LifePO4 Battery Bank - Compression needed?

[solardad]

New to LifePO4 world, still waiting on my first cells (32 280ah prismatic aluminum cells, 2p16s).

My question is on the need for compression when constructing your battery bank.

Appears there are many different approaches - no compression, electrical tape, single large hose clamp, top and bottom plates with threaded rods, etc...
From my current system, Leaf cells, compression is a must so I am going into the LifePO4 world with that in the back of my head. Given the expense of these cells I want to make sure I am not going to regret adding or skipping this setup step.

Any thoughts?

 

[JoeHam]

Some advocate compression.

Some advocate leaving space for ventilation.

I really don’t know myself!

 

[Turd Furguson]

you'll get varying opinions.

For me, I support compression. The cells expand/contract slightly with SOC so compression limits potential movement of the terminals & busbars. Data sheets for CALB cells require compression; Aluminum case cells have the same internals with a different case... it seems logical that compression would also be recommended here.

Data sheets for prismatic cells (i.e. Higee) list compression values for internal testing. Other literature notes increased longevity by limiting movement potential & gas expansion room.

I am currently draining my cells down to compress prior to testing at full charge... I find it interesting that my clamps have loosened slightly as the cells deplete, and when I snug them up voltage increases slightly. Starting point was 3.295v; assumed around 50% SOC.

Other manufacturers (i.e. Fortune) provide space between cells for airflow. However, those cell cases could be designed differently given the extra weight.

 

[carlos1w]

Here is what I did: https://diysolarforum.com/threads/best-way-to-compress-cells-in-packs.11299/post-130784.
My question is: how tight should one tighten the bolts?

 

[grizzzman]

The aluminium cells I bought came with plastic seperators, There is about 3/8" separation between cells. Personally I think the plastic case would swell over time. They required being strapped. The aluminum does a much better job at keeping the pouches inside compressed. And with sepration bus bar issues would be reduced.

 

[RCinFLA]

Swelling is mostly cased by overcharging.

If you have too much pressure build up, either initially from too much compression or constrained swelling, the film electrolyte separator within the battery could be punctured and result in a cell short.

A spongy compression may help reduce internal delamination that may happen as battery ages. Any delamination will reduce effective capacity.

Compression would not be my first concern.

My first concern is corrosion of aluminum case. The bottom of many cells are not well sealed with plastic tape. Some are totally exposed on bottom. It would be a good idea to get some mylar tape wider then cell width and tightly seal off the bottom of cell to reduce aluminum exposure. Some have reported the bottom of their LFP cell got severely corroded after being placed on pressure treated wood. I attached a chart I came across that grades different chemical corrositivity to aluminum. Bleach is very bad. Don't take asprin around your batteries. Outgassing of PVC plastic is corrosive to aluminum. You might have to do a little chemical cross search to find out common name for some materials that may land on your battery support structure. There has been several discussions on what to do with aluminum terminals and their threads. Not sure if there is a solid answer on this topic. Most are using stainless steel bolts. Still some debate on use of grease used for electrical wiring with aluminum wire. Plain old Vasoline may work as good or better. Whenever there is dissimilar metals in contact there is cause for concern.

My second concern is one cell going short and taking out next door neighbors from heating. Some thermal isolation between cells might be a good idea but be careful of material selected causing corrosion. Leave at least one side open to air to avoid too much heat buildup. The original plastic cased prismatic cells had gap ridges to give some thermal isolation and air space cooling channels,. Nothing with aluminum cases.

 

[Dzl]

Some advocate compression.
Some advocate leaving space for ventilation.

EVE advocates compression, goes so far as stating in their datasheet an additional 1000 cycles with 'fixture' (300 kg of force applied, whatever that means) so for anyone with aluminum EVE cells, 'fixture' what I assume means light compression is probably a good default approach to take unless you have reason not to.

 

[RCinFLA]

660 lbs of force !! That is a lot of pressure on micro-porosity polypropylene separators in LFP batteries.

Lead-sulfate and sulfuric acid is very corrosive to aluminum so keep that in mind before plopping down LFP cells in location that you just removed your old lead-acid batteries from. Don't reuse cables with lugs that show signs of lead-sulfate corrosion.

Baking soda (Sodium Bicarbonate) commonly used to neutralize battery acid is also corrosive to aluminum.

 

[Dzl]

660 lbs of force !! That is a lot of pressure on micro-porosity polypropylene separators in LFP batteries.

It does seem like a lot doesn't it? I'm just reporting what the datasheet says.

Though as has been discussed before in other threads, its unclear specifically what EVE means by

"'Fixture': the battery is under the action of 300kgf, the battery is under the action of preset 300 kgf"

 

[grizzzman]

660 lbs of force !! That is a lot of pressure on micro-porosity polypropylene separators in LFP batteries.

Lead-sulfate and sulfuric acid is very corrosive to aluminum so keep that in mind before plopping down LFP cells in location that you just removed your old lead-acid batteries from. Don't reuse cables with lugs that show signs of lead-sulfate corrosion.

Baking soda (Sodium Bicarbonate) commonly used to neutralize battery acid is also corrosive to aluminum.

That equates to less than one-foot lb of pressure. Or 86.796166297691 inch lbs.

 

[Dzl]

For anyone that finds this thread now or in the future that hasn't been following the other thread on compression.

I wrote a long post yesterday (post 195) summing up all the info to date, and linking to all the various scattered bits of conversation on the topic I could find. In the conversation that followed, and additional parallel tracks of inquiry Ghostwriter and I were pursuing, a lot of new and useful info came to light (Posts 195, 208, 215-217, 233 of the thread linked above). I recommend reading from 195-present for an up to date understanding of what we know about compression.

 

[Guardianwill]

EVE advocates compression, goes so far as stating in their datasheet an additional 1000 cycles with 'fixture' (300 kg of force applied, whatever that means) so for anyone with aluminum EVE cells, 'fixture' what I assume means light compression is probably a good default approach to take unless you have reason not to.

300 kg is like 600 plus pounds... Tight squeeze... But what you say is true they do definitely state that. I don't know I think my clamps go to 600 max...