# Box design - Battery hold down



## Yukon_Shane (Jul 15, 2010)

Okay, it's battery box design time for me and I'm having a bit of trouble understanding exactly how folks keep their parasmatic LiFePO4 batteries from launching into orbit when they hit a pot hole or speed bump.

The 100 Ah TS cells I just received have next to no space on the battery terminal side to incorporate a hold down apparatus: the posts are very close to the edge so that won't work; and, the area between the two posts on each battery is where my BMS will be located (and not terrible large either).

I've now gone through about 20 pages in the Garage section of the forum looking for examples of how folks are designing these boxes without much luck. Occassionally I find an example that uses some sort of nylon strapping to hold the batteries down but for the most part I'm finding alot of pictures of battery packs with no visible hold downs on the top of the pack.

I'm sure I'm missing something here. Any examples of designs that folks would be willing to share would be greatly appreciated.


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## steven4601 (Nov 11, 2010)

Hi,
Do you have space above the cells? 

Nylon straps and a lid/cover can work wonders. I had the same problem but with a simple aluminium frame & nylon bands I can keep the cells from going air-borne. For prismatics you might not want to use aluminium on the inside of the lid. But PC or ABS might work for the inside profiles.

Here's what I did ( not finished yet, the top of the lid itself is missing)


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## Yukon_Shane (Jul 15, 2010)

steven4601 said:


> Hi,
> Do you have space above the cells?
> 
> Nylon straps and a lid/cover can work wonders. I had the same problem but with a simple aluminium frame & nylon bands I can keep the cells from going air-borne. For prismatics you might not want to use aluminium on the inside of the lid. But PC or ABS might work for the inside profiles.
> ...


I've got plenty of room above the cells so some sort of insullated "buffer" like what you've used would work really well. It looks like you've basically used an empty set of you're headway battery holders to seperate the lid from the cells and still run wires. Nice set-up.

The trick for me would be finding something that is: strong enough to hold, non-conductive, and won't get in the way of my BMS or connections.

I can use a non conductive lid for safety (which I likely will) but that won't protect the cells (and the BMS with it) from being driven up into the lid unless I have something between the lid and the cells (which avoids the posts and BMS) physically holding the cells down. 

maybe nylon strapping *is* the best solutions...


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## steven4601 (Nov 11, 2010)

Hi,

Polycarbonate is very tough, easy to work with. So is Nylon. Or what about PU? cast Poly-urethan pieces yourself! I think buying profiles of your material of choice would be easiest for rapid development. 

I know they are not in the picture, but ill use 10 pin Mini-fit Jr connectors to connect to the cells for the BMS. There's plenty of space. Wrapping the wires in High-temperature non/hard to tear fabric (fleece?) tape can protect the traction & bms wires & connectors. For Isolation between the three stacks I used FR4 sheets and Mylar on the inside of the box.


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## NabilAhmad (Feb 26, 2011)

Yukon_Shane said:


> Okay, it's battery box design time for me and I'm having a bit of trouble understanding exactly how folks keep their parasmatic LiFePO4 batteries from launching into orbit when they hit a pot hole or speed bump.
> 
> The 100 Ah TS cells I just received have next to no space on the battery terminal side to incorporate a hold down apparatus: the posts are very close to the edge so that won't work; and, the area between the two posts on each battery is where my BMS will be located (and not terrible large either).
> 
> ...


Shane, I am glad you are asking this question; though the question really goes further than just battery mechanical restraint. I have been preparing a reference for this purpose. I have been working at at sign fabrication shop for a decade and am very comfortable and familiar with building square aluminum boxes and wiring them. Our shop participates in UL's program so I am familiar with the big no-no's. The two greatest considerations I would like to offer anyone would be sealing against moisture and wire strain relief. 

To answer your question directly, the recommendation I propose is to have at least two inches of empty space in the box above the top of the cells. Install a non-conductive non-brittle block around the the perimeter of that empty space. Size the block as thick as possible, yet sufficient to clear the terminals and wiring/BMS. Use one block per side of the box using multiple fasteners. Place the fasteners centered between each cell and one at each end; e.g. - 5 cells wide x 1 cell deep would have 6 fasteners on each side with none directly adjacent to any terminal. Countersink the fasteners into the block such that no tool would be able to touch a terminal and fastener accidentally; a plug over the countersink hole would be ideal. If possible, it would be preferable to thread the blocks and insert the fasteners from outside the box so that there would be no exposed fasteners inside the box; and would prevent the possibility of a loose fastener rolling around inside. Overall, this system employs mechanical redundancy for each cell. Cover the whole box with an aluminum lid lined with an insulator and seal it with a gasket tape. To hold the lid on, what I've fallen in love with is a buckle, similar to what you'd see on a cargo trunk or tackle box, that has a degree of adjustability and is operable without tools and has no loose parts when disengaged. NEVER RELY ON THE LID ALONE TO HOLD THE CONTENTS OF THE BOX IN PLACE; though I would recommend that it can apply some pressure to the previously mentioned blocks. A recommendation I'm going to play with is using synthetic wood decking for the blocks.

NEVER COVER THE VENTS IN BETWEEN THE TERMINALS. Today I witnessed a headway cylindrical cell vent for several minutes and was quite glad I had seen videos where the vents were blocked. A blocked vent will change a controlled venting into a more catastrophic rupture event.

With regard to to wire constraint and drainage, put two holes into the bottom of the box between 3/16" and 3/8". And mechanically restrain any wire leads from movement within 6" of the terminals. I would recommend a nylon "P" clip if you have a lead going across the box. Use panel bushings with strain relief for all wires penetrating the box. Use something like this: http://www.heyco.com/Liquid_Tight_Cordgrips/product.cfm?product=Snap-In-2&section=Liquid_Tight_Cordgrips It cannot be understated how important it is to restrict movement on terminal connections. 

Additionally, you may wish to toss in some packets of moisture beads to help retard corrosion on the terminals.


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## Thumper (Nov 24, 2010)

Here's a couple of pics of how I did my battery box.

Here is the box before installing in the car. Notice the 1/2" lip on the centre section. The middle row of batteries are held down by the lip.









The front and rear rows are held down by 1-1/2x3/8 aluminum bars. I drilled the box and welded nuts on the backside for the bolts that hold the bars in place. I used a strip of rubber weatherstripping under the bars to hold the batteries down snug.








I was able to drill and tap holes in the tops of the bars to hold my controller heatsink. This design really cuts down on the lengths of cables I needed.

More info on my build at http://www.szott.com/zot2/zot2.html


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## Yukon_Shane (Jul 15, 2010)

NabilAhmad said:


> Shane, I am glad you are asking this question; though the question really goes further than just battery mechanical restraint. I have been preparing a reference for this purpose. I have been working at at sign fabrication shop for a decade and am very comfortable and familiar with building square aluminum boxes and wiring them. Our shop participates in UL's program so I am familiar with the big no-no's. The two greatest considerations I would like to offer anyone would be sealing against moisture and wire strain relief.
> 
> To answer your question directly, the recommendation I propose is to have at least two inches of empty space in the box above the top of the cells. Install a non-conductive non-brittle block around the the perimeter of that empty space. Size the block as thick as possible, yet sufficient to clear the terminals and wiring/BMS. Use one block per side of the box using multiple fasteners. Place the fasteners centered between each cell and one at each end; e.g. - 5 cells wide x 1 cell deep would have 6 fasteners on each side with none directly adjacent to any terminal. Countersink the fasteners into the block such that no tool would be able to touch a terminal and fastener accidentally; a plug over the countersink hole would be ideal. If possible, it would be preferable to thread the blocks and insert the fasteners from outside the box so that there would be no exposed fasteners inside the box; and would prevent the possibility of a loose fastener rolling around inside. Overall, this system employs mechanical redundancy for each cell. Cover the whole box with an aluminum lid lined with an insulator and seal it with a gasket tape. To hold the lid on, what I've fallen in love with is a buckle, similar to what you'd see on a cargo trunk or tackle box, that has a degree of adjustability and is operable without tools and has no loose parts when disengaged. NEVER RELY ON THE LID ALONE TO HOLD THE CONTENTS OF THE BOX IN PLACE; though I would recommend that it can apply some pressure to the previously mentioned blocks. A recommendation I'm going to play with is using synthetic wood decking for the blocks.
> 
> ...


NabilAhmad: thanks for your response; this is very valuable information (you should add some of this to the wiki). I hadn't thought about the implications of covering the vents between the terminals so thanks for that. 

I'll follow you're advice and build the box higher then I need and use some sort of non-conductive block around the edge of the pack to hold it down.

thanks again,

Shane


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## Yukon_Shane (Jul 15, 2010)

Thumper said:


> Here's a couple of pics of how I did my battery box.
> 
> Here is the box before installing in the car. Notice the 1/2" lip on the centre section. The middle row of batteries are held down by the lip.
> 
> ...


Nice looking box. I think this is very much the type of system I'd like to build. My 100 AH cells have such small space between the terminal and the edge that I don't think I'd be comfortable using a conductive metal like aluminum to hold down the pack but otherwise I think this would work really well for me. 

Thanks for the pictures, it's realy helpful to see how a box like this can be built.


Shane


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## Thumper (Nov 24, 2010)

Shane. 
My cells are TS 100Ah same as yours.
The cells can't move close enough to the aluminum bars to touch, you just need to be very carefull installing the bars.
When I installed mine, I put all the cells in the box, then temporarily taped over the terminals with duct tape (just in case). After the bars were installed, I taped over them too and then removed the duct tape from the cells and installed all the jumpers.


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## Yukon_Shane (Jul 15, 2010)

Thumper said:


> Shane.
> My cells are TS 100Ah same as yours.
> The cells can't move close enough to the aluminum bars to touch, you just need to be very carefull installing the bars.
> When I installed mine, I put all the cells in the box, then temporarily taped over the terminals with duct tape (just in case). After the bars were installed, I taped over them too and then removed the duct tape from the cells and installed all the jumpers.


It definately looks like you've got plenty of space between the holddown's and the terminal. I guess the trick is to just make sure that the cells can't move around much. 

thanks


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## Thumper (Nov 24, 2010)

Yea, the box is tight. I didn't want to use the end plates and straps that came with each group of 4 cells. It would have made the pack too wide to fit in the frame so I made the box tight. Somebody told me that taking the plates and straps off voids the warranty but I'm not sure if that is true.
Trouble is that if a cell swells, I'll have a hell of a job getting it out as they will all be jammed in there.
When I first put the cells in, each cell could wobble around a bit but not enough to touch the aluminum bars. Once I put the copper connector straps on the cells, it became one large unit and didn't move at all.


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## EVfun (Mar 14, 2010)

To hold my Thunder Sky cells in place I took the end plates off, then drilled and tapped 2 holes in the center, 1 between the lowest to threaded holes for the straps and the other between the top 2 holes. I then rebanded my cells into the groups I wanted (blocks of 8 mostly) and held each block down with the end plates using the holes I drilled and tapped.


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