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Battery Box Design

68K views 249 replies 28 participants last post by  tomofreno 
#1 ·
I started out wanting this to be about battery heating....but after a little thought, I decided to throw out this title. With the recent discussion of temperature in the BMS "debate" and my impending need to re-stuff my EV full of lithium, I don't really want to pull them out again for more mods.

For a top performing battery, in all climates ( I realize yours may be different than mine) the battery box should control temperature...therefore perhaps heat and perhaps cool as needed (depending where u live)... both when connected to grid or when parked outside/at work/wherever (at least prior to driving).

Charging at temp is also of some concern since many bulk chargers don't have temp compensation.

For all these reasons, I've been visualizing different designs and have a whole bunch of random thoughts that need to get pulled into one (drawing perhaps) place.

Some of the hardware for heating.....
Farnam heating pads, soil cable, water bed heaters (I kind like this one), RV adhesive heating pads, temp controllers....etc etc.

Some of the ideas for cooling....
Forced air in/around/through batts... Sucking air out (opposite - this has some advantages to be discussed) Getting air movement under/through etc.

Other....
Winter vs summer changes to box if any.
Distribution of heat ie Aluminum sheet.
Best materials to use..
What to line box with top, sides, bottom.

Powering...
12VDC
Pack voltage DC
Grid 120AC and/or 240AC
Dual power....?

Yes, this was dumped to get you thinking... it all has to get organized somehow. I'm gonna throw out a caveat up front. Please be aware and thoughtful of others' situations and that what works in Sacramento, may not work as well in Winnipeg. There are lots of ways to skin a cat... hopefully we can gather a lot of what works and what doesn't and perhaps some new ideas all into one place since all my searching has not turned up a lot of info or consensus.

Thank you all in advance. :)

I'll throw out some of my ideas soon.
 
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#2 ·
A couple things that come to mind right away as a basic for heating/cooling is keeping all the batteries at a fairly even temperature. They could be in larger boxes, smaller boxes and mixed. For both heat and cooling.
I'm going to talk about LiFePo prismatics right now since that is my current need (selfish as it is :eek::p).
- I can see aluminum sheet under batts for a couple reasons. It will help to distribute the heat in larger boxes as well as when used with heating elements smaller than the battery area sitting on it. It has good thermal conductive properties and is relatively light.

- I can also see the opportunity to use the spacing created by the ribbed battery cases to allow air (heat/cool) to move through the pack/stack or whatever.

- assuming prior point has some value, a space is required between Al sheet and batteries that allows air movement. (have a few ideas here TBD). Space may reduce contact with Al sheet... but heat still rises off of the entire sheet... oh yes... forgot to mention, this sheet/space mentioned is on the bottom!:rolleyes:

- assuming prior two points have some value.... I would propose that cooling through ventilation (fan) would be best handled by drawing air out of enclosed bat box as opposed to forcing it in. I say this because I am envisioning the need to distribute forced air along a thin space at the bottom as opposed to venting this bottom space along the edges (and closing off in winter) and creating suction or exhausting possibly out the top. This would draw air up through all the passages between the battery cases.

That's enough for now... Thoughts? holes? flawed thinking?? :rolleyes::D
 
#3 ·
I'd like to see where this goes, I haven't designed my boxes yet (for Headways) but I will need some sort of heating. I grabbed a cheap waterbead heater off ebay, it's pretty sturdy, completely sealed slightly flexible I think it would work well when you are plugged into 120, and could probably be setup to run off of pack voltage as well. The one I got is 120V @ 1A, (120w) there are also 350w versions but that might be too much. I've already designed temperature sensors into my charging boards that will be in each battery box, so that might be able to cycle the heater as necessary.
 
#4 · (Edited)
Yes, I think water bed heaters are one of the better options. Couple things l like about them...as you mentioned flexible and waterproof and readily available as well as inexpensive. Most come with control also. One thing that may need tweaking is the temp range...as most are in the 70 - 100F range. Perhaps an additional thermal switch would do it. The other thing (for me) is that it would work good for my bigger boxes, but I have a couple of smaller ones that may not be suitable.
The other thought I had was to pick up one/some designed for Europe market and be able to run from 240VAC or my pack voltage of 210VDC or so.
Here is another interesting option...although, not quite as cheap.
http://www.kimhotstartheaters.com/category/565146

Ahhhh, look what I found... :) http://www.standbypowerservicecompany.com/battery_pads_wraps_specs.pdf

Apparently, the Silicone ones are not for use on batteries..... http://www.standbypowerservicecompany.com/silicone_hot_pads_specs.pdf

Here's another one... but it says not to use in an insulated battery box... http://www.padheaters.com/battery_heater.html

Another one... http://www.alpha.ca/web2/products/battery-accessories/battery-heater-pad.html

http://www.warehouseautoparts.com/Specialty_Line/Kat/kats_battery_pad_Blanket_heaters.htm

and another... http://alpha.ca/Documents/Brochure/Battery%20Heater%20Mats_10_A002.pdf . . and this one.... http://www.alpha.ca/web2/products/battery-accessories/battery-heater-mats.html
 
#8 ·
Hi canadian guys!..:D

Yes the cold wheater will be an issue.

You can see bellow a parts of my future headway battery pack.
I will use 3/8 lexan at the end, at base and for the upper. Both side will be aluminium sheet metal.

For hot temperature, I will think use two small 12v fans. I plan the holes, but i'm not sure to use it because overheat will maybe not a issue with a small car like a Smart.

Concerning cold winter, I think use Easy heat roof de-icing cable or Pipe Tracing Heating Cables. They are rated at 7w/feet and I think use 18' for 126w and less than 1A at 156v. They cost 30-40$.


 
#9 ·
Have you considered heating the air you are moving? It could be a recirculating system when heat is needed, with the air drawn out by a fan and heated then directed back in the other side. When cooling is needed open the loop drawing fresh air in and exhausting it (heater off naturally.)
 
#37 ·
This could be done with an external blower and dryer hose or small insulated AC flex duct. Some fine expanded metal under the batteries would facilitate good air flow but may need a small space between it and the box bottom to get enough air flow. Have a diverter duct valve of some sort you could open for warm weather.

Mr. Jack built some boxes out of carbon fiber. Wonder if he did a video on it? I'd have to have a pot of coffee though to get through it. His videos make me sleepy! But a cf box if sturdy enough would be a good weight saver and much better at heat retention than aluminum. And cheaper probably than aluminum. I think it would be easy to build them as well using card board for a form.
 
#11 ·
I’m still chewing on this: http://www.che.sc.edu/faculty/popov/drbnp/website/Publications_PDFs/Web26.pdf

I hate the authors by now pretty much. With their endless repeating and using sentences that never seem to end. And words they chose to confuse normal people on purpose.

I’ve found a few interesting things so far. But a lot is still not clear to me. Cell formation is one of the interesting things. But is not important in this topic. It’s like formatting a hard disk.

There is a lot about temperatures. The aging test are done at different temperatures: -10, 0, 25 and 50C. The absolute degradation of the cells is the worst at -10. But relatively it’s worse at 50C. The initial capacity at -10 is from the beginning much lower than at 50C.

I haven't read yet what the capacity of these cells that where aged at -10 is at 25C after the >5000 cycles. That's very important. But I’m slow.

My gut feeling tells me: high temperatures (but below 60C) is good for short term performance. You have higher capacity and more power available at that particular moment. But at the cost of earlier long term capacity and power losses.

You have to choose between these two firstly before designing a battery box. Do you want maximum performance right now? Or do you want your cells to last as long as possible?

Also take in consideration that the cells are heaters by themselves. If the insulation is sufficient you don’t need extra heating. Only the first few miles might be a bit under powered.
 
#13 ·

My gut feeling tells me: high temperatures (but below 60C) is good for short term performance. You have higher capacity and more power available at that particular moment. But at the cost of earlier long term capacity and power losses.
I agree that up to a point heat is good, as long as you don't get electrolyte break down and stay below that point I'm not sure what the mechanism is that would cause early losses.
 
#12 ·
I'm thinking of having some built using aluminum. This time I want boxes with a top too. I'm thinking of insulating bottom and sides with this stuff. Its' 5/32" thick and about R4 I think. Diamond plate is expensive I heard and after a while will be dingy anyway. I was wondering what else I should use. How about billet sheets?

Thermostatically controlled muffin fans with louvers in the top of the sides would help remove heat in summer and soil heaters could warm them in the winter.
 
#15 ·
I didn't think about the top as my old box is more of a rack with no top. But yea the top would be good too in winter. In "the video" they say to limit the temp to 55-60C I think. Calb said keep it below 60 which I think you saw that in the post I made with their reply on another thread.

I'm thinking of modifying the rack and setting these boxes on the modified rack frames.
 
#19 · (Edited)
The manual for the Thundersky LFP260ah batteries shows they only loose about 10-12% range at -25C (-13F) compared to 25C (77F).

They loose about 20% range at -45C (-49F). But that is COLD. :eek: :eek: (even for Canada)

The small reduction in range doesn't seem like much to worry about. :)

I am sizing the batts ~25% big so they will still meet the range requirement as they age (or when the temps drop).

LFP seem less affected by cold than lead.


--sorry, that's what happens when U use the back button to edit... :|
 
#22 ·
The manual for the Thundersky LFP260ah batteries shows they only loose about 10-12% range at -25C (-13F) compared to 25C (77F).

They loose about 20% range at -45C (-49F). But that is COLD. :eek: :eek: (even for Canada)

The small reduction in range doesn't seem like much to worry about. :)

I am sizing the batts ~25% big so they will still meet the range requirement as they age (or when the temps drop).

LFP seem less affected by cold than lead.
Either headways are affected very differently or the Thundersky data isn't quite right. (Or they include self heating which means the cell doesn't actually stay at any of the numbers quoted)
At 0degC it looks like headways are at about 95% Capacity
At -10C about 82.5%
At -20C about 60%
I would be willing to bet there isn't much capacity at -45C

This is based on a 0.3C discharge which doesn't produce enough heat to raise the cell above ambient temperature. Most vehicles are going to draw more than that so there will be some self heating.
 
#20 ·
Naturally, my thoughts are going to be out on the extreme edges, but maybe there's something in them that can benefit someone. :rolleyes: One thing that may help is the fact that I am trying to keep the cost of the cases down to allow for easier adoption of newer technologies. I'm planning on nano-tech LiPo cells, and want heating/cooling because of how hard I want to push the pack.

I'm currently leaning towards liquid climate control. I'm toying with the idea of using rectangular aluminum radiator tubing to build an egg crate-like divider structure that would flow liquid around the individual cell packs. The same system could take care of the controller and battery pack. With a small radiator in a ducted box and a heating element inside, it could cool or heat.
 
#23 ·
I checked the manual and it specifies that for the test the cell should be normally charged and then left for 12 hours in -25C before the discharge test of .3C.

I do not think they are counting on cell heating.

Does anyone have testing or experience to add about how much range is affected using LIFEpo4 in the cold?


-They do specify that during the AK-47 shooting test no flames should appear, but smoke is allowed. :D
 
#24 ·
Also consider that charging the cell when cold the resistance will be higher, so voltage would increase faster before the cell is actually full. So if you stop charging at similar voltage when the pack is cold you'll get less charge into the cells, and because of increased resistance you'll have greater voltage sag on discharge and pull greater amps for the same power output. At the very least you probably want warm cells when charging even if you don't have warm cells when discharging.
 
#25 · (Edited)
Wow, miss a day .. miss a lot. :) Thanks for all the feedback. Some good ideas in there. I hadn't thought much about recirc air heating and dump for cooling. hmmmm. I was thinking radiant for heat and then a quick mod of some kind to allow air in and suck out for cooling. Have to think about this. As for temp impact, I did spend some time looking at this and there are quite a few users of Lifepo who can attest to fairly significant losses in colder temps. There may be some benefit with LiFeYPo...but so far no data... just "seems better" from some folks. I know lead chemistry is best suited to around 75 F. As for lifepo...well, guess that is being discussed as to wants and wishes...peak performance vs pack life/charging/range etc.:)

My boxes are steel angle iron frame with exterior grade plywood, treated and painted black. I've been wanting to go away from the wood this time around, not that it won't likely outlast the truck...but just seems a bit out of place. I know a lot of folks have used the plastic "sign board" material...and I was thinking along this line but wanted something different. One thing I want is to have it black so it doesn't stand out when looking under the truck. I dont think paint is going to stick to the plastic sign boards very well. I found this "foam board" from "Elmers" (the glue ppl). It's black and has some fine foam inside and covered both sides with a plastic of some kind. It's crazy light and I bought a sheet for experiment. I took a couple of pics... not sure if they will be revealing, but I'll upload. Oh, just Google it...there are lots of pics...
I'm thinking of this for the sides. I may put something else inside for more insulation...or look at thicker board (what I have is 5mm).
 
#27 ·
For those who would add some type of electric element to heat... what are your thoughts on powering?
115VAC, 230VAC (or whatever your supply is?)
Pack voltage so you can heat when away from a plug?
Dual power so you can do either?
 
#29 ·
For those who would add some type of electric element to heat... what are your thoughts on powering?
I'm using Farnham pads under the cells, separated by aluminum sheet to diffuse the heat. They are all powered by 110/120v AC, plug in to the wall when parked. Idea is that draining during driving will keep them up to temp. But I also don't need to keep the vehicle parked remotely all day.
 
#28 · (Edited)
Here are a few pics of what I did the first time. . . Just for discussion/interest. Oh, and all my boxes are bolted in with SS hardware so they can be removed (thank god I did this!) lol and modified.
 

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#40 ·
We seriously need to look at carbon fiber containers. Check out this site. Look at the 13.5lb cantilever structure holding up 500lbs! This stuff is unbelievably strong AND light. Expensive to buy but not to make. Apparently a sheet or two of glass make for a good mold to make uniform flat sheets.

Here's a DIY article for sheet making. This stuff is neet! Check out this guy and what he's built! It needs further evaluation as I know NOTHING about this stuff until tonight.
 
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