# Battery Box Frame Support



## PeterH (Mar 20, 2009)

Hi there,

I'd like to bounce this design off of the community to make sure I'm not making any fatal errors. Take a look at the two photos below. They show a view of the frame I've made to support my battery boxes. The steel is 1/8th inch thick, 1 x 1 angle mild steel. 

The frame is positioned so that it projects down from the frame no more than 3 inches. Meaning I've lost only 3 inches of clearance. I can make up that clearance later if needed, by adding a 2 inch lift kit. This is on my Geo Tracker and there are loads of options for lifting the body for off road use... which I won't be doing with $8K worth of batteries hanging on the underside!!

Each box has a frame around the bottom edge. the entire frame is then bolted to the frame at 8 points. You can see the connection points along the front edge of the frame. The photos show only the parts tacked in place. I've since taken it down and properly welded the joints and painted the frame with POR15 to keep it from rusting.

The bolts are 5/16th inch stainless steel screwed into nutserts set into the frame. Feels like it will support the weight of 30 CALB 130 cells... but I'm not a structural engineer so I thought it might be a good idea to see what everyone else thought about my design. So... thoughts?

Thanks,
Pete


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## clonezero (Oct 16, 2009)

To prevent swing on the connection point extend the strap to the top of the frame and put 2 bolts in.


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## PeterH (Mar 20, 2009)

That's a good idea... I didn't think of lateral movement like that. However, I think I'll add another piece of steel at a 45 degree angle between the existing weld and the frame. Adding another threaded nutsert forward of the existing one, by about 3 or 4 inches will be much easier just due to the cramped space. I think I can add four new pieces like that without too much trouble.

Thanks,
Peter


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## DJBecker (Nov 3, 2010)

Remember that bolts don't holding things together. It's usually friction that does that. The bolts just create the clamping force to create the friction.

The rough surface (undercoating?) isn't going to create consistent clamping force. That will leave the fastener carrying a shear load, which will quickly fatigue it. Stainless steel is weaker than steel, and often more likely to crack.


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## PeterH (Mar 20, 2009)

Well... glad I asked about this!

How should I improve this so I'm not leaving a trail of batteries?

The options seem to be:

1. remove any coating on the frame rail so there is a flat friction surface

2. replace those bolts with bolts with a better shear strength

3. just weld the battery box frame right to the cleaned vehicle frame and forget the idea of being able to remove the frame in the future

Can anyone think of other ideas or think option 3 is a bad idea?

Thanks,
Peter


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## DJBecker (Nov 3, 2010)

Can you put a loop of flat steel over the frame rail and bolt the battery tray to the loop?


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## PeterH (Mar 20, 2009)

DJBecker said:


> Can you put a loop of flat steel over the frame rail and bolt the battery tray to the loop?


That is a good idea... I'll see if I can do that, tonight when I crawl under my build.

I'll update this thread when I have the final design complete, just so there is a useful conclusion to this.

Thanks,
Peter H.


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## rwaudio (May 22, 2008)

PeterH said:


> That is a good idea... I'll see if I can do that, tonight when I crawl under my build.
> 
> I'll update this thread when I have the final design complete, just so there is a useful conclusion to this.
> 
> ...


how about simply welding a bracket to the frame then use a bolt in a non sheer direction to attach it (assuming you want it removable)


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## dtbaker (Jan 5, 2008)

you REALLY don't want just one bolt taking the load in shear if it loosens. very minimum add another bolt... and BETTER if you can extend that strap up and over the frame rail into a U or at least a J at the top to take the load in case Bolt loosens.

or, commit to the mount, and weld that sucker in. You could always cut/grind it out if you change batteries in 8 or 10 years.


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## TomA (Mar 26, 2009)

Hmmm...

Bolts work best in shear. If you have the choice, put them just the way you have them. That is their failsafe mode for coming loose.

Second, having a good fit of your bracketry against the frame is important, but not the most important consideration.

I have two other concerns that I think are more troubling:

First, the nutserts. I know they are guaranteed and demo'd to hold fast and all that, but I don't like them on a steel frame because of corrosion and flex, both of which are serious problems here. You don't need them anyway. Just open the holes in your frame and through bolt your brackets. you can also make the frame holes so that you can weld in a tube to keep the frame from crushing when you tighten the through bolts.

Second, I would make at least half your eight box brackets out of large triangles of thicker steel, 3/16" should do it, and three times longer in the horizontal dimension as the vertical. This then provides a strong, gradual ramp from the bottom of the frame down to the bottom of the battery box. I would extend those brackets 3/8" lower than the bottom of the battery box rail. Now you'll have skid skegs that will hopefully transfer a bump load into the frame before whatever you don't have the clearance for hits your battery box and shears it off...

It also gives you a much wider mounting scheme into the frame, with the bolts probably 6" apart.

I know very few EVers build this way, but I'm always leary of these battery boxes that hang down in defiance of the inevitable, with a flimsy corner perpendicular to forward motion. Its easy to protect that with a ramp of steel that can save the day over a speed bump or negotiating a low shoulder.

Just my $.02...

TomA


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## DJBecker (Nov 3, 2010)

TomA said:


> Bolts work best in shear. If you have the choice, put them just the way you have them. That is their failsafe mode for coming loose.


That statement isn't quite correct.

Bolts have about 60% of their tensile strength in shear. So even if it's a one-time shock or pull, you would still want to pull on the bolt rather than apply shear force.

The real consideration is fatigue life, and you address that by putting the bolt in tension such that any change in force is a small percentage of the tension. For instance, a varying pulling force will change the clamping pressure but not change the bolt tension. Since the bolt barely flexes or stretches, there won't be a fatigue life issue.

If you have a shear connection, the bolt should clamp the two surfaces together and have mating surface friction carry the entire load. That way even with vibration, the bolts sees only the constant tension load.

If the bolt can't establish a continuous clamping force, perhaps because of undercoating or just not being tightened, it's going to fatigue either in shear or (intermittent) tension.


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## shoup (Feb 10, 2009)

TomA said:


> Hmmm...
> First, the nutserts. I know they are guaranteed and demo'd to hold fast and all that, but I don't like them on a steel frame because of corrosion and flex, both of which are serious problems here. You don't need them anyway. Just open the holes in your frame and through bolt your brackets. you can also make the frame holes so that you can weld in a tube to keep the frame from crushing when you tighten the through bolts.


I agree with TomA. Nutserts should go. Through bolts will be much lower risk. 

When this is combined with a U-plate welded around the frame rail you should have a very solid connection. The steel used in unibody frame construction typically is not that thick and you are now putting it in shear. This could cause it to tear/stretch over time if you do not weld in some kind of plate.


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## PeterH (Mar 20, 2009)

Hello everyone. Thanks for all the good feedback and ideas. I spent a good 2 hours working on this last night and accomplished zero... because I just wasn't happy with my options.

I built 4 brackets to go over the top of the frames but discovered that the two on the rear side of the battery box frame won't work because it is at that point that the frame starts to curve upwards for the rear wheels. The front side would work. I just haven't taken the next step yet.

At somepoint before this is all over, I will have a skid plate of some sort protecting this because I am also uncomfortable with all that money hanging out beneath the vehicle. Besides, I will be taking this out to my wood lot when it is time to cut firewood for next winter.

Back to the basic problem. 

I've considered just welding the support frame right to the vehicle frame. If I can't come up with a better idea, that is exactly what I'll do.

Currently considering two other ideas:

1. Welding a tab to the bottom of the vehicle frame rail that I can bolt to.

or (just thought of this one)

2. Drilling holes all the way through the vehicle frame, from the top to the bottom and using a long bolt (probably 3 on each frame rail) to basically hang more support for the entire battery box frame. In this case, I'd retain the nutserts as additional support, but add this new hanging support to the mix.

To support those vertical bolts, I'll have an additional piece of steel on the top of the vehicle frame as sort of a "super washer" to prevent any chance of pull-through.

On the bottom end of the bolt will be another piece of steel, welded to the battery box frame, that will run from side to side, below the boxes. That cross piece of stell will likely be 1/4 inch thick by maybe an inch wide, mild steel.

So my new question, with this in mind, what grade of bolts are best suited for this application? I would expect very close to zero shear action in this configuration. Just a butt load of tension on the bolts.

In addition to all of the above, I failed to mention that at some point, the actual battery boxes will also be attached to the floor of the cab with rivets to help seal up the gap between the boxes and the rest of the vehicle. Not that I'm expecting those rivets to hold the batteries in place, in the event of a major support problem.

That major skid plate concept will be added once I have this frame secured and will probably consume another inch or so of clearence. I'm thinking at some point, I'll be looking at a suspension lift kit to regain some of that lost clearence, but that will only be added as a last resort.

Have any further thoughts to add? They are always appreciated... my wife only seems to comment on the mess I'm making...

Thanks,
Pete


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## epyon (Mar 20, 2008)

I was gonna bolt down my trays too .But I was gonna use rubber lock washers an torque the 15 ARP head studs an bolts to 50 Ft Lbs . I'm making the car electric and I lost the reset to return the ARP studs .


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## Ace_bridger (Nov 22, 2011)

A good quality 5/16 bolt is rated as having an Ultimate Axial Holding Power of 1600 lbs. Whilst I don't agree with the word 'power' one bolt should hold 1600 lbs, maximum.

I would feel comfortable with a factor of safety of 2 so assume each bolt can hold 800 lbs in tension.

A rule of thumb might be that the shear strength is 60% of the tensile strength so assume one bolt can hold 800 x 0.6 = 480 lbs in shear.

I'm not sure how much that battery pack weighs or how many bolts you have holding it up but you could use the above to calculate how comfortable you feel .

My main concern would be, as has been suggested, that the pack will shift if you brake hard or come free in a crash. You need to stiffen everything up by adding extra bolts or members to your frame.

Ideally hang the pack in it's box from the chassis members, again, as has been suggested.

Welds in tension are liable to cracking under fatigue so I would try and keep your welds in compression and back them up with a good quality bolt. That is an expensive cargo and it would be a shame if you scrapped it in a smash!!

Cheers,

Ace


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## Woodsmith (Jun 5, 2008)

If I was fitting that battery box to my chassis I would weld in two full width cross members made of 2"x3"x1/8" to the underside of the chassis frame. They would have triangular gussets to brace them to the chassis frame to make then solid and rigid.

The battery box can then be bolted to the crossmember.

The exisiting set up seems to rely on the stiffness of the angle iron to prevent them from sagging downwards as they are cantilevered from the outside of the chassis.

No matter how strong your bolt fixings are the outer two boxes would bend the box frame angle.

Do the boxes need to be removable for filling or can you put the batteries in without removing them?


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## PeterH (Mar 20, 2009)

Hello everyone. Thanks again for all the feedback.

I've been working on this issue, on and off for the past week or so. I need to post some images of the current status once I finish up the steel work.

I had a pile of 1/2 inch steel u-bolts that had a flat "u" and two parallel legs that are about 4 inches apart and long enough to go thru the frame rail and almost reach the bottom of the rack. They look sort of like big staples. At the moment, I've installed two of these, one on each side, in the middle of the front-to-back length of the box support frame. I added a piece of 1/4 inch steel plate on top side of the frame as a sort of super washer, in an attempt to prevent any chance of things pulling through the frame rail.

Then I fabricated some additional supports to hold up the frame at the mid point, using those bolts that extended down. Used 1/4" steel plate for those items. These support brackets aren't welded to the box frame, they just hug it up against the vehicle frame. I'm guessing someone will recommend I weld them to the box frame.

That frame rail metal is seemingly very thin. I haven't put a gauge to it, but I'd bet it was no heavier than 16 gauge steel. I'll have to check for sure. I know the flooring sheet metal is only 20 gauge.

I'll be adding two more single 1/2 inch steel bolts like this at the leading and trailing edge of the support frame on both sides. At that time, they will also go through another piece of 2 inch wide by 1 inch tall U-channel steel (under everything) that will extend about 12 inches behind the box frame and at least that far forward of the box frame and be bent up to meet and then bolted to the vehicle frame rail again. They will act as additional support for the boxes and a sort of skid rail. That is a little difficult to describe with just words... the pictures will help once I have them posted.

The boxes don't have to be removed to load them up with the batteries. In fact, this past weekend I couldn't stand the suspense and I loaded all my batteries in the boxes to see just how much the suspension 'sat down'. Learned that with all 45 cells in place, the body sat only 3/4s of an inch lower. Of course, that is without my fat ass in the seat nor any passenger weight. But I do think that is a good sign... because I have plenty of options for lifting the suspension if I need to.

I could try to bolt the boxes to the frame but I think that might be a bad idea. They are 1/8th inch aluminum boxes that will be attached to the flooring sheet metal with revits eventually. I'm afraid any flex that might happen, would just rip through the box sides.

I'm still trying to come up with a good non-electrical heating arrangement to keep things warm enough to be functional through a cold Idaho night.

I'll post pictures after this next weekend's efforts.

Thanks,
Peter


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## PeterH (Mar 20, 2009)

I wanted to provide a bit of closure to this thread in case anyone might benefit from this discussion in the future.

It has taken me several weeks and several thousand dollars to re-engineer my battery box mounting rack. The nutserts are gone. They are a nice thing to have when you need them, but they weren't right for this application.

Much of the past 3 weeks was spent waiting for my new BendPak 4 post auto lift to arrive, which accounts for all that cost. I decided that there was just no way I was going to be able to do this job on my back on a creeper...

You can see here a wide shot of the finished rack. 










I've welded two 36 inch lengths of 2 inch wide x 1 inch tall x 1/8th inch think "U" channel to the original rack. Those U channels are attached securely by about 12 inches of weld on both sides of each pair of frames. That isn't easy to explain but take my word for it, the rest of the rig will be long gone before those rails come away from the rest of the frame. 










It only took me about 10 tries to get it all in place correctly but with that lift, that wasn't as bad as it sounds. Here you can see it in place, shown from below.










Not only is this more secure since it isn't just hanging by a few straps of metal, it is also closer to the frame. Meaning I only lost 2 inches of clearance instead of 3 or more inches that I would have lost in the original design. However, the boxes do stick into the cab a bit more but I can deal with that easily enough.










In this next shot, you can see the rack mounted to the rail beneath the driver's seat.










And of course, the same shot but beneath the passenger seat.










Both sides are bolted to the vehicle frame by 4 bolts that run all the way through the frame. I expect zero shifting with this method but of course, I'll be keeping an eye on the system as part of routine maintenance... got to have something to check if I'm not changing oil all the time. 

Now back to the next focus... batter temperature control!

Pete


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## Yukon_Shane (Jul 15, 2010)

looks really great Pete.

well done


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## shoup (Feb 10, 2009)

Looks excellent. Thanks for posting all the details. I wish I had a lift.


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## skooler (Mar 26, 2011)

Wow!

That's an inspiration!

I'll be stealing some ow your ideas for my conversion.

Just hope I can do as good a job as you have.

Thanks for the pics, all too often we get a thread like this with no conclusion!

Cheers,

Mike


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## PeterH (Mar 20, 2009)

Thanks everyone... I'm glad this might help someone else... I get a lot from this site so maybe this is a little 'payback'... 

I agree, it is a bit frustrating to follow a thread only to find out it just dies quietly just before we learn 'who did it'.


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