DIY vs. 'plug-n-play' LFP config / construction

[memilanuk]

So... a couple things that have been bugging me for a bit now:

Nearly everyone seems obsessed with compressing the cells in DIY battery packs; but pretty much every tear-down video of a commercial / 'plug-n-play' battery appears to show cells simply taped together and jammed in a box, sometimes with some padding along the sides. Why is it so important for EVE/CATL/Lishen/whatever cells to be compressed, when it apparently isn't important for Battleborn, SOK, Renogy, etc.?

Again, most of what I can find on the forum indicates that cells should ideally be kept in an upright / vertical orientation (lugs/terminals on top), while I've seen more than one instance of commercial batteries such as Battleborn mounted on a bulkhead with the terminals perpendicular to the ground, or even completely upside down, with no obvious problems. My understanding is that the reason for keeping the DIY cells vertical is to make sure that the electrolyte is distributed as originally intended; but why is that seemingly not an issue for the commercial batteries?

 

[upnorthandpersonal]

I hope this thread doesn't lead to hundreds of posts on tons of pages like last time this was discussed

My opinion is that the cells need a fixture (compression is a bit of a misnomer - again, my opinion). According to the datasheet, cycle capacity is increased if put into a fixture compared to not, and previous versions of the datasheet made this explicit (datasheets before that didn't mention it at all). In recent versions, it is under the assumption that a fixture is used in all cases. The term compression stems from the datasheet where an explicit force was mentioned that the fixture would exert on the cells.

Long story short: If your fixture prevents cells from expanding, you're good. If not, you can still be good but might have a thousand cycles less. I believe that these cells were always intended to be used in a fixture, and that later datasheets revised this because cell cycle tests indicated that if they were not, they had lower cycle count.

The reasons why commercial manufactures do/don't put them in a compression/fixture has many facets. Some will say it's so that their packs die faster so they can sell more, others will state that the information wasn't known before the packs were designed. Yet others will state that the manufacturers simply didn't know since it may/may not be needed to do with smaller cells compared to larger ones. There will be many a long thread to come to discuss this I fear...

 

[memilanuk]

If your fixture prevents cells from expanding, you're good.

So... would putting the cells inside a hard/rigid case be sufficient to limit expansion? Specifically, I'm looking at doing something like what @SolarKimmy did in his video (I've got the boxes and the BMS, but got caught up in the ongoing cell drama unfortunately).

 

[upnorthandpersonal]

Basically, yes. You want to prevent cell terminal stress primarily, and a proper, rigid fixture will provide 'compression' as well.

 

[memilanuk]

Any thoughts/opinions on the orientation aspect?

 

[upnorthandpersonal]

Always straight up, never on the side.

 

[memilanuk]

I was more referring to why it's a big deal for the DIY cells, while the commercial batteries don't seem to care - some even tout being able to mount in alternate orientations as compared to FLA.

 

[upnorthandpersonal]

We've seen on this forum that at least one battery company that had cells on its side had leakage - check Will's video on that one. There are just as many cell manufacturers that state cells should be used right side up. Even Winston cells:

Blog - Winston | shop.GWL.eu

shop.gwl.eu

 

[mrzed001]

Check out how they are in a Tesla battery pack: