Best practice for 300kgf ‘fixture’ 280Ah cells

[fafrd]

This chart shows that bell curve, symmetrical to a point, but too much pressure would damage the cell so there is a limit on the high end unlike the low end (or that was my takeaway). The above graph shows the curve for A123 cells, but the info given by the EVE engineers matched what is shown here for the most part.

The top line takeaway for me was: shoot for ~12psi, but if you can get in the ballpark of 6-17 you are in the right ballpark, and if you are going to err one way or the other, definitely err on the side of less pressure. Too much pressure = possible damage


I thought it was pretty clear they were saying that limit was ~17 PSI (maybe we are saying the same thing differently?). To use the phrase ghostwriter used, 'above 17psi things start getting pretty crushy' ?

‘Too much’ compression seems to only be an issue if you are applying too much pressure to a cell while clamping it into a fixture (resulting in crushed cells causing degradation through mechanical deformation).

‘Too little’ pressure means you were unable to prevent cells from swelling (resulting in degradation through mechanical degradation).

After reading-reading what the EVE engineer wrote, I think my plan will be to apply 12-13psi of force with a ridgid non-conforming structure when the cells are ~50% SOC or thereabouts (not swelled, not sunken) and call it a day.

The evidence seems much more clear that cell cycle life is reduced when the cell is allowed to swell as it moves well past 50% SOC towards full than it is that charging cells to full in a ridged structure can cause sufficient stress/force to damage them and reduce cycle life.

Among other things, a further benefit of clamping cells in a rigid structure is that stress on the terminals caused by cell swelling becomes a non-issue...

 

[Dzl]

‘Too much’ compression seems to only be an issue if you are applying too much pressure to a cell while clamping it into a fixture (resulting in crushed cells causing degradation through mechanical deformation).

If I understand what you are positing, you are saying so long as the pressure applied at the time of fixture is not excessive, there is no way too much pressure could be applied after the fact (at high SOC in a rigid fixture)?

This was not my takeaway, but its an interesting idea, and if true it would simplify things for us somewhat.
Can you point out what specifically the engineer at EVE said that gave you this impression? I went back and looked and didn't see anything to that effect but its hard to keep track of all of the various details of this thread.

Is my understanding of your position correct: that you are saying if its in the happy range 6-17psi when placed in fixture, there is no risk of too much pressure?

To use an extreme example to test the idea, say you have 16 cells in series (inline). You apply rigid fixture with a pressure equivalent to ~16psi at the storage/shipping SOC (lets say ~30%). Even in this situation you think that you would not be in danger of exceeding what is safe/healthy for the cells?

Am I missing important parts of your thinking or misunderstanding something? I feel that I probably am.


My basic understanding is:

1. Target 12psi at the middle of your usable SOC bandwidth, or the SOC your pack will spend the most time near (the engineer implied the former, but I suspect the latter is equally valid or more valid in some contexts)
2. But ensure that the upper limit does not exceed 17psi, and ideally the lower limit should not exceed ~6psi.
3. The closer the pressure range is to 12psi the better from the POV of cycle life
4. But anywhere in range of 6-17psi should be substantially better than no fixture.

 

[fafrd]

If I understand what you are positing, you are saying so long as the pressure applied at the time of fixture is not excessive, there is no way too much pressure could be applied after the fact (at high SOC in a rigid fixture)?

This was not my takeaway, but its an interesting idea, and if true it would simplify things for us somewhat.
Can you point out what specifically the engineer at EVE said that gave you this impression? I went back and looked and didn't see anything to that effect but its hard to keep track of all of the various details of this thread.

Is my understanding of your position correct: that you are saying if its in the happy range 6-17psi when placed in fixture, there is no risk of too much pressure?

To use an extreme example to test the idea, say you have 16 cells in series (inline). You apply rigid fixture with a pressure equivalent to ~16psi at the storage/shipping SOC (lets say ~30%). Even in this situation you think that you would not be in danger of exceeding what is safe/healthy for the cells?

Am I missing important parts of your thinking or misunderstanding something? I feel that I probably am.


My basic understanding is:

1. Target 12psi at the middle of your usable SOC bandwidth, or the SOC your pack will spend the most time near (the engineer implied the former, but I suspect the latter is equally valid or more valid in some contexts)
2. But ensure that the upper limit does not exceed 17psi, and ideally the lower limit should not exceed ~6psi.
3. The closer the pressure range is to 12psi the better from the POV of cycle life
4. But anywhere in range of 6-17psi should be substantially better than no fixture.

Your understanding of my position is spot-on.

And it was arrived at by reading one of ghostwriters exchanges with an EVE engineer - I think it basically said their fixture is rigid, does not apply constant clamping force through the full charge/discharge cycle and that firm enclosure in a rigid case will work fine.

There are so many threads on this that I can’t easily track down the thread/post that left me with that understanding but my takeaway was that we are all making this more complicated than it has to be, EVE never suggested the use of springs, we did, and the basic point is that cycle life can be extended if the cells are enclosed snugly enough to prevent bloating when filled.

My read of ghostwriters translation in that the EVE engineers are saying don’t deform your cells by clamping with too much force initially - nothing about bloat-preventing force as the cells approach full...

I don’t believe EVE has even ever measured such forces, only the camping force when half-full cells are originally clamped into place.

After reading through the threads others linked to, I wrote a comment to this effect so I need to give that post and then go backwards to find the threads/posts that left me with that understanding..:

 

[fafrd]

Your understanding of my position is spot-on.

And it was arrived at by reading one of ghostwriters exchanges with an EVE engineer - I think it basically said their fixture is rigid, does not apply constant clamping force through the full charge/discharge cycle and that firm enclosure in a rigid case will work fine.

There are so many threads on this that I can’t easily track down the thread/post that left me with that understanding but my takeaway was that we are all making this more complicated than it has to be, EVE never suggested the use of springs, we did, and the basic point is that cycle life can be extended if the cells are enclosed snugly enough to prevent bloating when filled.

My read of ghostwriters translation in that the EVE engineers are saying don’t deform your cells by clamping with too much force initially - nothing about bloat-preventing force as the cells approach full...

I don’t believe EVE has even ever measured such forces, only the camping force when half-full cells are originally clamped into place.

After reading through the threads others linked to, I wrote a comment to this effect so I need to give that post and then go backwards to find the threads/posts that left me with that understanding..:

I think the post from ghostwriter that left me with this understanding is this one: https://diysolarforum.com/threads/e...spaced-for-expansion.7892/page-26#post-185041

But having read through that entire thread now, it seems my understanding was dead-wrong.

Rereading ghostwriters translation of what the EVE engineers stated, seems like with a rigid fixture, what you want to do is:

1/ clamp to 12 psi at ~50% SOC (or midpoint of your target range)

2/ charge up to full SOC and back off to 17psi if pressure exceeds that level (and leave as is if it does not).

I hadn’t seen the post about EVE using a calibrated hydraulic mechanism to maintain 300kgf through the full charge/discharge cycle nor their statement that pressure above 17psi can damage the cells.

So targeting the 12psi / 300kgf of force near the midpoint then checking max pressure near full and assuring it does not exceed 17psi seems like the safer approach.

Looks like I need to track down my torque wrench...

 

[Hedges]

2/ charge up to full SOC and back off to 17psi if pressure exceeds that level (and leave as is if it does not).

I hadn’t seen the post about EVE using a calibrated hydraulic mechanism to maintain 300kgf through the full charge/discharge cycle nor their statement that pressure above 17psi can damage the cells.

So targeting the 12psi / 300kgf of force near the midpoint then checking max pressure near full and assuring it does not exceed 17psi seems like the safer approach.

Looks like I need to track down my torque wrench...

OK ...
How do you propose to do that?
Assemble the pack with a load cell in it, and leave it there?

 

[fafrd]

OK ...
How do you propose to do that?
Assemble the pack with a load cell in it, and leave it there?

Balance cells to ~50%.

Clamp pack at 12psi (w/ torque wrench).

Assemble battery (bus bars, BMS, etc...).

Charge battery, using torque wrench to monitor pressure as it progresses.

If/when pressure exceeds 17psi, reduce pressure to 17psi.

Repeat as necessary until reaching maximum charge target.

 

[Bob B]

Balance cells to ~50%.

Clamp pack at 12psi (w/ torque wrench).

Assemble battery (bus bars, BMS, etc...).

Charge battery, using torque wrench to monitor pressure as it progresses.

If/when pressure exceeds 17psi, reduce pressure to 17psi.

Repeat as necessary until reaching maximum charge target.

It would be a lot easier just to put some springs on there .... Sorry just couldn't resist

 

[fafrd]

It would be a lot easier just to put some springs on there .... Sorry just couldn't resist

Well you guys get it all figured out and if it’s easy and cheap, I’ll follow in your footsteps.

The spring design still causes me concern regarding stress on the terminals...

 

[Gazoo]

I think the only sure way to prevent stress on the terminals is to use braided busbars no matter how the cells are mounted and whether or not they are stationary. Maybe I am paranoid.

 

[Bob B]

I think the only sure way to prevent stress on the terminals is to use braided busbars no matter how the cells are mounted and whether or not they are stationary. Maybe I am paranoid.

Just because your paranoid ... doesn't mean they aren't out to get you LOL

 

[Hedges]

Balance cells to ~50%.

Clamp pack at 12psi (w/ torque wrench).

Assemble battery (bus bars, BMS, etc...).

Charge battery, using torque wrench to monitor pressure as it progresses.

If/when pressure exceeds 17psi, reduce pressure to 17psi.

Repeat as necessary until reaching maximum charge target.

How do you propose to do that? (monitor pressure using torque wrench)

 

[fafrd]

How do you propose to do that? (monitor pressure using torque wrench)

Oh, that’s easy. Go forward and if the nut turns before 17psi, back off to original position. If the nut doesn’t turn forward by 17psi, loosen it and retighten to 17psi...

 

[fafrd]

I know there are some calculations floating around somewhere in some threads on this subject, but since 1/4” bolts seem like the most commonly-used, I’m hoping someone knows the answer and can save me the trouble of scanning those large threads.

using 4 standard unlubricatef 1/4” bolts, what level of torque (inch pounds) should be applied to get about 12psi of clamping force (and about 142% that much torque for 17psi, correct?)?

 

[upnorthandpersonal]

/me cries in metric

 

[Mark44]

Yeah, mine swelled too when I parallel top balanced. The swelling reduced quite a bit when I hooked the cells in series and and fully discharged them. I charged and discharged several times before I mounted them in a fixture and noticed they do like to "breathe".

My fixture consists of 1/2 inch plywood and six 5/16th threaded rods for my 8S pack. What I discovered in the end was hand tightening the nuts with no lock washers when fully discharged, the cells still swelled when charged. Someone pointed out the threaded rods act like springs. I would not have believed that had I not seen it for myself.

I am going to redo my pack and see if I can't get it to be more square. A couple of my cells came undulated as @AussieSim pointed out in this thread. The gap between my cells widens at the bottom. I am also concerned about this putting stress on the terminals. I think the only way around that is to use braided bus bars. Even short flexible cables would be better than stressing the terminals using the solid busbars I received with the cells.

Yes there are several threads discussing this and I have read through all of them including your springs idea thread @Bob B . I like your idea but I am open to other ideas as well. And I think the only way to prevent stress on the terminals is to use braided busbars or flexible cables with lugs no matter what method is used? I have to think through about the best way to deal with my problem and I can't proceed until I receive the replacement cell I need because I stripped one of the terminals on mine....stupid me.

So Gazoo,

Given the price of braided busbar, why would you not just use 1/0 or 2/0 gauge Wire ( I would think cheaper)? Not sure why everyone is so welded to solid bars? (yes pun intended) (ducking quickly)

Sorry, I just saw where you mentioned short flexible cable! (palm planting face). Still a valid question at the end though, what makes everyone like solid bar so much?

 

[fafrd]

So Gazoo,

Given the price of braided busbar, why would you not just use 1/0 or 2/0 gauge Wire ( I would think cheaper)? Not sure why everyone is so welded to solid bars? (yes pun intended) (ducking quickly)

Sorry, I just saw where you mentioned short flexible cable! (palm planting face). Still a valid question at the end though, what makes everyone like solid bar so much?

Honestly, if I convinced myself that I’m more concerned about mechanical stress on the terminals from swelling than I am about increasing cycle life from >2500 to >3500, I’d just position my cells with space for swelling between them (at the outer limits of the busbars) and be done with it.

This DIY LiFePO4 battery stuff is complicated enough as it is and ‘only’ 2500 cycles translated to almost 7 years at 1-cycle per day. I highly doubt all of the other elements of my system are going to last 7 years and 2500 cycles is already 2-5X the cycle life of LA batteries I was planning to use before I stumbled upon this DIY LiFePO4 stuff...

 

[Mark44]

Honestly, if I convinced myself that I’m more concerned about mechanical stress on the terminals from swelling than I am about increasing cycle life from >2500 to >3500, I’d just position my cells with space for swelling between them (at the outer limits of the busbars) and be done with it.

This DIY LiFePO4 battery stuff is complicated enough as it is and ‘only’ 2500 cycles translated to almost 7 years at 1-cycle per day. I highly doubt all of the other elements of my system are going to last 7 years and 2500 cycles is already 2-5X the cycle life of LA batteries I was planning to use before I stumbled upon this DIY LiFePO4 stuff...

Yes I agree with this assertion. I still wonder though what the genus for solid bus bars is?

I thought that stranded copper was a poorer conductor, but have also read somewhere that this is incorrect?

Your thoughts or knowledge please? 'Cause I think this guy is looking at cable bus vs. solid to eliminate issues with swelling and terminals.

 

[Dzl]

/me cries in metric

Everyone keeps referencing 300Kgf and 661Lbf, but what I really want to know is how many Stones force we are talking.

So Gazoo,

Given the price of braided busbar, why would you not just use 1/0 or 2/0 gauge Wire ( I would think cheaper)? Not sure why everyone is so welded to solid bars? (yes pun intended) (ducking quickly)

Sorry, I just saw where you mentioned short flexible cable! (palm planting face). Still a valid question at the end though, what makes everyone like solid bar so much?

My guess is simplicity (as in its just a piece of copper, its cheap, and they usually come with the cells)

Short, large AWG cable seems common for connecting a bunch of LA batteries, but with the high number of cells and very short distance between cells, large gauge wire seems like more trouble than flat bars, braided busbars, or the CALB style busbars (which I think are a balance between simplicity, cost, and tolerance)

This is a CALB style busbar:

The left and the right are the same thing (the right side is just deconstructed) the CALB bars are many thin layers with some curvature to allow some small amount of flexing/expansion.

Here are CALB style busbars next to some very high quality looking braided bars.


And here is an example (I think, its hard to tell) of using wire instead of busbars as you suggest:

Then there are the busbars that come with EVE and other commodity cells (not: I was told by an EVE rep, EVE does not manufacture or supply hardware for their cells). These seem like the cheapest and most basic, and probably least well suited for either high c-rates or handling expansion/contraction or other mechanical stressors. But as mentioned they are "free", come with the cells, and are good enough for most people (with fixture and low c-rates, I doubt there is any cause for concern, but we love to over-engineer and overthink don't we.. ). It does appear (these ones at least) are slotted.

 

[fafrd]

Yes I agree with this assertion. I still wonder though what the genus for solid bus bars is?

I thought that stranded copper was a poorer conductor, but have also read somewhere that this is incorrect?

Your thoughts or knowledge please? 'Cause I think this guy is looking at cable bus vs. solid to eliminate issues with swelling and terminals.

Yes, I believe he’s considering stranded bus bar mainly to provide flexibility for swelling / reduce mechanical stress on terminals.

I don’t know about conductivity of stranded busbars versus solid but what I do know it that a solid copper busbar of sufficient thickness will be able to handle peak currents required and this is very easy to calculate / check (plus they are less expensive than pretty much any other option).

 

[Gazoo]

Honestly, if I convinced myself that I’m more concerned about mechanical stress on the terminals from swelling than I am about increasing cycle life from >2500 to >3500, I’d just position my cells with space for swelling between them (at the outer limits of the busbars) and be done with it.

This DIY LiFePO4 battery stuff is complicated enough as it is and ‘only’ 2500 cycles translated to almost 7 years at 1-cycle per day. I highly doubt all of the other elements of my system are going to last 7 years and 2500 cycles is already 2-5X the cycle life of LA batteries I was planning to use before I stumbled upon this DIY LiFePO4 stuff...

I totally agree with everything you said except one point. A couple of my cells are diluted and there is isn't any space at the ends of the busbars. They reach but just barely. I noticed this when I parallel top balanced. When I get around to reassembling the pack, I hope to make it more square.

I am not at all concerned about increasing cycle life because my cells are going to be used for back up in the event of a power failure. I will cycle them probably monthly. My only concern is preventing stress on the terminals.

This battery stuff is complicated but I am trying to keep it as simple as possible and stay away from the weeds....in other words not trying to be absolutely perfect...lol.