Cell Terminal Strain Relief, bus bars, and compression

ohthetrees

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I'm sure everyone is tired of talking about cell compression. I know I'm tired of reading about it (I've read many posts on many threads about it).

What I'm hoping to discuss in this thread is compression and bus bars, but only as it relates to the integrity of the terminals. I have 16 EVE 280ah from Amy on the way, and I'm coming up with my pack design, which will be a 4P4S design. My original plan was to build a threaded rod box with aluminum end plates, snug them lightly at 50% SOC, and use the bus-bars supplied by Amy (2mm x 20mm). However, all this reading about compression has got me worried about the strain on the terminals when the cells expand and contract.

What I'm asking for is 1st hand experience or informed opinions on whether the fixturing discussed above would mitigate strain on terminals with solid bus bars. Since I'm building the pack for a boat, and I'm already slightly nervous about the physical durability of these big 280ah cells, my first priority is reducing any physical strain on the cells. Improved cycle life is far down the list.

I just don't have a sense of how much force the terminals would experience (Usually charging less than 0.2C and discharging less than 0.3C) with such a setup.

At this point, I'm getting so concerned about terminal strain that I'm leaning toward interconnecting with curved pieces of wire and lugs, maybe 1/0 or 2/0. Alternatively, I'm also thinking of putting spacers between each cell (corners and edges) and letting the middles of the cells "breath" even though I'd give up cycle life, maybe this would reduce strain and I could keep the rigid bus bars.
 

upnorthandpersonal

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You could go with braided bus bars. I make mine myself, but you should be able to find them off-the-shelf. If you're in a high vibration environment, these large cells do have the potential to translate a lot of the vibration to the terminals when you have rigid bus bars. My personal opinion is not to have rigid bus bars at all with these cells, but maybe that's just me being overly cautious...

nickel_pated-jpg.39049
 

ohthetrees

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You could go with braided bus bars. I make mine myself, but you should be able to find them off-the-shelf. If you're in a high vibration environment, these large cells do have the potential to translate a lot of the vibration to the terminals when you have rigid bus bars. My personal opinion is not to have rigid bus bars at all with these cells, but maybe that's just me being overly cautious...

nickel_pated-jpg.39049
Very nice looking. What is the ampacity, and do you have a resource to share describing the construction? I'm on a boat, and have a small vice, but no press, and I worry that without a gas-tight connection, that resistance would get worse over time. The appeal of the wire+lug approach, though far bulkier and uglier, is that I know how to make a great gas-tight connection between wire and lug.
 

ohthetrees

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the whole point of compression is to prevent strain on the bus basr ( as well as prevent bulging )..
if done correctly, the should not be any strain...

if this wont work for you, take a look at @upnorthandpersonal strandled copper flexible busbars
Do you have personal experience with compression effectively stopping strain, or a good source to cite? It seems to me that even compressed lightly at 50% SOC, that a battery bank ranging over 10-100% SOC could result in some movement as the cells expand and contract.
 

upnorthandpersonal

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Very nice looking. What is the ampacity, and do you have a resource to share describing the construction? I'm on a boat, and have a small vice, but no press, and I worry that without a gas-tight connection, that resistance would get worse over time. The appeal of the wire+lug approach, though far bulkier and uglier, is that I know how to make a great gas-tight connection between wire and lug.

These are 150A (48V system, so plenty) - but you could double up on the braid. In your case, if you don't have the right tools, I would buy them off-the-shelf. Wire + lug is a possible alternative.

I have some posts here somewhere describing how to make them. Essentially: annealed copper pipe compressed over the braid. Then nickel plate them with nickel-acetate.
 
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HRTKD

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Do you have personal experience with compression effectively stopping strain, or a good source to cite? It seems to me that even compressed lightly at 50% SOC, that a battery bank ranging over 10-100% SOC could result in some movement as the cells expand and contract.

A snug fitting compression fixture isn't going to allow enough movement to be an issue with the bus bars. I have two batteries, each in a compression fixture made out of threaded rod with 3/4" plywood ends. A couple months ago I checked all my terminal connections and found only one screw needed a very small amount of retorquing. My cells sit in the front interior compartment of my camper. I take the camper down some rough Forest Service, BLM and county roads. My trailer is far from a Campground Queen. If anything was going to shake loose, it would have already.

I'm currently using the bus bars and screws provided with the batteries. No thread locker or lock washers. My cells were charged to 95%+ the first few months of the implementation.

Don't overthink things.
 

ohthetrees

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A snug fitting compression fixture isn't going to allow enough movement to be an issue with the bus bars. I have two batteries, each in a compression fixture made out of threaded rod with 3/4" plywood ends. A couple months ago I checked all my terminal connections and found only one screw needed a very small amount of retorquing. My cells sit in the front interior compartment of my camper. I take the camper down some rough Forest Service, BLM and county roads. My trailer is far from a Campground Queen. If anything was going to shake loose, it would have already.

I'm currently using the bus bars and screws provided with the batteries. No thread locker or lock washers. My cells were charged to 95%+ the first few months of the implementation.

Don't overthink things.
Thank you for this first hand experience. Reassuring. Though you found one slightly loose screw, I wonder how you know that there isn't tension on the terminals, strain being taken up by the terminals themselves as the cells "breath"? Tension can still exist with all the screws remaining tight.
 

HRTKD

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If the cells were moving, the bus bars would be moving back and forth. The bus bars have elongated holes. Frequent movement of the bus bar in relation to the terminal would have resulted in a loose screw.

Note that when I say one screw got retorqued, it was very minimal. I didn't use a torque wrench for the subsequent check, just a screw driver. I might have turned the screw driver 5°. The screw on the other terminal of the same cell was fine. So I chalked it up to an inadequate initial torque. If I had found that more than one screw needed tightening, I would have retorqued them all. I use a 1/4" digital torque wrench that measures in inch pounds as well as newton meters. As I recall, my initial torque was 30 inch pounds. I didn't write it down though. I was shooting for something less than 4 nm but more than 3 nm. 30 inch pounds was a nice round number.
 

ohthetrees

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If the cells were moving, the bus bars would be moving back and forth. The bus bars have elongated holes. Frequent movement of the bus bar in relation to the terminal would have resulted in a loose screw.
I'm not trying to be pedantic, or over skeptical, but my concern really isn't bus bars moving, or screws loosening. Asking another way, what if the reason your cells aren't moving at all is because the rigid bus-bars are holding the cells in place? Terminals could be experiencing strain even if nothing is moving. In fact, I'm very sure this effect happens, I'm just not sure whether it is so minor I don't have to worry about it, or it is a serious concern.
 

HRTKD

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The bus bars would hold the cells only at the top. There are pictures of bloated cells that aren't held within a fixture. The bus bars in some cases are still intact and the battery pack looks like an accordion.

It could be argued that too much torque on a bus bar prevents movement, which could cause strain on the terminal. But that's applicable only if the cells are allowed to expand. If my cells are expanding, I doubt it's enough that can be measured.
 

Bob B

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I personally think @upnorthandpersonal 's approach to bus bars is the ultimate solution. Stress on the terminals will be as small as possible.

Another personal opinion of mine is that if the cells are encased in such a way that they cannot expand at all .... The pressure is probably too great at full charge .... or too little when fully discharged ..... and the cycle life of the cells is being reduced.

EVE's spec sheet indicates that each cells will expand and contract by .5mm between 30 and 100% SOC .... with optimal compression which calculates out to about 12PSI. (300 Kgf) So, a 4S pack could be seeing 2 mm of movement even when perfectly compressed.
 

Marinepower

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I had the same concern as you re. soild bus bars. I used the 70mm version of these instead. Note the holes are for M8 bolts, so a bit oversized. They also carry braided interconnects.


However, you are presumably going to be using lugs and copper cable for the pos and neg terminals to connect to the boat.. I think just using lugs between cells would also be fine and a simple solution.

UPDATE: since this post was first made - I have recently had problems with the bare copper oxidizing on this CALB bus bars. I think a nickel plated alternative would be better....
 
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Colonel.lp

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My first experience of assembling a pack of eve 280 cells on the bench and charging absolutely schooled me in solid bus bars being potentially bad. My experience is going to go against the grain of thinking a clamped pack minimises terminal strain.

My 4s pack was clamped between 10mm aluminium end plates, with 4mm aluminium plates between the cells, and 10x 6mm rods clamping it all together probably tighter than it should have been (cells at 50% charge).

After first full charge, on removing terminal nuts, there was a very noticeable click as every terminal moved relative to busbar. I repeated this.

Not only that, on full discharge, the end plates were loose despite being very well tightened at 50%

The braided busbars are a great solution if you can make them, they are bloody expensive to buy, so I've gone for cable and lugs.

This is my second 2p4s pack that I've done differently.
IMG_20210827_001722_684.jpg
 
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ohthetrees

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However, you are presumably going to be using lugs and copper cable for the pos and neg terminals to connect to the boat.. I think just using lugs between cells would also be fine and a simple solution.
That's the direction I'm leaning toward. The busbars might be fine, but I don't think I'd sleep easy. My boat is my family's home, and I have no margin for error. The braided copper looks really nice, but:
a) I am not sure about sourcing the materials in my timeframe
b I don't have a big enough vice on the boat that I'm confident would give me a gas-tight connection, between the ends and braid, and I fear increasing resistance over time. Corrosions on a boat is a big concern.

So I think I'm settled on the wire. Downsides are the labor of making the wire connectors, and the fact that the current capacity (before damaging the conductor) of bare bus bars is higher than insulated wire for the same amount of copper. I wonder what the thickest possible wire I can use before it is simply too stiff to work with and curve between adjacent terminals.
 

ohthetrees

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My first experience of assembling a pack of eve 280 cells on the bench and charging absolutely schooled me in solid bus bars being potentially bad. My experience is going to go against the grain of thinking a clamped pack minimises terminal strain.

My 4s pack was clamped between 10mm aluminium end plates, with 4mm aluminium plates between the cells, and 10x 6mm rods clamping it all together probably tighter than it should have been (cells at 50% charge).

After first full charge, on removing terminal nuts, there was a very noticeable click as every terminal moved relative to busbar. I repeated this.

Not only that, on full discharge, the end plates were loose despite being very well tightened at 50%

The braided busbars are a great solution if you can make them, they are bloody expensive to buy, so I've gone for cable and lugs.

This is my second 2p4s pack that I've done differently.
This post is absolute gold, thank you. Those clicks as tension releases confirms my growing suspicion of solid bus bars and your slop at low SOC throws into doubt how good an idea a simple (no spring) clamping fixture is for mobile/offroad situations.

I like your new pack solution, and I think mine will look similar. What gauge wire is that? It looks fairly small, and I'm expecting some high amperages from time to time, so I'm trying to figure out the biggest wire I can use that isn't too stiff to work with. You seem to have gone with a lot of small rods and springs, rather than fewer large ones. What was your thinking? And what was your source for springs?
 

Colonel.lp

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The wire is 25mm² (2awg) welding wire, it's spec'd at 170amps nominal from the suppliers. And the pack is 2p4s, so theoretically if the cable between the 2ps was bigger it could handle 340amps, but I've used 25mm² there too. I'm not sure I would trust it drawing anywhere near that, but it's for a camper and the biggest load I have is a 1200VA inverter that's rarely used.. I'm lucky if I'm drawing more than 5 to 10amps.

I may have a spark of inspiration and make braided busbars at some point, but Im not sure I can make them to the spec I'd like, the biggest problem being I'd like them tinned if they are going up against aluminium, and the only way I can see that working without making a soldered mess would be to have a solder pot.
 

Nardus01

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25mm² = 4AWG isn`t it ?
35mm² = 2AWG

25mm² will do 1500w/115Amps @ 13volt
35mm² will do 2500w/190Amps @ 13volt

You have to match the wiring to your BMS, so 25mm²: max 120 amp BMS, 35mm² : max 200A BMS
 

Steve_S

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Interesting thread. A couple of observations though...
1) No one has mentioned the use of Lock Washers on the busbars - I use & recommend Serrated Washers as they are superior to split washers and keep an even balanced pressure.

2) Properly Binding & Securing your cell blocks is essential it is the correct method to use with the cells. Please Follow the cell Specification Data sheet information. Different brands & casing types have varying requirements.

3) I should point out that if people look at properly assembled commercial battery packs, quite often the busbars are welded to the cell pads and that those same cell packs are properly bound and blocked. Not only is that shown in some of Will's teardown but others as well. Not talking about the cheaper Chinese pre-builts (even some of them are starting doing it now) but the Higher end stuff.

A thought on Cable/busbars, that is 2 lugs (2 crimps) per, 2 possible points of issue, resistance etc.
Lithium Based Batteries are Not "Brute Force" like Lead, remember they ARE MILLIVOLT Sensitive and that goes for their sensitivity to Resistance !
REMEMBER why everyone is so picky about Matched Cells and Equal Resistances ! KISS Rules are always Best ! Time Tested & True. Do consider consequences and the potentials, sometimes a Good Solution may not actually be because of the side-effects. Also, all it takes is one bad crimp to overheat and cause a potential fire, no different than a loose bolt on a busbar.

4) Flexible Busbars are a good idea IF properly made from the correct materials, relative to application & environment. In a Marine Environment, Tinned Pure Copper mesh & lug-ends with Insulation is the way to go. See below. You can buy tinned mesh, sleeving as well and DIY them. Same with untinned, BUT you have to be confident that the connections/compression is "perfect". A Solid busbar has no weaknesses. It's easy to say and not hard to do but it is not easy to Make Properly, look closely at the commercially made ones & you'll see. (errant copper strands wreak havoc)

1630057010029.png
1630057067517.png
These are identical flexbars from Erico
REF: https://www.erico.com/category.asp?category=R2918

These & Others available at:


Hope it helps, Good Luck
Steve
 
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Colonel.lp

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25mm² = 4AWG isn`t it ?
35mm² = 2AWG

25mm² will do 1500w/115Amps @ 13volt
35mm² will do 2500w/190Amps @ 13volt

You have to match the wiring to your BMS, so 25mm²: max 120 amp BMS, 35mm² : max 200A BMS
Yup you are right, I don't work in awg, wrong line on table. And I'm just going on the manufacturer spec for welding cable. It's still above my usage.

I don't have to match my cable to my BMS as it's an electrodacus. So it's sized for the cutoff relays and fuses, and I'd argue just because you have a BMS that can supply a certain current, doesn't mean you have to use it, you spec your cables with overhead for your use and fuse accordingly.
 
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