can start, even though you've
got the HV battery sitting there just a few feet away.

The 12V battery does not have to provide starting amps. Once the various
computers are booted, the HV battery comes on line, provides both HV and
12V power, and MG1, (Motor Generator 1) is used as a starter for the ICE.

So, that's the 101 level course on the Prius' 12V battery. :rolleyes:

I got a chance to work on my improved battery warmer.

Basically, it's just a tray of hardboard with spacer strips of plywood over the top of it. The heat tape runs between the spacer strips, and a layer of aluminum goes over the top of the whole thing.

Plug it in and the heat tape warms up the aluminum, which spreads out the heat to the batteries sitting on top of it.

I'll let you know more when I actually get it in my battery box!

http://farm3.static.flickr.com/2736/...94c4b05460.jpg

Here's the video!

http://www.youtube.com/watch?v=Y_L1OhD2K0k

I also tossed this up onto INSTRUCTABLES as just the video.

EV Battery Warmer Part 1

Looks pretty good Ben.

http://www.youtube.com/watch?v=Lm5IATVw3WI

While I built a battery warmer for the Electro-Metro a while back, I never got it installed.

There was some sort of a problem that was setting off the GFI on my charging power outlet AND the GFI in the car.

I took the cover off the warmer, fixed the problem, and installed the warmer in the car.

Running the warmer for an hour or so seemed to bring the batteries up to "Not Cold" temperatures, which is all I am trying to do - just get the batteries somewhere between 50 and 70 degrees F.

I will still need to experiment to find out what the ideal run-times for the battery warmer are at various temperatures.

Since the battery box is INSIDE the car, any heat escaping from the battery box should help to heat the cabin (at least a tiny bit!)

orange4boy recently posted up on my similar thread at PRIUSchat.
Perhaps my reply has some value for this thread:

FWIW, here is my list of thinnish heating products most likely to work
either under the whole battery box, or between the box and the HV
battery itself -- some of them are pretty "out there." :o

Self Adhesive Engine, Oil, Block Pre-Heating Pad

Wolverine Engine Oil Heaters - Products

ClearMirror® : The original fog free bathroom mirror defogger - From $69

Electric Foot of the Bed Warmer - Stops Cold Feet!: CozyWinters

http://www.comfortheat.com/files/product_ffm.pdf

Thermocouples, Coil Heaters, Mica Heater Bands, Ceramic Heaters
The “FuzzyLogic” Logic Temperature Controllers warrant a look, but are
probably prohibitively expensive.

These two items are 12v, so would require a 120vAC to 12vDC transformer.
At any rate they are too neat not to pass along: ;)

OEM Products, 12 Volt Heating, Carbon Fiber Dual Temp Waterproof Single Pad Seat Heater Kit

This for blowing hot air through the back seat HV battery inlet vent:

Moose Racing UTV Cab Heater - - Motorcycle Superstore