Plastic case prismatics, strapped but no compression?

[Sparky_SC]

I am about to make a 4P16S LifePo4 pack using used tested cells. The cells were in a OEM enclosure in 10S configuration with stainless steel strapping (4 bands) around each group of 5 cells with aluminum plates at each end. I have seen a few other pictures on the net of other plastic cell OEM configurations that also use steel strapping around the cells. 90% of the info here on the forum is about EVE cells, thus my questions.

It would appear that the intent is to prevent expansion more so than compressing the cells. I have read everything I can find on the subject but there just isn't any info out there on steel banding plastic case prismatics.

My present thinking is to follow what the OEM manufacturers did but rather than steel straps use plates with threaded rod similar to the common method with springs, just no springs, snug up the nuts and quit there.

Here is a link to a post here on the forum showing the strap configuration (post #27) by WInston battery : https://diysolarforum.com/threads/c...e-batteries-together.27575/page-2#post-331261

And a picture of one of my battery banks showing the OEM steel straps. There is no info available from the manufacturer, they are out of business. The straps have a crimp where each overlaps with a screw clip to prevent separation. I have taken a over all length measurement of the banded cells as a reference point to use.

What are the thoughts here?? Just use rods, no springs, snug the nuts up and basically duplicate what the OEM's did only with threaded rod instead of strapping? Thinking of tightening just enough to obtain the same overall length as the OEM setup which appears very consistent.

 

[Bob B]

As far as I know, there was no compression requirement in the specs for the older plastic cells. Any strapping was likely to prevent movement of the cells.
Looks like some pretty substantial terminal connection ... and I like those braided bus bars.

 

[toms]

I was the one that posted the Winston pic. When i first started buying them the end plates and strapping were supplied along with bus bars and bolts.

They gave no compression, and many of these packs are now approaching ten years of service.

Your idea of end plates and threaded rod just snugged up will work fine.

One interesting thing with Winston that i don’t hear much of anymore is they made a special cell if you wanted to stack the cells on their side. To me this implies the standard cells weren’t designed to take external pressure (ie compression)

If i was building using EVE cells i wouldn’t compress them either. I haven’t seen any evidence that slight compression in those cells extends lifespan compared to prevention of expansion.

 

[grizzzman]

I would take a look at this site. https://marinehowto.com/lifepo4-batteries-on-boats/

 

[Bob B]

If i was building using EVE cells i wouldn’t compress them either. I haven’t seen any evidence that slight compression in those cells extends lifespan compared to prevention of expansion.

While I agree that there is no compression necessary for the older plastic cells ....
I really don't understand why you keep denying that the EVE spec sheets that recommend compression of their cells to improve cycle life is correct ..... do you have any evidence that it isn't correct? Why would they recommend something like that if it weren't true.

Also not sure why you introduced that into this thread that has nothing to do with the EVE cells.

 

[Sparky_SC]

I would take a look at this site. https://marinehowto.com/lifepo4-batteries-on-boats/

Lot of great info on that site, thanks ! He shows and mentions compression for the plastic case cells but no details at all.

 

[toms]

While I agree that there is no compression necessary for the older plastic cells ....
I really don't understand why you keep denying that the EVE spec sheets that recommend compression of their cells to improve cycle life is correct ..... do you have any evidence that it isn't correct? Why would they recommend something like that if it weren't true.

Also not sure why you introduced that into this thread that has nothing to do with the EVE cells.

I didn’t introduce EVE cells into the thread.

I have evidence of compressing prismatic LiFePO4 cells causing reduced life.

I have no evidence of compressing LiFePO4 cells increasing life.

I’m saying what i would do, i won’t be at all offended if others tread a different path.

 

[Bob B]

I didn’t introduce EVE cells into the thread.

I have evidence of compressing prismatic LiFePO4 cells causing reduced life.

I have no evidence of compressing LiFePO4 cells increasing life.

I’m saying what i would do, i won’t be at all offended if others tread a different path.

If you want to start a new thread, I would like to see the evidence you have .... but don't want to discuss it here.

 

[Roswell Bob]

If you want to start a new thread, I would like to see the evidence you have .... but don't want to discuss it here.

Did this topic go anywhere? I'm preparing to build battery and this is interesting.

 

[Bob B]

Did this topic go anywhere? I'm preparing to build battery and this is interesting.

Not that I know of ..... if so, please post a link here.

To me, it seems clear that we need to use the EVE spec sheet guidance unless there is clear evidence that it isn't correct.
EVE has released multiple versions of the spec sheet that shows the benefit of proper compression.

 

[Roswell Bob]

Not that I know of ..... if so, please post a link here.

To me, it seems clear that we need to use the EVE spec sheet guidance unless there is clear evidence that it isn't correct.
EVE has released multiple versions of the spec sheet that shows the benefit of proper compression.

I'm planning on using springs

 

[grizzzman]

Lot of great info on that site, thanks ! He shows and mentions compression for the plastic case cells but no details at all.

I think your idea of bolts will work fine for you. Here is another site for you. https://nordkyndesign.com/protection-and-management-of-marine-lithium-battery-banks/

 

[Sparky_SC]

I think your idea of bolts will work fine for you. Here is another site for you. https://nordkyndesign.com/protection-and-management-of-marine-lithium-battery-banks/

Since starting this thread, I exchanged a few emails with a China manufacturer of plastic case cells. Eventually he sent me a couple videos showing how they compress in the factory. One was a common plastic banding machine, air operated, same as used for banding skids and packages. The other was a homebrew hydraulic press with a 5 ton bottle jack, they pumped the jack until the cells were compressed then installed steel straps between the end plates with pop rivets. The steel bands and the end plates had the holes for the rivets predrilled so that is what determined the ultimate compression, not the jack. He did not give me a spec for compression and I highly suspect they don't have one ! They just pull them up nice and tight and thats it.

The battery modules I have use 4 steel straps to compress the cells (rows of 5). Taking a measurement before the bands were cut and after revealed the modules are compressed about 1/8 inch total (1/16 per side). You can see a slight bulge about 2/3 of the way up from the bottom when they are uncompressed.

So........ presently, the game plan is to use threaded rod and compress until the dimension is equivalent to what the factory bands produced. They were very consistent.

 

[grizzzman]

Since starting this thread, I exchanged a few emails with a China manufacturer of plastic case cells. Eventually he sent me a couple videos showing how they compress in the factory. One was a common plastic banding machine, air operated, same as used for banding skids and packages. The other was a homebrew hydraulic press with a 5 ton bottle jack, they pumped the jack until the cells were compressed then installed steel straps between the end plates with pop rivets. The steel bands and the end plates had the holes for the rivets predrilled so that is what determined the ultimate compression, not the jack. He did not give me a spec for compression and I highly suspect they don't have one ! They just pull them up nice and tight and thats it.

The battery modules I have use 4 steel straps to compress the cells (rows of 5). Taking a measurement before the bands were cut and after revealed the modules are compressed about 1/8 inch total (1/16 per side). You can see a slight bulge about 2/3 of the way up from the bottom when they are uncompressed.

So........ presently, the game plan is to use threaded rod and compress until the dimension is equivalent to what the factory bands produced. They were very consistent.

Good to see you have a plan.

 

[toms]

Not that I know of ..... if so, please post a link here.

To me, it seems clear that we need to use the EVE spec sheet guidance unless there is clear evidence that it isn't correct.
EVE has released multiple versions of the spec sheet that shows the benefit of proper compression.

Take a look at the video posted by mikefitz in the spring compression thread.

It clearly shows a pack being built with a fixed outer housing, ie not constant pressure.

If you were pedantic, you could find out the SOC and initial force applied.

I know if my LiFePO4 manufacturer was building his own packs in a different manner to what he was advising me i’d be on the phone pretty quick looking for an explanation.

 

[toms]

Since starting this thread, I exchanged a few emails with a China manufacturer of plastic case cells. Eventually he sent me a couple videos showing how they compress in the factory. One was a common plastic banding machine, air operated, same as used for banding skids and packages. The other was a homebrew hydraulic press with a 5 ton bottle jack, they pumped the jack until the cells were compressed then installed steel straps between the end plates with pop rivets. The steel bands and the end plates had the holes for the rivets predrilled so that is what determined the ultimate compression, not the jack. He did not give me a spec for compression and I highly suspect they don't have one ! They just pull them up nice and tight and thats it.

The battery modules I have use 4 steel straps to compress the cells (rows of 5). Taking a measurement before the bands were cut and after revealed the modules are compressed about 1/8 inch total (1/16 per side). You can see a slight bulge about 2/3 of the way up from the bottom when they are uncompressed.

So........ presently, the game plan is to use threaded rod and compress until the dimension is equivalent to what the factory bands produced. They were very consistent.

When i was using the Winston straps i used a woodworking sash clamp to get the straps to fit - the clamping force was very low, what you are proposing will be fine.

 

[Bob B]

Take a look at the video posted by mikefitz in the spring compression thread.

It clearly shows a pack being built with a fixed outer housing, ie not constant pressure.

If you were pedantic, you could find out the SOC and initial force applied.

I know if my LiFePO4 manufacturer was building his own packs in a different manner to what he was advising me i’d be on the phone pretty quick looking for an explanation.

If that's all you have .... you have nothing.
I'll continue to use the EVE specs as the gold standard for their cells.

 

[LB3]

I used stainless steel straps to constrain my GBS battery build.

Found this on Flee-bay:

 

[toms]

If that's all you have .... you have nothing.
I'll continue to use the EVE specs as the gold standard for their cells.

Fair enough, you are comfortable doing what they say - i’ll stick with doing what they do.

 

[Bob B]

Fair enough, you are comfortable doing what they say - i’ll stick with doing what they do.

You know virtually nothing about that video except some robots dropped cells into a fixture and came out with a battery.
You don't know what that battery was going to be used for ..... was there really any concern over cycle life for it's application. You don't know anything about how much compression the fixture was going to provide.

So.... to say that you have evidence that compression actually reduces cycle life .... based on a very short clip of a pack being put together is pretty ridiculous and reduces your credibility in general.