Best practice for 300kgf ‘fixture’ 280Ah cells

[Tradewinds63]

Yeah, 300 number only applies to the 270 to 320 ah size cells. My personal opinion is that fixed- none spring loaded should be from firm contact to 8 psi and spring or foam loaded 10 to 12 psi. Cells will keep expanding after the initial charge. So un clamping would be counterproductive imho. On a hot day and full cycle they really want to expand. The end game here is that the layers of the “jelly roll” are kept from rubbing past each other as the cells cycle or straining.

They were some claims made by an EE in some interview on the topic. The video I found online somewhere last year. Don't recall where at this time.

 

[upnorthandpersonal]

the removal of the blue insulator film that comes with them and tying all the cases together

I know we did that in the past in a test set-up in the lab hen we first got blue prismatic cells (this was before my time, so I don't know the details). I know I've checked the original EVE LF280, the LF105 and some cells from GWL, but some people claim to have had a high current between case and one of the terminals. As I said, I've never seen real evidence of this.

 

[Luk88]

Personally I think if you are using a clamping fixture that doesn't allow lots of travel (up to 1mm per cell long term). You have to keep inspecting the pressure fairly frequently.

How? By measuring additional spring compression and/or bolt torque.

As many said. Too much is way worse than not enough.

I'm currently in a process of figuring out how can I switch my springs for much longer ones. For a couple of my batteries a woodworking clamp is all that's needed. But I have one battery this is much more difficult to do with. So to anyone building a new DIY battery. Use long springs (or a very high stack of disc springs) in the first place.

 

[Hedges]

I tested my cells before settling on the construction method and after finding only a naturally occuring chemically produced polarity between the Cathode terminal and the case, I wasn't convinced additional insulators were needed. I was actually considering the removal of the blue insulator film that comes with them and tying all the cases together, in effect, puposely grounding them to one another.

I think that will accelerate the failure mode some people's packs have experienced.

In each cell there are two electrodes, relative to each other they are nominally 3.4V apart.
Both of those electrodes is soaked in liquid, and the liquid is also in contact with the cell cases.
Unless there is an insulating wrap around outside of electrode roll, one electrode has much more surface area with thin layer of electrolyte to case.
If you try connecting each terminal to the case externally one at a time, I would expect contact with the terminal not having intimate electrolyte path to case to result in some current flow, might grow an oxide layer.

If you connect 16 cells in series, first and last cell are nominally 16 x 3.4V = 54.4V apart at the terminals.
The cases will have electrolyte path to terminals 15 x 3.4V = 51V apart.

You have now created a cell consisting of one case, electrolyte, cell electrode, 51V source, cell electrode, electrolyte, other case.
And you have bonded the cases together, perhaps to ground as you suggest.

51V / 2 = 25.5V across case, electrolyte, cell electrode.
I expect that to break down anything in the way, conduct current, etch through either the case or the electrode. Due to applied voltage polarity to each of these two parasitic cells, I expect one to etch through the case, the other to etch through the electrode.


Would you lay a wet towel on top off all your cell terminals and a sheet of metal on top of that?

 

[Tradewinds63]

Personally I think if you are using a clamping fixture that doesn't allow lots of travel (up to 1mm per cell long term). You have to keep inspecting the pressure fairly frequently.

How? By measuring additional spring compression and/or bolt torque.

As many said. Too much is way worse than not enough.

I'm currently in a process of figuring out how can I switch my springs for much longer ones. For a couple of my batteries a woodworking clamp is all that's needed. But I have one battery this is much more difficult to do with. So to anyone building a new DIY battery. Use long springs (or a very high stack of disc springs) in the first place.

The official experimental testing used fixed clamps with no allowance for travel. Same applies to another independent test that resulted in the same.
I have yet to read about any experimental testing results that incorporate a spring apparatus or call for an allowance for movement. I see people discussing it and speculating about it but at this time I see no paper on the matter of spring clamping.

 

[Tradewinds63]

I think that will accelerate the failure mode some people's packs have experienced.

In each cell there are two electrodes, relative to each other they are nominally 3.4V apart.
Both of those electrodes is soaked in liquid, and the liquid is also in contact with the cell cases.
Unless there is an insulating wrap around outside of electrode roll, one electrode has much more surface area with thin layer of electrolyte to case.
If you try connecting each terminal to the case externally one at a time, I would expect contact with the terminal not having intimate electrolyte path to case to result in some current flow, might grow an oxide layer.

If you connect 16 cells in series, first and last cell are nominally 16 x 3.4V = 54.4V apart at the terminals.
The cases will have electrolyte path to terminals 15 x 3.4V = 51V apart.

You have now created a cell consisting of one case, electrolyte, cell electrode, 51V source, cell electrode, electrolyte, other case.
And you have bonded the cases together, perhaps to ground as you suggest.

51V / 2 = 25.5V across case, electrolyte, cell electrode.
I expect that to break down anything in the way, conduct current, etch through either the case or the electrode. Due to applied voltage polarity to each of these two parasitic cells, I expect one to etch through the case, the other to etch through the electrode.


Would you lay a wet towel on top off all your cell terminals and a sheet of metal on top of that?

I said I contemplated or toyed with the notion. I didn't say I did it. Let's take a look at why I didn't do it. And it isn't exactly for the reasons you expressed and your final comparative of a wet towel on the top of the terminals is an absurd comparative as such a casing bond isn't even remotely similar.

The current construction of the prismatic battery uses folded materials to create anode and cathode cells about an electrode separator membrane. Each side of the cell contains different chemistry, the anode chemistry and the cathode chemistry. These foldes are repeated several times to fashion a battery from many cells. Technically, the big blue things are batteries, not cells and we need to understand the differences and what we are actually discusing here in order for it to be discussed without abstract citations. And I'm not going to entertain arguments to the contrary here about the blue things being being batteries and not cells.

This is an important distinction because we are discussing the current POTENTIAL that is present between the outermost cathode cell backs and the metal case, we'll cite them and end artifact cells because they arent providing power in the main battery cells. The construction folds must end with both cathode or both anode folds on each end, otherwise it creates a shorted cell and the power drains right through the case from the end anode to end cathode via an internal series discharge. Now, the above isn't a perfect description, it's basic description. I'm not going to write a book about it here.

As initially expressed in the previous post discussing this matter, the case and anode terminal potential is derived through a single cathode chemistry and the end cathode electrodes back sides and the case. This means the potential amperage or power between the case and electrode is limited to two improper cells consisting of anode chemistry only. And if you want an example of that potential amperage, we're talking .2 amps for an 80 square inch area between two end wall artifact cells. These aren't the cells that produce the electricy you use in the battery. If you bond the external cases to one another, you deplete the potential in the end artifact cells only. It doesn't drain the power from between the central anode and cathode cells. Once that potential is drained, the end artifact cells become neutral and won't pass anymore current.

The reason I didn't do it is because it can cause interference within the outermost bi-chemistry cells. That's it, not because it will drain the cells like a wet towel over the top of all the terminals. That's an absurd comparative because you are introducing a direct short through the entire potential of the battery and not just an isolated short in the end artifact cells.

 

[Recovering EE]

The way I learned this is that an electrochemical *cell* exhibits the characteristic electrical potential for that particular chemistry, and a *battery* is a combination of cells, whose electrical potential is an integer (2 or more) multiple of a single cell's nominal characteristic electrical potential.

The characteristic nominal electrical potential of a LiFePO4 cell is 3.2V. You can increase the energy storage capacity of that cell by increasing the surface area of the anode and cathode. This can be accomplished by making a single layer anode/cathode combination that is exceptionally large and flat (not practical), or you can get "tricky" and wrap them around like a jelly roll, or fold them. In either case, you still have a single electrochemical cell that still exhibits the chemistry's nominal electrical potential. It's technically *not* a "battery". However, describing it as a "battery cell" or even just a "battery" is quite common.

It's a lot like the misuse of the word "theory" in popular parlance.

 

[Luk88]

The official experimental testing used fixed clamps with no allowance for travel. Same applies to another independent test that resulted in the same.
I have yet to read about any experimental testing results that incorporate a spring apparatus or call for an allowance for movement. I see people discussing it and speculating about it but at this time I see no paper on the matter of spring clamping.

What official experimental testing? Can you post a link or citation if you know of a real research paper? I was under an impression there was no publicly available scientific testing on the subject.

If there is and they indeed used fixed clamps, how would they maintain proper force as the cells naturally swell with age? Eve datasheets for MB3X claim 6 tons of force comes from aging related swelling. Where does this force go in a fixture that allows no movement? It either deforms the fixture if its made with thin sheet metal. Or it increases the pressure inside the cells bursting their vents and spewing their electrolyte out...

Edit: About posts directly above. Don't forget many of today's lifepo cells like MB31 from EVE don't use the rolled jelly roll, but stacked construction method. Where catode/anode and other elements are stacked one atop another like sheets of paper in a printer tray.

 

[aldavid88]

Hi guys, I am very new to the solar and batteries. But because I live in Ukraine and we have very unstable situation with electricity because of war I have to build my own system. I don't want to do a mistake and also one more difficulty is related to the metric system we use. I read carefully few threads, but anyway I am not quite sure if I understood everything correctly. I have 8 cells EVE MB31 and want to build 24v battery, so all 8 will be in series. I have full metal DIY case with 4 6mm (~1/4") 20 tpi steel studs. I am not quite sure if I need to use square of the case side cover (which is 96 in²) or the battery (55,8 in²) for calculation. I used battery size, but correct me if I am wrong.


The square of 1 battery side is 55,8 in². 55,8 in² * 12 psi = 670 lbs. 670 lbs / 4 = 167,5 lbs. So, I need to apply 8 in-lbs per one or 0,67 ft - lbs (0,9 n-m in my case).

Is it correct?

 

[Hedges]

It would be area of battery face, not of your metal end plate. Because pressure on battery.

No springs, just bolts clamping? I think manufacturer spec was at a particular SoC.
Some people used springs to maintain compression over travel. The manufacturer built packs I have do not, just a pop-riveted steel case.

Some people put insulating tubes over the threaded bolts. Yours seem far enough away from cells not to touch.

At lower SoC, the cells may shrink and simply slip out the bottom. Do your end plates bolt to a bottom plate that cells can rest on?

Between the cells some insulator is advisable, rather than just relying on the blue plastic wrap.

Sometimes even those two-hole terminals have broken off at the weld. Maybe if you restrain them with a wrench while tightening bolts, that will avoid it. Put insulation on wrench handles to avoid shorting to other cells.

 

[HRTKD]

8 in-lbs sounds in the ballpark of what I did on my 4s battery pack. I think I might have used 12 in-lbs. That was 4 years ago, so my memory of it is a bit fuzzy.

Good looking build so far!

 

[aldavid88]

No springs, just bolts clamping?

Yep, only bolts.

Some people put insulating tubes over the threaded bolts.

This one I also added. It's transparent, but you can notice it on the photo.

At lower SoC, the cells may shrink and simply slip out the bottom. Do your end plates bolt to a bottom plate that cells can rest on?

Yes, there are grooves for the bottom plate. So you can choose the needed size and tight the bolts.

Between the cells some insulator is advisable, rather than just relying on the blue plastic wrap.

It's not visible on the photo, but I added 0,5мм epoxy plates below the batteries and case and between the cells. But somewhere I read that EVA Foam is much better, so I ordered 3mm fire resistant EVA Foam to replace epoxy plates.

8 in-lbs sounds in the ballpark of what I did on my 4s battery pack. I think I might have used 12 in-lbs.

To be honest 8 in-lbs for me sounds also like "not enough", but this is the number I calculated 2 times. I have a bit of time, so will try to figure this out. Because my torque wrench starts with 20 in-lbs. I need to get something with lower range

 

[HRTKD]

Because my torque wrench starts with 20 in-lbs. I need to get something with lower range

Yeah, you don't want to use that. I did that once on transmission pan bolts with a 1/2" click torque wrench, thinking I could extrapolate down. Nope. Broke the bolt right off and had to have someone drill it out. Lesson learned, so I bought a 1/4" digital torque wrench.

 

[Luk88]

Hi guys, I am very new to the solar and batteries. But because I live in Ukraine and we have very unstable situation with electricity because of war I have to build my own system. I don't want to do a mistake and also one more difficulty is related to the metric system we use. I read carefully few threads, but anyway I am not quite sure if I understood everything correctly. I have 8 cells EVE MB31 and want to build 24v battery, so all 8 will be in series. I have full metal DIY case with 4 6mm (~1/4") 20 tpi steel studs. I am not quite sure if I need to use square of the case side cover (which is 96 in²) or the battery (55,8 in²) for calculation. I used battery size, but correct me if I am wrong.
View attachment 250444

The square of 1 battery side is 55,8 in². 55,8 in² * 12 psi = 670 lbs. 670 lbs / 4 = 167,5 lbs. So, I need to apply 8 in-lbs per one or 0,67 ft - lbs (0,9 n-m in my case).
View attachment 250447View attachment 250446
Is it correct?

Hi. Others already mentioned, but I'll expand on it. It is not a great idea to just do compression this way where you set it once and not allow any future movement. Cells do swell slightly as they age. The force of this is very large and even though you may calculate the correct torque now it will become far too large in time. Thin steel will most likely deform in time and you end up with most of the force on the edges of the end cells and that may be quite dangerous.

Also using 8lbf (or 6Nm) is absolutely bonkers unless you're using very large diameter rods. For M6 you need under 1Nm (0.8lbf),for M8 it is 1.2Nm. Very rarely you'll find a torque wrench that can measure this.

(divide 300 by 4 =75kg per screw, assuming no lubrication and stainless steel, it is 1.1Nm for M8 or 0.8Nm for M6)

So how come people do 8lbf and such? Most likely their case deforms. Personally I suggest either correct poron foam (you can't make a mistake with this if you select the correct type) or springs.

If you can't do any of that use at least disc springs under your screws which are very cheap. However, whatever you do. If there is compression forget about using 1~2mm sheet steel. I'm using 10mm steel (with springs) as 3mm I used before was deforming a lot well before 300kg.