# Battery Box Work in Progress



## iti_uk (Oct 24, 2011)

Nice box, but...



swoozle said:


> One of the beauties of this design is that if I design the strut bar attachment right, the box can break away the fore/aft linkage and pivot from the bar in the event of a frontal or rear collision.


What would be the advantage of this? At best you would be introducing an unknown variable into a frontal crash, at worst (if I understand you correctly) you are allowing the battery pack to move toward the impactor and risk physical damage to the battery, along with the dangers that a punctured cell include.

The strut bar is not designed to hold something hanging from it. If you were to replace the strut bar with a properly triangulated piece designed to carry a load such as you propose, then fine, but please don't go designing pendulous crash structures especially if they include high voltage electrical connections.

Another small point to note is that frontal impact simulation and testing sees negative accelerations (very) easily of the order of 10g+. The strut bar in your first picture would snap like a twig in a crash were you to hang a several-kg battery box from it. When designing mounts like this, imagine that someone has thrown the weight of what you plan to support (motor/battery boxes, etc) at the speed you plan on travelling at you and you have to catch it (or, imagine you are standing on the side of the road and someone throws a battery box out of the window of a car passing by at 30mph (for example), would you be able to catch it? How would your mounting structure perform in those circumstances?). Think intuitively of how strong you have to be in order to catch it, then take a look at the structure you are designing and consider whether or not you would trust it to be as strong.

Of course nothing should be totally solid - too stiff a structure results in very high deceleration spikes during an impact. There should be deformation to absorb the crash energy, and this deformation should, as much as possible, be controlled. That bar will not just deform, I would bet it would snap as if it weren't there. I daresay that you would see deformation (drooping, sagging) even at a standstill were you to hang several kilos worth of battery boxes from it.

Chris


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## swoozle (Nov 13, 2011)

Wow. 

While I appreciate the general thrust of the comment re taking crash loads into account when making battery boxes....that's a little over the top.

I'd be surprised if any of the boxes on this site can take 10g crash loads without letting loose of their mounts. Nobody can design a box carrying 200 pounds (typical) to a 10 g requirement by intuition. There will always be asymmetric loads that will cause failure in unexpected ways unless you are a true designer with analysis and probably crash testing under your belt. 10G exceeds crash survivability requirements for modern aircraft interiors, for instance.

The advantage is something you yourself mention: reducing load spikes during a crash. I don't plan on making the bottom fore/aft mounts weak, but if they rip away first and allow some box movement, that reduces the load spike. And in a 10g crash situation, the LAST thing i care about is damage to the battery or a potential puncture. Keeping the box within the confines of the engine compartment (in this case) would be success as far as I'm concerned.

"...I daresay that you would see deformation (drooping, sagging) even at a standstill were you to hang several kilos worth of battery boxes from it..."

That exact strut bar is in use in the same type of design in Jon's car and isn't "drooping" or "sagging" with quite a few more batteries than I plan on hanging. Are you familiar with that bar? Do you know what it's made of? Apparently not.


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## iti_uk (Oct 24, 2011)

Sorry, I don't want to sound too negative, but if you're considering crashworthiness then it is wise to be realistic.

10g indeed high for aircraft interior requirements, but we're designing a car. Aircraft aren't designed to crash. Spikes of 13g are commonplace in 30mph head on accidents in cars.

To put some context behind what I am saying, I am a crash analysis engineer by trade and I heard you speak about wanting to have your battery box swinging from a strut bar during an accident. And that bar could be high strength boron steel tubing, but it is simply the wrong shape for that kind of loading. The fact that other people have done their conversions like this, or have made structures likely to break off during an impact is not necessarily something to aspire to.

I'm not trying to rain on your parade here, and I'm sorry if I'm coming across as an asshat, but I'm just trying to point out something I think could be dangerous for you. It's true that real-world crash testing, or even simulation would make the design process easier, but it's not that hard to get in the right ballpark on intuition.

Chris


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## swoozle (Nov 13, 2011)

iti_uk said:


> Sorry, I don't want to sound too negative, but if you're considering crashworthiness then it is wise to be realistic.
> ...
> I'm not trying to rain on your parade here, and I'm sorry if I'm coming across as an asshat, but I'm just trying to point out something I think could be dangerous for you. It's true that real-world crash testing, or even simulation would make the design process easier, but it's not that hard to get in the right ballpark on intuition.
> 
> Chris


OK, I'll accept the critique of the design I floated. That is, after all, why I floated it. (But seriously, "drooping" and "sagging"? That's just trolling. But that wasn't you.)

I dispute that you can get in the "ballpark" for a 10G load (or more) on a 200kg box of rocks by intuition. 
Since you are the expert, please do more than just dump and offer some constructive guidelines for battery box design and anchoring. Looking at the mounting schemes floating around, there's a lot more DIYers than just I that could benefit.

By the way, did you look at the "bar"? Did you notice that it's actually two "bars", one of which is tied to the firewall? Before you guys go off thinking I'm swinging a 200kg load from a 1.5" D thin wall aluminum conduit with a 1.5 meter span, please look at the details. As well as what is not defined yet. And offer constructive advice, not just crapping on it.


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## iti_uk (Oct 24, 2011)

Yeah, it was the look of the bar which made me make my original statement, unless you are planning to attach the box to both the cross-member and the ones which attach to the firewall, but given your statement about swinging the box forward during a crash, I assumed you wouldn't be doing this.

Like I said, it's not too hard to get in the right ball park. For example, the engine/motor mounts are designed to withstand such impacts. For example (forgetting about the deformability of the strut bar for a moment), take a look at the way the bar is mounted - the size of the bolts used particularly, compared to those of the motor/transmission mounts. Would you be happy mounting the motor using only the fasteners used for that strut bar? Keep in mind also that you are asking those fasteners not only to control the mass of the battery box, but also to continue doing their original job.

In your case, I would be tempted to make use of the motor mounts - maybe some reinforcement and/or larger or higher grade fasteners, and use that tripod to support the battery box rigidly. I certainly wouldn't design the system to allow the box to break away during an impact, given the nature of it's contents.

By the way, when I mentioned "drooping" and "sagging", I was talking about millimetres of deflection, not bending like a diving board ;-)

Just trying to be helpful. I would hope that if I was doing something you knew to be dangerous, you would step in and tell me that I was doing it wrongly. Sorry if I come across too harshly, my job has taught me to speak bluntly about design.

Chris


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## Guest (Apr 12, 2012)

Do you have some intuitive examples you can show the crowd that is watching this? I'd be interested if I could SEE some examples of what your talking about when putting in battery boxes in a converted vehicle. Mostly it is stuff batteries where you can and be done with it with no real thought about the worst case encounter with a brick wall.


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## Ziggythewiz (May 16, 2010)

Maybe we should all get our vehicles crash tested...or just not drive into brick walls.


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## steven4601 (Nov 11, 2010)

Interesting solution.
Towerstruts,, while they can support the (front) weight of the vehicle from the the side of the spring/damper, towerstruts usually are NOT structural! Don't try bolting something onto them.


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## swoozle (Nov 13, 2011)

steven4601 said:


> Interesting solution.
> Towerstruts,, while they can support the (front) weight of the vehicle from the the side of the spring/damper, towerstruts usually are NOT structural! Don't try bolting something onto them.



This statement demands a little more explanation, please. Strut towers take the full static weight of the car and react all of the vertical dynamic loads from the car to the wheels. I'm having trouble grasping how they could be more structural. 
The size and method of attaching a strut bar and whether it's up to additional battery loads, that part I get the importance of. The rest, not so much.


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## steven4601 (Nov 11, 2010)

varies a little from brand to brand, but some brands tower struts are thinner than the trunkfloor sheet metal. The strength comes from the fact metal is very strong when its pulled on. hence their shape... a tent like shape where the forces are concentrated in the center upward. bolting something foreign to a towerstrut is not free of risk.


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## iti_uk (Oct 24, 2011)

steven4601 said:


> varies a little from brand to brand, but some brands tower struts are thinner than the trunkfloor sheet metal. The strength comes from the fact metal is very strong when its pulled on. hence their shape... a tent like shape where the forces are concentrated in the center upward. bolting something foreign to a towerstrut is not free of risk.


Almost. Tubes are good in tension and in compression, compression being the more apparent mode for a strut bar. The ideal strut bar would be a straight line from point to point, not a tent shape. Any bends in the bar introduce bending moments to the equation, something that tubes are not very good at doing. This is why, for example, F1 car wishbones (tubes, essentially) are VERY strong in the modes that they are designed for, but you could snap one by standing on it.

As for shock towers not being structural, that is about as far from reality as it could get from a chassis design point of view. The structural members of a car MUST include all suspension mounting points, otherwise you would have a very wobbly and fragile car...!?! Bolting things to them is fine, as long as you use suitable hardware.

As for what I am proposing - I know that in conversions, especially with regards to batteries in EVs, battery placement is as much about "stuff it where it'll fit" as anything else, but to state that you are designing breakaway structures for you batteries rings alarm bells in my ears. I can't do drawings on my laptop - I'm useless with a trackpad, but try to imagine a large box above the motor in the first pic of the OP of this thread, supported by beams running from the motor mounts to the front two corners and mid-rear of the box. The box would also have to be made fairly strong, but this would at least be secure and un-breakaway-y.

Sorry, it's been a long day at work and my brain is fried.

Chris


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## Guest (Apr 12, 2012)

swoozle said:


> This statement demands a little more explanation, please. Strut towers take the full static weight of the car and react all of the vertical dynamic loads from the car to the wheels. I'm having trouble grasping how they could be more structural.
> The size and method of attaching a strut bar and whether it's up to additional battery loads, that part I get the importance of. The rest, not so much.


Shock tower cross bars are designed to reduce flex and are a bolt on item. It is not a built into the car item. As such I would leave the cross bars alone and let them do the job they were intended to do. I would not bolt up any battery boxes to the shock tower cross bars. Introduce another force upon the cross bar and you may have trouble.

Build mounts and connection points to the main structural components of the vehicle. 

In the event of a Wall Tackle all bets are off. Even with a stock ICE vehicle with no modifications.


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## swoozle (Nov 13, 2011)

OK, this is a great discussion and I'll certainly take into account the various opinions here, but I'll repeat that I do not intend (and never said) that I was designing breakaway battery box mounts. I don't think anyone can design to 10G crash loads by intuition and I certainly don't think someone (let alone I) can design a multi-point breakaway dynamic crash load mount system by feel and have any confidence that it will function even remotely as hoped.

As you said, in the end we're all stuffing batteries where they'll fit and making the box mounts as strong as we deem appropriate. No doubt not up to NHTSA/DOT/TLA standards but appropriate to the need.

I think I'll still keep some aspects of the strut bar mount idea. You all can testify at my civil negligence case. Whether I share anymore of the metamorphosis of the details (that will really determine whether it works or not as opposed to the "hanging a hippo from a soda straw" part) is another question....

In the end it amazes me how people can look at a crappy 2" x 3" scaled down photo, of a part(s) they've never actually seen before, with a lot of the important details missing and make dramatic and sweeping statements about drooping and sagging and breaking away and snapping and death and destruction and the degradation of the hallowed institution of marriage and loss of respect for your elders..... Help and guidance is one thing, chicken little is another. The message is as much about how you say it as what it is.


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## drgrieve (Apr 14, 2011)

10G is nothing. In Australia for my EV I need to build to restrain 20G frontal

2.3 Battery Restraint
The batteries that power the vehicle must be fixed in position so that they will not easily break
free in a crash and thus create a hazard to the driver, passengers or other road users. The
battery restraint system must adequately withstand at least the following crash accelerations:
Front impact – 20 g (i.e. 20 times the battery weight);
Side impact – 15 g;
Rear impact – 10 g; and
Vertical (rollover) impact – 10 g.
An impact sensing (G force) switch should be fitted to the vehicle so that the traction circuit is
opened in the event of an impact.


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## iti_uk (Oct 24, 2011)

swoozle said:


> OK, this is a great discussion and I'll certainly take into account the various opinions here, but I'll repeat that I do not intend (*and never said*) that I was designing breakaway battery box mounts.





swoozle said:


> One of the beauties of this design is that if I design the strut bar attachment right, the box can break away the fore/aft linkage and pivot from the bar in the event of a frontal or rear collision.


Just sayin' ;-)



swoozle said:


> You all can testify at my civil negligence case.


Lol I'm sure that won't be necessary! 




swoozle said:


> In the end it amazes me how people can look at a crappy 2" x 3" scaled down photo, of a part(s) they've never actually seen before, with a lot of the important details missing and make dramatic and sweeping statements about drooping and sagging and breaking away and snapping and death and destruction and the degradation of the hallowed institution of marriage and loss of respect for your elders..... Help and guidance is one thing, chicken little is another. The message is as much about how you say it as what it is.


The detail was in your words, not the photos (other than the strut bar - there's a pretty clear photo of that). When you know what to look for, you don't need studio quality photographic evidence. ;-)

drgrieve, I'm not surprised by those numbers, fairly standard for OEM crash testing. I think it's a little unfair that it's a requirement for your conversion where you are, given the lack of possible analysis. Still, in the right ball-park is always where you want to be. ;-)

Chris


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