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Discussion Starter · #1 ·
How thick does your battery box need to be?

I'm looking at the space I hope to fill and it would be great if I could get by with a box wall of 1" in thickness (and no more). However, I will need to do some sort of battery box heating in the winter and I was wondering if that leaves enough insulation to keep it reasonably warm.

My thoughts were 1/2" of a painted/sealed MDF as the structural part of the endcaps (which is where I'm 'tight') and put a layer of this on the inside and outside of the box in order to help keep my cells warm on the -20F nights.

the 'lid', bottom, front and back of the 'box' would be 1" MDF with the same (or more) insulation plan and something like one of these at the bottom of the box helping to keep it >-10F or so.

After writing the above, I did some research/calculations and with a box surface size of 1.9 sq meters, an outside temp of -20 and a desired inside temp of 0 and an R-value in the 1.5-3 range (SI units) I'd be losing about 10-20W from my box...

No problem!

Anyone see a problem in my logic/plan?
 

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well.... I would try to keep batteries no colder than 40 deg f overnight. They say you should NOT charge below 0 deg F, also you could use them, and they'd warm up a little.

You might want to add some heat at the bottom of the boxes with either the Farnum pads available, or a product from HeatLine.com which is custom cable that puts out self regulated 1-8 watts per foot.
 

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Discussion Starter · #3 ·
well.... I would try to keep batteries no colder than 40 deg f overnight.
Keeping my batteries over 40F all year would be extremely inefficient given the average high here is less than 40F for 5+ months of the year.

They say you should NOT charge below 0 deg F, also you could use them, and they'd warm up a little.
Specs from CALB seem to state charge over 0C and discharge over -20C and store over -20C. So given wiggle wrong and seat-of-pants talking, 0F seems about right. Meaning I'll need to keep them at that temp... and that seems considerably more reasonable than 40.

TS seems to test their cells down to -45C (and shows they have about 82% capacity at -45C), so perhaps they are a better cold-weather winner? Or perhaps the CALB cells are just great and simply are more conservative in their specs (as they are in their AH).

I'd charge right after using, so I'd assume internal temps would be above the charge low-temp limit and that charging would keep them there.

You might want to add some heat at the bottom of the boxes with either the Farnum pads available, or a product from HeatLine.com which is custom cable that puts out self regulated 1-8 watts per foot.
Great idea! I hadn't heard of heatline before... *scampers off to do some looking*
 

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I would look for material other than MDF for building the battery box. It has proven to be overly heavy and required special fasteners to maintain structural strength in my experience. It has a density of 48 lb/ft3. You might be able to use a thinner baltic birch plywood.
 

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some marine plywood might have a good R value, but requires paint/sealing. I am liking the boxes I made using 1/4" thick polypro with external steel 1x1 'edges'. Next time I might go aluminum to avoid prime/paint/undercoat of the steel. The poly is pretty easy to cut and heat-weld

If very cold in winter, I would glue a layer of 'space blanket' mylar first, then some rigid insulation, with heatline at the bottom.

for a look at the rear rack construction check several images around here: http://www.envirokarma.org/ev/gallery/110228_rearpack.htm

or the original build for lead earlier in my gallery
 

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I'm pretty new here, actually this is my first post! I've been in the planning stage of my EV project for about 2 months now and the subject of keeping my batteries from freezing is a major issue where I live (Winnipeg, MB Canada) as we see night time temps drop way below -30C in the winter.

My thought on insulation thus far was simply to use a couple of old sunbeam electric heating blankets and basically wrap them around the battery compartment. My thinking on this was that they only draw about 20 watts, and have built in thermostats and safety switches in the event they over heat or malfunction. Also their size was a big plus, I'm thinking one queen blanket could easily keep 10 batteries at a comfortable 10C or so overnight.
 

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Discussion Starter · #8 ·
Welcome! I'm pretty new too -- but with everyones help I think I'm starting to wrap my brain around what's needed. What a bunch of great people here!

Were you thinking of putting them IN the battery box, or around the outside of the battery box? I would think around the outside would involve lots of wasted energy, no?
 

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Welcome! I'm pretty new too -- but with everyones help I think I'm starting to wrap my brain around what's needed. What a bunch of great people here!

Were you thinking of putting them IN the battery box, or around the outside of the battery box? I would think around the outside would involve lots of wasted energy, no?

My plan involved putting the blanket inside the box, then installing the batteries on top of the blanket, I'm not an expert but I can't think of any reason this wouldn't work. Has anyone else ever tried something like what im proposing?
 

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First, I just want to say Thanks lowcrawler. I've been following your threads, and it's helped me a lot.

I live in the south... so, I'm speaking from ignorance! Don't you have like plug-ins everywhere up there for block warmers? You can plug in the heaters when the car is not on the road, and when driving I think the batteries can keep themselves warm. I think the Mylar and foam are great ideas for keeping the heat in as long as the box is sealed well. Some type of Thermostat on the heater would be good too..
 

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My plan involved putting the blanket inside the box, then installing the batteries on top of the blanket, I'm not an expert but I can't think of any reason this wouldn't work. Has anyone else ever tried something like what im proposing?
household electric blankets are intended to run on 120vAC.... are you planning to plug them into AC only when charging? I would NOT recommend wiring to your DC pack as the on/off switch would likely not last long and could arc and be a fire hazard. Similar for the typical 'ground warmer' cables used in greenhouses to bring dirt up to 74 degrees. People have had some bad/scary experiences using the thermostat/switches intended for AC wired to DC.

This issue of thermostat/switching is what led me to decide to try the heatline.com product wired direct to the traction pack. NO switches, no thermostat, and not too expensive at $3.50/ft. It looks to be highly customizable in length and easy to wire to several separate boxes.
 

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First, I just want to say Thanks lowcrawler. I've been following your threads, and it's helped me a lot.

I live in the south... so, I'm speaking from ignorance! Don't you have like plug-ins everywhere up there for block warmers? You can plug in the heaters when the car is not on the road, and when driving I think the batteries can keep themselves warm. I think the Mylar and foam are great ideas for keeping the heat in as long as the box is sealed well. Some type of Thermostat on the heater would be good too..
Actually yes, we do have block heater plugs everywhere but not in public parking lots, only private ones. I had figured that the heating blankets should at least have the option to run at all times, just in case I leave the car in a public lot for several hours. If it costs me a couple watts of power, I think the trade off is worth it, as letting the batteries freeze would probably lower the lifespan considerably.
 

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household electric blankets are intended to run on 120vAC.... are you planning to plug them into AC only when charging? I would NOT recommend wiring to your DC pack as the on/off switch would likely not last long and could arc and be a fire hazard. Similar for the typical 'ground warmer' cables used in greenhouses to bring dirt up to 74 degrees. People have had some bad/scary experiences using the thermostat/switches intended for AC wired to DC.

This issue of thermostat/switching is what led me to decide to try the heatline.com product wired direct to the traction pack. NO switches, no thermostat, and not too expensive at $3.50/ft. It looks to be highly customizable in length and easy to wire to several separate boxes.
You make a good point about the blankets being AC, and I suppose I had forgotten to mention that because of the low amp draw on the blankets the idea was to run the blankets off the cars 12v system with a low power inverter and use a battery monitor to shut down the blankets in the event the 12v battery got too low. I'll take a look at the heatline product tho, as it might be a better option, the whole heating blanket idea just came about due to simplicity and availability.
 
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