Another compression using springs question!

[DIY Senior]

Hi all,
I need a logic check on my compression calculations for my new battery builds. I am building 2 24v batteries with the new EVE MB30 306ah cells. These are going into a RV so I am limited by space constraints. I plan to use 1 enclosure with 2 parallel packs of 8 cells clamped with 1 compression plate using 9 rods with die springs. With 1 battery I would be looking for about 674 lbs of force spread across 6 springs or 112 lb load each but since I am trying to compress twice the area don’t I need twice the force? That would work out to a load of about 150lb per spring across 9 springs if evenly divided or do I need 112 lb on the outer springs and 224 lb in the inner 3? Which logic is correct or is the answer neither? My late night thinking could be being affected by lack of sleep and 1 too many beverages!

Comments and corrections appreciated! Thanks

 

[Bob B]

I'd go with 150 lbs per spring for 9 springs ..... but, I'm not a mechanical engineer.

 

[DIY Senior]

Thanks, that's the way I am leaning but hoping someone can give me the scientific proof. I was about to order the 150 lb springs then read a post where someone was doing a similar build and was doubling the middle springs but no formula or explanation.

 

[S Davis]

Thanks, that's the way I am leaning but hoping someone can give me the scientific proof. I was about to order the 150 lb springs then read a post where someone was doing a similar build and was doubling the middle springs but no formula or explanation.

Make sure the springs have enough travel to absorb the expansion of eight cells and not bottom out and apply too much pressure. I only have single cell stacks so less expansion shorter springs.

 

[hwy17]

How do you plan on spreading the compression? My pack is 2 wide and I was going to use 1/2" aluminum plate. But I ended up going no compression.

For a mobile application I would want compression, but just for the purposes of securing them together firmly. I would still use 1/2" plate but with just bolts no springs. Not that you asked about that part.

My laymen's interpretation is the same, double the force for double the surface area. But the risk and consequence of deflection in the end plate gets higher with that much force. If a lesser end plate bends it could crush the corners of the cells.

 

[DIY Senior]

@ S Davis... Yes I am researching springs (most likely 2 in plus) with enough pressure and working distance to reach my target load with a minimum of at least 1/8 in of remaining travel.

 

[DIY Senior]

@ hwy17... My plan at the moment is to use 3/4 in treated plywood and aluminum C channel for hopefully extra rigidity with less weight. I plan to run the threaded rod thru vinyl tubing so I can keep it close to the cells to keep horizontal leverage to a minimum on the edge of the cells.

 

[Luk88]

My laymen's interpretation is the same, double the force for double the surface area. But the risk and consequence of deflection in the end plate gets higher with that much force. If a lesser end plate bends it could crush the corners of the cells.

Yes, definitely double the force. One can think about this in terms of pressure. A definition of pressure is force times an area. Not for example force times area squared etc. So if you're doubling the area you need to double the force for the same pressure.

I can't say anything informed about the strength of wood to withstand force and not bend, but intuitively 3/4 ply seems OK? That Al C channel, it will depend on the cross section, but I suppose it might be more of a decoration rather than contribute to stiffness unless it is really big.

I initially liked the idea of using vinyl tubing, but then I thought, what about high temperature resistance? Pvc (vinyl?) is usually good up to 70C. What if one cell develops a defect and heats up a lot before venting? It is unlikely, but that's why we use FR4 between cells.

I have the same problem with one of my batteries. I used polyethylene sheet to insulate sides from steel channel (there is a small gap, but still) and although PE is good up to 90C it is a far cry from fr4.

 

[DIY Senior]

Good point with the vinyl. I am using FR4 material between the cells and did think of using it on the outside also but was concerned about movement from vibration etc. Maybe FR4 and tape? The vinyl has 1/16" walls so if it did melt at 1 point there should still be spacing due to it running full length. Rethinking though, looking at what else might be practical. Thanks

 

[DIY Senior]

@Luk88...You got me curious about the vinyl tubing (stated working 50psi at 70F) so I put a section in the oven at 250F for 15 minutes. No distortion and no apparent difference from normal, just warmer. I then upped the temp to 350F and put it back in the oven for 15 more minutes. This time it started to sag a little to a more oval shape but the walls were still consistent and tough to penetrate with a sharp object. Conclusion is that the BMS would probably shut the battery down before it melted the tubing and even if the tubing disappeared the battery would still have to distort 1/16" to contact the rod so unsure about a problem.

 

[DIY Senior]

Found an answer to tubing over the rods concern!

High-Temperature Abrasion-Resistant Soft Rubber Tubing for Food, Beverage, and Dairy ​

• Flexible
• Compatible with Barbed Tube Fittings
• Soft (Durometer 65A)
• Temperature Range: -60° to 450° F