EVE-280 cells should these be clamped tight or spaced for expansion?

[John Frum]

"Any compression is better than no compression. BUT NO compression is better than compressing it above 18 PSI."

I don't think both of those statements can be true.

 

[DThames]

Agreed it is about preventing bulging with charging. I'm not clear if the longevity issue is related to the cells themselves or to repeated flexing of the terminals due to the bulging. My plan is to use 1/4" 6061-T6 aluminum plate with threaded rod and a Lucite top, similar to what many have done including mainesail at marinehowto.com. Tighten lightly at low SOC, then expansion with charging will create its own pressure due to the rigid structure. Of course that is a variable and unknown amount of pressure, but it seems like a tried and proven solution.

As for the 300kgf force, I hope we can get some more information from EVE about that. If it really is critiical I will use some compression springs like these:
https://www.leespring.com/compression-springs-hefty?search=LHL625D

These are compression springs that apply 380lbs force per inch of travel. These ones are 1.5" extended and 1.02" full compressed, so you could get up to about 190lbs of compression per spring. One at each corner of a stiff plate could provide up to 760lbs to the battery bank. I did buy some LHL 625D 03 springs just in case, they are only $4 apiece. But the protruding springs interfere with my desired installation, so not sure if it's worth doing or just stick with a more rigid setup that everyone else has used. I'll decide after (if) I get any more info from Eve on their "fixture" and any recommendation on applying the 300kgf force they mention in their specs. For now, still waiting on my cells...

I just started reading this thread, so someone might have mentioned this already...."When your compression spring application requires a high load in a small space, Lee Spring’s Belleville Washers can be the solution." I work for an electric motor MFG and for certain customers that want the shaft to move a little along its long axis but only move under a given force (high volume air blowers/compressors) we stack these up, facing each other or spooning to get the tension we need.

 

[Deleted member 9967]

But its important to note these are pouch cells, so there are no cell walls to crush.
Point being 17 is not equal to 0 on the U curve, it is equal-ish to ~6 in terms of cycle life, which is substantially higher than 0 in terms of cycle life. The reason 18+ is said to be worse than 0 is because it is the upper limit due to the mechanical strength of the cell, not because it is the mirror image of 0 on the U curve. "Anything close to 17" is just as good as anything close to 6, so long as you don't surpass 17, which due to the consequences you should try very hard not to do and stay away from .

Ok, so as long as the outer ultra thin aluminum is not crushed then basically we are good to go. Got it.

 

[Colonel.lp]

"Any compression is better than no compression. BUT NO compression is better than compressing it above 18 PSI."

I don't think both of those statements can be true.

Any compression within the range 6 to 17 is better than no compression.

 

[ArthurEld]

Any compression within the range 6 to 17 is better than no compression.

I think there are a lot of fixtures with more then 17 psi compression. It is difficult to tighten a 1/2" nut against a piece of plywood to 6 - 17 psi even with a torque wrench. My threaded rod from Home Depot is dinged up. I have to put a wrench on the other end and I can't even tell which nut is turning. At these low torques little things make a difference like whether a washer is used.
I think it makes a difference if you are compressing one cell compared to 16 cells. It makes a difference if two bulged cells are placed facing each other compared to if they turned away or against a concave cell.
People want to add springs and honeycomb aluminum.
All of these things make a difference and there are many other variables that I forgot or didn't think of.

 

[Colonel.lp]

I think there are a lot of fixtures with more then 17 psi compression. It is difficult to tighten a 1/2" nut against a piece of plywood to 6 - 17 psi even with a torque wrench. My threaded rod from Home Depot is dinged up. I have to put a wrench on the other end and I can't even tell which nut is turning. At these low torques little things make a difference like whether a washer is used.
I think it makes a difference if you are compressing one cell compared to 16 cells. It makes a difference if two bulged cells are placed facing each other compared to if they turned away or against a concave cell.
People want to add springs and honeycomb aluminum.
All of these things make a difference and there are many other variables that I forgot or didn't think of.

All true. And it's why I'm going for springs, it a very easy measurement to set up and monitor at different charge levels.

As you mentioned about uneven sides, I think this is the biggest hurdle, I can see on my cells there many places they are not touching. Whether over time they will fill out or compress slightly I have no idea. My C rates will normally be very low, with the occasional 0.5C to 0.75C for short bursts, so to me this is just indulging my geek.

What would be perfect is a separator that deforms to fill the gaps, but I'm thinking that would need something that collapses at a specific pressure close to 17psi but can maintain pressures lower than that. I thought about neoprene sheet, but my experience of that is it looses it's spring too quickly. And it's an insulator, so negates my aluminium separator plates.

 

[BiduleOhm]

but my experience of that is it looses it's spring too quickly.

So what? it would actually be a good thing here I think

 

[ArthurEld]

I'm curious how much it costs for a torque wrench that can accurately measure 6 psi or 12 psi

 

[Colonel.lp]

So what? it would actually be a good thing here I think

If it loses its ability then the cells will become loose and desired compression will be lost, possibly resulting in stress on the battery terminals if in a place of vibration like my old truck.

 

[Colonel.lp]

I'm curious how much it costs for a torque wrench that can accurately measure 6 psi or 12 psi

Aren't you confusing torque with pressure.

Torque is force X distance
Pressure is force per unit area

I think I worked out something like 12psi over an eve cell is equivalent to 33inlb torque which would be spread across the number of clamping points, but that's for a m6 or 1/4" rod. It changes for different size rods and nuts.

This is why I opted for springs, clamping alone has too many unknown or difficult to calculate variables.

 

[BiduleOhm]

If it loses its ability then the cells will become loose and desired compression will be lost, possibly resulting in stress on the battery terminals if in a place of vibration like my old truck.

Oh ok, I thought about the case where it turns solid, that shouldn't be a problem. But in other cases it might, good point

 

[ArthurEld]

Aren't you confusing torque with pressure.

Torque is force X distance
Pressure is force per unit area

I think I worked out something like 12psi over an eve cell is equivalent to 33inlb torque which would be spread across the number of clamping points, but that's for a m6 or 1/4" rod. It changes for different size rods and nuts.

This is why I opted for springs, clamping alone has too many unknown or difficult to calculate variables.

I thought pounds per square inch are the same as inch pounds.

 

[ArthurEld]

Good thing too loose is better than too tight

 

[ArthurEld]

Has anyone ever accurately measured 12 psi on one of these compression fixtures?

 

[Colonel.lp]

I thought pounds per square inch are the same as inch pounds.

Psi is lb/in² i.e 10psi would be 10lb sat an an inch square
Torque is lb x inches. 10inlb would be the moment of force created by hanging 10lb off a 1inch bar, or 1lb off a 10inch bar.

T = KDP

• T = Torque (in-lb)
• K = Constant to account for friction (0.15 - 0.2)
• D = Bolt diameter (inches)
• P = Clamping Force (lb)

Calculation of Clamping force from bolt torque

I'm trying to calculate the clamping force resulting from torquing a nut and bolt to a particular level. I have found this formula in various forms in a lot of places. $$T = KDP$$ $T$ = Torque (...

engineering.stackexchange.com

 

[vtx1029]

I just started reading this thread, so someone might have mentioned this already...."When your compression spring application requires a high load in a small space, Lee Spring’s Belleville Washers can be the solution." I work for an electric motor MFG and for certain customers that want the shaft to move a little along its long axis but only move under a given force (high volume air blowers/compressors) we stack these up, facing each other or spooning to get the tension we need.

While a good idea you would need to stack a ton of belleville to compensate for the cell expansion and contraction. And then at that point is it cost effective when you can buy a die spring that will do the job for the price of one belleville washer?

 

[ArthurEld]

Psi is lb/in² i.e 10psi would be 10lb sat an an inch square
Torque is lb x inches. 10inlb would be the moment of force created by hanging 10lb off a 1inch bar, or 1lb off a 10inch bar.

T = KDP

• T = Torque (in-lb)
• K = Constant to account for friction (0.15 - 0.2)
• D = Bolt diameter (inches)
• P = Clamping Force (lb)

Calculation of Clamping force from bolt torque

I'm trying to calculate the clamping force resulting from torquing a nut and bolt to a particular level. I have found this formula in various forms in a lot of places. $$T = KDP$$ $T$ = Torque (...

engineering.stackexchange.com

Well that is not at all the same. I am curious how you would accurately measure 12 psi on one of these fixures?

 

[Colonel.lp]

Well that is not at all the same. I am curious how you would accurately measure 12 psi on one of these fixures?

I wouldn't, I'd measure the torque required to provide 300kgf clamping force on the side of the cell. I.e. 33inlb over 6 x m6(1/4") nuts. So 5.5inlb on each of 6 nuts + drag for my nyloc nuts.

But really I wouldn't do that because it's fraught with errors. Springs are the future man! ?

 

[JoeHam]

Springs are the future man! ?

And here I thought it was plastics ?

 

[Colonel.lp]

And here I thought it was plastics ?

Nah, plastic are yesterday's future. Plastics are now man/womankind downfall. ?