Guesstimating compression pressure

[BigBrosMo]

Hi All. I hope this doesn't anger too many people with its lack of refinement or accurate measurement. And maybe I might be better off to ignore compression altogether, but here I am!

I have an 8s configuration of EVE LF280K batteries in a 2x4 configuration. Ie. 4 rows of 2 batts. This fits nicely in the battery box I made. I "showed it off" and got lots of input on compression. Both from the "don't worry about it" and the "but you HAVE to" camps. The overall takeaway was "some compression is better than none but too much is worse." Ok cool.

So I've sandwiched my batts between two plastic cutting boards and added 6 stainless steel M4 threaded rods. 2 on each end, and 2 through the middle. (I used lengths of wire sheath to insulate the rods)

I don't have room for springs in the box I made and since I am on an island in Thailand, remaking it, or getting properly gauged springs isn't the easiest effort. So I am left to guesstimating.

Anyone who's compressed their bank to 12psi with some accuracy.... How tight is at 50% soc? Like should I tighten each of the 6 rods finger snug, finger tight as hard as I can, finger tight plus a couple turns of the wrench? Basically is there any sort of guideline I can use from your experience, or should I really just say forget about compression altogether if I can't be accurate?

 

[HRTKD]

If you don't have a torque wrench, I suggest you use a nut driver and get it good and snug with that. Tighten the nuts like you would the lug nuts on a wheel. A little on the left, a little in the middle, a little on the right, rinse and repeat.

 

[Zil]

My understanding of "compressing cells". The idea is not to press the cells with some guessed 'torque'. The purpose is to restrain the cells from expanding. If the cells are tightly restrained by plates firmly held by threaded rod with ny-loc nuts that will not loosen, torque doesn't enter into the solution.

 

[Atomic Ed]

If I'm not mistaken, didn't Will say in his videos that he doesn't bother with compression? He thinks it is not really necessary is what I took from it.

 

[KevinD]

If I'm not mistaken, didn't Will say in his videos that he doesn't bother with compression? He thinks it is not really necessary is what I took from it.

Do you have a link for that video? He’s got so many videos…

 

[Atomic Ed]

Do you have a link for that video? He’s got so many videos…

I don't recall in which of his videos he mentioned this. I believe it wasn't the premise of the video so it makes it harder to remember. Not to mention I watched so many of his great videos thus far. Maybe Will can chime in as to which videos he mentions it.

In the meantime this other guy on YouTube who I feel is also very good with solar, did a video with details on whether to compress cells or not. Make sure you watch the entire video to get his opinions.

 

[time2roll]

Does a couple zip ties pulled real snug count? That's all I got.

 

[Ampster]

The issue with Zip ties or tape is that unless there is a backing plate of some kind the corners of the cells on each end could compress and the those cells could bulge on one side.

 

[KevinD]

Wow, this opens a whole new can of worms. I’m thinking, for me, I’ll strap them together with strapping tape for now. Then in the fall when I’m reinstalling my heaters, I’ll revisit the issue.

Anyway, that’s my decision today, my battery cables are in my mailbox in Florida, I need to get them shipped to my RV in Oregon.

 

[VideoPipeline]

re: "Guesstimating compression pressure"
Assuming that you have a repeatable assembly process, you could just buy a 300kg shipping scale and then you'd know for sure
(pressure = weight reading divided by the area of the side of the cell).
Less than $80 on Amazon, e.g.: B08DNYGF7J / B09BFXT9FW / B08M3MD2MC

 

[time2roll]

The issue with Zip ties or tape is that unless there is a backing plate of some kind the corners of the cells on each end could compress and the those cells could bulge on one side.

I agree it is best to have some type of backing plate with zip ties. Also for the concentration of pressure on the corners.
Tape seemed complicated to unwrap if that becomes needed. Also I have heat pads on the sides that would also complicate the build.