# Using an EV in a real winter



## lowcrawler (Jun 27, 2011)

So here is the question...

It's routinely <0F here for weeks on end. And <-20F or <-30F isn't unheard of.

I don't know enough about heat transfer math to do this myself: What kind of load am I going to need to dump into keeping the batteries at 50+F using something like the flexi-watt system here: http://www.diyelectriccar.com/forum...heater-control-system-p334095.html#post334095
Note that I can't really put more than about 1" of styrofoam as insulation... and only a piece of plywood on the bottom.

Also, I should say, I notice a huge increase in voltage sag at 60.... but if I could keep them at 50, I'd be happy. My MiniBMS beeps at me (at 2.7) almost every non-babied acceleration and my top speed is like 50-55 if my batteries are under 40-50 degrees. (vs 80+mph in the summer). I've started putting a space heater in the car to keep them warm... but that's not very 'efficient'.

FWIW: it's a 1970 Bug with 48 (153v) 100ah CALBs.

At this point, I'm thinking I'm simply going to not to use it during the winter....

Do any of you actually use your car in a 'real' winter? Most people I see here talk of winter as mid-40's or so... and that's simply not my winter reality. (shoot, I might go 4+ months without seeing a 40 degree day)


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## gdirwin (Apr 7, 2009)

I drive my RAV4-EV conversion every day (for about 2 years now). I am in Winnipeg - we regularly get -40C or colder for a couple weeks every year - average temps in Jan are -23C (-10F).

I have heating pads underneath the batteries (plugged into a timer so they come on a few hours before I drive away in the am) and I plug it in at work. The pads are about 90W in each of 2 battery banks (front and back).

I also pre-heat the interior before I drive (simple AC interior warmer) which consumes way more power (800W?). I also installed 12V seat heaters. To keep the windows clear, I have 2 x 1500W ceramic heating elements (installed in the original heater core position) - these are enough to keep the windows clear/safe - they heat up quickly (no wait like in an ICE car warming up) but do not keep the car cozy warm like a fully warmed up ICE. A friend here drives his EV on the highway (from his country home to work in the city) and has 6000W of heat to keep the cabin warm!

Performance with the pre-warmed batteries is acceptable (lower than summer, but still okay) - I use 46x200AH TS cells with a 1000A Zilla in an AWD RAV4. Range is noticeably lower in winter (as much as 40% less), and there is more sag/reduced performance. I also tweak BMS and charger settings a bit in winter vs summer.

No noticeable problems so far!

Hope this helps!


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## mora (Nov 11, 2009)

I'll share my experiences too. I have 45x 90Ah TS/Winston cells, about 1 1/8 inch of insulation around batteries and no heating pads. Pack is split in two, one in front and other one behind backseat. I have my car parked outside all the time. I have experience from two winters, this is going to be third winter. Temperatures are below -10C for months and there is always a few week period when its -30C or less.

Voltage sag is bad if battery box temp is below zero celcius (~32F). Last weekend I didn't drive my EV at all and box temp went to -10C (14F). It was -22C outside all the time. Guess batteries were at -10C too. At 1C load cells dropped to 2.8V per cell. 2C was less than 2.5V per cell. BMS screamed but I didn't care. I knew cells were charged. Cells warmed pretty quickly by themselves and after 10km drive battery box temp was above zero and voltage was sagging way less. I get almost the same range than during summer. Only heater use will cut my range dramatically. I charge only when battery box temp is above zero, which usually means I have to charge right after parking. My boxes stay above zero for 24h if its -25C (-13F) outside.

I have 4x 1500W ceramic heater elements in place of original heater core. It puts out good heat no matter the outside temperature. 2x 1500W is not enough to warm the interior but it keeps windows clear. It will do if temps are close to zero. Below that 4x 1500W will give real heat. I have no seat warmers. I'm thinking of adding them for this winter.

I really should have heating pads installed inside battery boxes but I can manage without them. Maybe for next winter then, hehe. I'm happy as I don't have to worry about car not starting after a really cold night. Normal routine is to turn heater and blower fully on, wipe off excess snow and get back to car. Usually car is really warm inside when I'm done cleaning windows from outside.


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## tomofreno (Mar 3, 2009)

Don't think you will do much better than that response from gdirwin for relevancy. I'll add some experience in more mild temp's...lows here are typically in the 20's F, some weeks in the teens, few nights in the single digits. I have ten 35W heaters (Farnum) for each group of 4 to 5 cells, so 350W total, but they don't remain on all the time. A controller with tc feedback cycles them to maintain the 65 F set point, don't know actual energy consumption. Battery boxes have 1/2" thick insulation. The heaters kept them at set point temperature on two successive nights of -5 F (-20C). They will remain above 50 F for 4 -5 hours when the car is parked outside unheated when they were pre-heated to 65 F. I think having insulation on the bottom is critical since that is where the heaters are. You will loose a lot of heat through the plywood if the heaters are sitting directly on it, resulting in much higher required energy to keep the batteries warm over a given time. Also recommend mounting them to aluminum to spread the heat and protect the heaters from the cells scrubbing on them. I would use 1" thick insulation in your climate.


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## rochesterricer (Jan 5, 2011)

This page linked in the wiki has some good info:

http://www.econogics.com/ev/evcold.htm

I'm actually surprised to see some of you say that the two ceramic elements aren't enough, after reading this test by John Wayland:

http://www.evsource.com/articles/heater_compare.php

Based on what you guys are saying though, it looks like a 4 element heater is the hot ticket for a climate like mine.


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## tomofreno (Mar 3, 2009)

rochesterricer said:


> This page linked in the wiki has some good info:
> 
> http://www.econogics.com/ev/evcold.htm
> 
> ...


You shouldn't be, since he states that he tested in an ambient of 40 F. Just about any heater feels warm at that ambient. I don't even turn my heater on if its that warm! Takes a lot more to heat air at an ambient of 10 F, and much more still at -5 to -15 F. Plus he wasn't moving. Air infiltration cools the car faster when moving, especially at highway speeds. Faster you travel, the greater the heat you require. That's why gdirwin mentioned the guy who drove the highway to work had a 6kW heater.


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## lowcrawler (Jun 27, 2011)

Thank you all for the experiences. Sounds like my original plan (flexi-watt box bottom with aluminum pan, 3 1500W ceramic heaters, and 12V seat heaters) will actually probably do the job. Also sounds like the sag I experience at <50F isn't abnormal.

One last question -- if you sag your cells and the BMS screams because of it -- yet you know they are 'charged'... are you harming your cells by running them (albeit temporarily) below the 2.7V the BMS get's mad about?


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## dimitri (May 16, 2008)

lowcrawler said:


> One last question -- if you sag your cells and the BMS screams because of it -- yet you know they are 'charged'... are you harming your cells by running them (albeit temporarily) below the 2.7V the BMS get's mad about?


MiniBMS has temperature biased LVC, so LVC alerts are adjusted to lower voltage when the pack is cold. If your pack is around freezing temp, then LVC would come at 2.4V-2.5V , not 2.7V.

If you get LVC alerts due to deep sag when the pack is cold, then you are subjecting your cells to a higher C rate than normal. You can adjust the trimpot on the BMS head board to make a longer alarm delay, say 10-15 seconds. This should give you extra time for heavy accelerations before LVC alert comes.

There is no harm to cells from deeper sag under load during cold temperature periods.


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## lowcrawler (Jun 27, 2011)

Thanks for the info, and great product, Dimitri...

The more I learn about miniBMS, the more I like it! I first thought it was simply the cheapest solution -- as I learn the features it has, I like it even more.


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## dimitri (May 16, 2008)

lowcrawler said:


> I first thought it was simply the *cheapest* solution -- as I learn the features it has, I like it even more.


I prefer the term "cost effective"


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