# Aluminium battery box fabrication



## Yukon_Shane (Jul 15, 2010)

Hi folks,

Can anyone recommend a reference source or offer advice on the fabrication of an aluminium battery box? 

My new build will require a fairly large box (approx. 30"x30") and I'm not sure how best to determine material thickness and whether or not some sort of bracing will be required. 

Is there a standard aluminium thickness typically used? Do folks often incorporate some form of reinforcement? 

I apologize if this has been answered elsewhere, I searched the forum but couldn't find much on the subject.

Thanks


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## piotrsko (Dec 9, 2007)

Machinists handbook, but that will give you a headache. Aluminum construction tion is usually a skin with supporting members doing weight transfer. I use .060 for heavy stuff like batteries or walk on skins, .030 for fairings, covers, or doors. 

I find that my amateurish attempts to save weight are for naught, but surplus aluminum is a good deal as long as it is a weldable alloy.


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## Old.DSMer (May 18, 2012)

Will you be using all aluminum and TIG welding it? Or steel with aluminum skin?

If you will be fabricating and welding all aluminum, try use minimum 1/8" material. Much easier to work with. Internal framing or bead rolling will probably be required to prevent excessive deflection of the bottom. I used .080 with .060 frame and it was difficult to weld. Too thin. And too flexible. It was OK for me since I have external supports. But I would not feel comfortable if it was solely holding all the weight.

If steel framing with aluminum skin, I would still recommend 1/8 for the steel. Thinner for the aluminum (and most seem to use rivets).

Do you have any sketches/pictures of your idea?


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## Yukon_Shane (Jul 15, 2010)

Old.DSMer said:


> Will you be using all aluminum and TIG welding it? Or steel with aluminum skin?
> 
> If you will be fabricating and welding all aluminum, try use minimum 1/8" material. Much easier to work with. Internal framing or bead rolling will probably be required to prevent excessive deflection of the bottom. I used .080 with .060 frame and it was difficult to weld. Too thin. And too flexible. It was OK for me since I have external supports. But I would not feel comfortable if it was solely holding all the weight.
> 
> ...


I was hoping to just use straight aluminium but if I have to I could put in some steel reinforcements.

I have a roughy sketchup model but really it's just a relatively square box with 60 x 100ah prismatic cells (5 rows of 12), not much engineering.

So maybe 1/8" aluminium box resting on 4 steel straps below each of the battery rows.


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## Old.DSMer (May 18, 2012)

Sounds like a reasonable plan. You will probably notice some "pop canning" of the bottom sheet, so most likely external structure will be required. Are you insulating and heating as well?


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## Yukon_Shane (Jul 15, 2010)

Old.DSMer said:


> Sounds like a reasonable plan. You will probably notice some "pop canning" of the bottom sheet, so most likely external structure will be required. Are you insulating and heating as well?


Definitely insulating and heating. My previous build used 3" rigid foam and a low watt heater, I'm thinking something similar here but the insulation will be on the exterior so I'm thinking of trying to put a fiberglass layer over it just to protect it from the elements.

Perhaps I should consider something like spray on box liner instead...more research


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## EV-propulsion.com (Jun 1, 2009)

We use 1/8 6061-T3 for our aluminum boxes. Either weld a "hat" to the bottom for strength, or weld in dividers inside for strength. Here's a couple of pictures from a recent conversion using dividers for structural support.
There are brackets for support on all four sides, plus four legs down to the frame for support also.
Mike
EV-propulsion.com


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## Yukon_Shane (Jul 15, 2010)

Hi Mike,

Thanks for the response and pics. That's a great looking box and conversion.

Dividers for strength make a lot of sense. I'll definately incorporate these in my design. 

So maybe I could get away with 3 steel "straps" under the box with dividers inside to keep the box rigid.

I'll work up a new sketchup drawing.

Cheers


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## Yabert (Feb 7, 2010)

I also like the dividers inside the box to add strength, but if like me you can't use dividers inside the box, you can add strength by the outside.
You can see the four stainless steel strap and the four reinforcing channel under my 1/8'' aluminum battery box here:http://www.evalbum.com/4155


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## Yukon_Shane (Jul 15, 2010)

Yabert said:


> I also like the dividers inside the box to add strength, but if like me you can't use dividers inside the box, you can add strength by the outside.
> You can see the four stainless steel strap and the four reinforcing channel under my 1/8'' aluminum battery box here:http://www.evalbum.com/4155


Hey Yabert,
My box will be in about the same location so this is helpful. I can see the straps in the pictures but where are the four reinforcing channels?

What's the size if this box?

Any particular reason for using stainless steel? Is it stronger?

Thanks


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## dtbaker (Jan 5, 2008)

Yukon_Shane said:


> Hi folks,
> 
> Can anyone recommend a reference source or offer advice on the fabrication of an aluminium battery box?
> 
> Thanks



I've had great luck with 1/4" polypro plastic and heat welding.... its less expensive, tough, non-conductive, needs no paint to natural white color. I've also used black ABS and heat welded, and found it slightly softer and more flexy, but equally easy to work with.


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## Yabert (Feb 7, 2010)

Yukon_Shane said:


> I can see the straps in the pictures but where are the four reinforcing channels?


You can see the 4 channels who reinforcing aluminum under my box here: http://www.diyelectriccar.com/forums/showthread.php/smart-fortwo-ev-high-power-version-51472p8.html



> Any particular reason for using stainless steel? Is it stronger


Not stronger than steel... but weather resitant.


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## z_power (Dec 17, 2011)

dtbaker said:


> I've had great luck with 1/4" polypro plastic and heat welding.... its less expensive, tough, non-conductive, needs no paint to natural white color. I've also used black ABS and heat welded, and found it slightly softer and more flexy, but equally easy to work with.


I'm considering polypro box in next build... I'm curious if you can see any signs of plastic aging after few years? UV exposition and thermal cycling combined with vibrations are probably most stressing factors.


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## dougingraham (Jul 26, 2011)

z_power said:


> I'm considering polypro box in next build... I'm curious if you can see any signs of plastic aging after few years? UV exposition and thermal cycling combined with vibrations are probably most stressing factors.


I made my battery boxes out of fiberglass and epoxy resin. The cost was probably a little less than aluminum but it weighs less and is stronger. For example my rear battery box can hold 36 100 ah cells but weighs less than 10 lbs. I turned it upside down and could stand on it before I put it into the car. The weight of the batteries would be something over 270 lbs. Working with glass and resin is not for everyone but I enjoyed it

With almost all plastics you need to provide UV protection. You can paint it black which will tend to hide any imperfections or if it is clear you can use one of the automotive clear coats with UV protection.

Best Wishes!


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## PorscheFan5 (Mar 24, 2015)

Doug,

I was also thinking about using either fiberglass or AL. Could you post some pictures of your boxes? What did you put fiberglass on? Plywood? What thickness?

I have made similar battery boxes on my boat with fiberglass but that was just two batteries...


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## dougingraham (Jul 26, 2011)

PorscheFan5 said:


> Doug,
> 
> I was also thinking about using either fiberglass or AL. Could you post some pictures of your boxes? What did you put fiberglass on? Plywood? What thickness?
> 
> I have made similar battery boxes on my boat with fiberglass but that was just two batteries...


I knew I would need insulation so I built my boxes out of the 1/2" blue foam you can get at home improvement stores. I used the 8 ounce bidirectional and unidirectional variants of fiberglass and West Systems epoxy. There are photos on my build photo album found here. RX-7 conversion start of battery box testing. There are captions on almost all of the photos. The battery box photos are scattered throughout the album so keep looking. In retrospect I was in model airplane mode trying to get the lightest possible resin to glass ratio and on my tests I ended up with the fiberglass coming right off of the foam. There is a plastic layer that can be peeled off of the foam but you also want to rough up the surface to give the epoxy something to mechanically bond to. I used white gorilla glue to bond the foam planks to each other. If you are a wood worker then plywood would be a good choice if you don't need insulation. Plywood is a composite material that has been used for a long time and its characteristics are well known. A 1/4" thick box with a few layers of glass inside and out would be plenty strong and weather resistant.

One of the great things about using foam and fiberglass is that you don't need any expensive tools. A utility knife, a long straightedge ruler (aluminum yardstick) high quality scissors, and sanding blocks are it. For the epoxy you will want the metering pumps but those are inexpensive. You can get reusable mixing cups but I found that a supply of throw away cups was a better solution because you don't have to clean them up. I do use reusable mixing sticks but again if you have access to something disposable that can save time and mess.

The downside (and there are always downsides): Working with epoxy resin is messy. You can be really careful and you will still get resin on clothing and floors and tables. You want to keep the liquid materials off your skin. With long term contact one will eventually become allergic to the product so wear disposable gloves. The dust when you are sanding is not good for your lungs and itches like mad on your skin. Carbon fiber is worse in this regard. Long sleeve shirts and pants are a must. I prefer sanding outdoors with the wind at my back or side so I didn't have to wear a mask. You will most likely have a couple of hundred dollars into it when you are all done but it is easy to spend that kind of money on aluminum and specialized tools or shop time as well.

I am quite happy with my battery boxes and I would build them this way again. If there is anything not clear I will be happy to clarify.


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## dtbaker (Jan 5, 2008)

z_power said:


> I'm considering polypro box in next build... I'm curious if you can see any signs of plastic aging after few years? UV exposition and thermal cycling combined with vibrations are probably most stressing factors.



no problems w the 1/4" thick polypro.... its the same material as the battery cases on lead-acid batteries, which was why I picked it. You cannot glue it, you HAVE to heat-weld it. But its really easy to work with. Heat-welding is no harder than a caulk gun.


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## PorscheFan5 (Mar 24, 2015)

Hi Doug,

Could you post some more info or a link for the "1/2" blue foam" you mention? IN your case how was the bottom of the battery box held to the vertical sides of the box? The batteries are heavy and I want to understand what held it together and bore the weight of the batteries...

BTW, what kind and how many batteries did you use? v per battery, AH, and # of batteries? It seems you used Netgain Warp 9 (which is what I plan to use). The curb weight of my Boxster is around 2900 lbs stock...so was making a benchmark/comparison...

Did you use an emergency shutoff in addition to the Maintenance switch?


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## dougingraham (Jul 26, 2011)

PorscheFan5 said:


> Could you post some more info or a link for the "1/2" blue foam" you mention?


I am not sure this the exact product I used but here is a link: Blue foam insulation

It is a little more than 1/2" thick although they don't say that in the advertisement.



PorscheFan5 said:


> IN your case how was the bottom of the battery box held to the vertical sides of the box? The batteries are heavy and I want to understand what held it together and bore the weight of the batteries...


The sides are attached to the bottom with White Gorilla Glue. It is fast drying, doesn't smell bad, stronger than the foam, and easy to trim off the overflow when it foams. But the foam itself won't support the batteries. All the strength is in the fiberglass. The foam acts like the vertical in an I-Beam and its only purpose is to keep the fiberglass from buckling under compression. 8 oz (per sq yard) fiberglass has something like 350 lbs per linear inch strength. A bag made of a single layer can easily support all my batteries. Fiberglass of this weight needs about a 1/2" radius corner if you want to get it to lay flat (and you do). So both inside and outside corners are rounded appropriately. I glassed the outside of the box upside down on the floor with wax paper protecting the floor. If I remember correctly there is a layer of uni running the long direction of the box and then one the short direction. I wetted out extra cloth on the wax paper so as to have three to four inches of lip. Two layers of cloth in each direction. Verticals have two layers and bottom has four. I reinforced the vertical uprights with about 4 inch wide strips of glass. Once all this is cured turn it over and repeat on the inside. You can see some of this in the battery box photos spread throughout my photo album.



PorscheFan5 said:


> BTW, what kind and how many batteries did you use? v per battery, AH, and # of batteries? It seems you used Netgain Warp 9 (which is what I plan to use). The curb weight of my Boxster is around 2900 lbs stock...so was making a benchmark/comparison...


I bought 60 of the GBS 100 AH cells. I ended up fitting 52 of them in the car. There are 33 in the rear and 19 in the front. They weigh about 7.5 lbs each so 390 lbs in batteries but about another 30 lbs of battery boxes, straps and screws. Last summer I put the car on he EVTV scales at EVCCon and it came in at 2424 lbs after the conversion. If I was doing this today I would use 94 of the 60ah cells instead. Easier to find places to fit and you end up with a system that is essentially immune to voltage sag.



PorscheFan5 said:


> Did you use an emergency shutoff in addition to the Maintenance switch?


I did put a contactor in the negative battery lead as an emergency shutoff. I don't have a slap switch, but the key switch opens this contactor. I am going to put an inertia switch in that will open this. And when drag racing the car there is a wire loop on a connector on the rear of the car that can be pulled out to open the contactor. It probably isn't really necessary because the Soliton has contactors internally that it will open if it detects a problem.


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## PorscheFan5 (Mar 24, 2015)

Thanks Doug. With 52 batteries, assuming they were std 3.2V you had total 166.4 Volts and 100AH making it 16.6KWH system, correct? If you used 94 cells of 60AH it would make it a 300V and 18KWH system. Understood on the smaller size of the battery and easy to locate them but does your charger and motor support 300V?

Here is what I am thinking of right now...

Equipment: Netgain Warp 9 with LogiSystems Controller - 192V, 1000Amps.
Proposed Batteries: 55 of the 3.2V 200AH LiFePO4 making it 176V, 35.2KWH...and 680lbs of battery weight! Each cell is 5.6Kg, it seems!

What do you think of that? How much distance do you think this will take me at full charge? Would you suggest another combination?


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## dougingraham (Jul 26, 2011)

PorscheFan5 said:


> Thanks Doug. With 52 batteries, assuming they were std 3.2V you had total 166.4 Volts and 100AH making it 16.6KWH system, correct? If you used 94 cells of 60AH it would make it a 300V and 18KWH system. Understood on the smaller size of the battery and easy to locate them but does your charger and motor support 300V?


The motor can handle up to approx 190 volts if you limit the kilowatts you feed to it. The question is not if the motor can handle it it is if the controller can. The motor controller will limit the motor voltage to whatever you set it to. The advantage of higher voltage is that the battery never sees the high current. If I have the limits set on the motor to 150 volts and 1000 amps and I have a battery that is 300 volts the battery only ever sees 500 amps. And sag doesn't matter because the motor never sees the voltage go below 150. If you have a battery with a nominal voltage of 150.4 (47 cells) the battery will be seeing full motor current and any sag will cause the voltage to go below 150.



PorscheFan5 said:


> Here is what I am thinking of right now...
> 
> Equipment: Netgain Warp 9 with LogiSystems Controller - 192V, 1000Amps.
> Proposed Batteries: 55 of the 3.2V 200AH LiFePO4 making it 176V, 35.2KWH...and 680lbs of battery weight! Each cell is 5.6Kg, it seems!
> ...


I would look for a Soliton 1 or a Zilla HV 1k so you can go to higher voltage. Use 100 AH cells instead. As many as 96 even. This would give you a more granular pack of 30.7 kwh. Most likely this will be limited by available space to something less than 96 cells. Lets assume that you can fit all 680 lbs of batteries in the car and it gains 400 lbs over stock. This means the car will weigh about 3600 lbs? This means 360 wh/lb and with 35.2 kwh of batteries gives a range of just over 100 miles with usable of around 70 miles.

It really depends on what you want to do with the car. If all you want is a daily driver and you need to do 60 to 70 miles per day this might be ok. If you want to drag race the car then this is not a good setup. If you want to autocross it you would not want nearly so much weight. I figured out the range I needed and then pretty much doubled that. As it turns out this was not a bad way to go. It kept the weight of the car close to stock. Adding hundreds of lbs of batteries is somewhat counter productive. In a sports car it can turn a fun to drive car into a pig.


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## Duncan (Dec 8, 2008)

Hi nathan7884

Please respond with a sensible reply - you have 24 hours or I will assume that you are a bot


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## steveclunn (Sep 5, 2011)

we make our own..and sometimes have them shipped from Tulsa... make an email connection with our office and she will help you if she can.. Greenshedconversions.com


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