Lifepo4 bus bars

[Bart]

So the bus bars that came with my lifepo4 280ah batteries are the solid rigid plate type

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Im worried that with battery expansion when charging that overdue pressure will be put on the battery bolts.
Would it make any difference to charging / resistance etc if I made my own bus bars (all the same length) out of 25mm2 cable with lugs on each end.

 

[autorouter]

What's the thickness of these busbars? They are 20mm wide, so if 2mm thick the cross-sectional area is 40mm2. The idea of a busbar is that they are so thick that there is negligible voltage drop along any part of it. If you use standard cable the (small) voltage drop is enough to make the voltages across them unequal, unless you take extra steps to equalise them.

 

[RogerIvy]

Use threadbars to prevent/restrict too much expansion. Do your top balancing with the cells under compression. Then when they are fully top balanced add the busbars.
We place our cells into light compression when they are about 30% SOC. when I say light, we tighten the Allen key using the short arm so it’s pretty light. No spanner’s! When the cells discharge completely they can move a bit in the enclosure. When they are fully charged they don’t move at all.
Experiments that EVE have performed show that with correct compression cells give up to 1000 more cycles. Too much compression is worse than no compression.
Without compression two things go wrong:
1. Internal delamination of the electrodes reducing the lifespan of the cell
2. Structural issues resulting from the rigid busbar. This can be overcome by using flexible busbars. Easy to make your own.

 

[RogerIvy]

…but having said all of the above, if your cells are bad quality they could swell too much and not last long no matter what you do. Even if you compress using springs on threadbars and flexible busbars.

 

[Raul]

You could make them out of roofing copper band, or soakers. The soakers are 0,7mm times 4 at 25mm wide, gives you 70mm2 bar. Put a half round bend in the middle , any two holes in the ends, and you got your links, without introducing two crimps resistance.
I agree that light compression is beneficial, but if you treat them under 0,2C charge/discharge, the compression will not add or take away nothing. Good bms is more important.

This here, gives you 6strips at 25mm by 300 long

SOAKER ROOFING STRIPS - Slate Stone Roof Copper Brass Aluminium 300 x 100 x 50 | eBay

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[Two on Tour]

One thing that is always lacking in cross section is the copper tube crimp terminals on the end of your pos and neg cables attached to your battery posts.

 

[EML]

…but having said all of the above, if your cells are bad quality they could swell too much and not last long no matter what you do. Even if you compress using springs on threadbars and flexible busbars.

Just subscribed to your YouTube channel. Linked it for anyone else who's interested - looks good.

 

[Bart]

Use threadbars to prevent/restrict too much expansion. Do your top balancing with the cells under compression. Then when they are fully top balanced add the busbars.
We place our cells into light compression when they are about 30% SOC. when I say light, we tighten the Allen key using the short arm so it’s pretty light. No spanner’s! When the cells discharge completely they can move a bit in the enclosure. When they are fully charged they don’t move at all.
Experiments that EVE have performed show that with correct compression cells give up to 1000 more cycles. Too much compression is worse than no compression.
Without compression two things go wrong:
1. Internal delamination of the electrodes reducing the lifespan of the cell
2. Structural issues resulting from the rigid busbar. This can be overcome by using flexible busbars. Easy to make your own.

Ty for the super informative info, what voltage would you say is 30% soc

 

[Bart]

Use threadbars to prevent/restrict too much expansion. Do your top balancing with the cells under compression. Then when they are fully top balanced add the busbars.
We place our cells into light compression when they are about 30% SOC. when I say light, we tighten the Allen key using the short arm so it’s pretty light. No spanner’s! When the cells discharge completely they can move a bit in the enclosure. When they are fully charged they don’t move at all.
Experiments that EVE have performed show that with correct compression cells give up to 1000 more cycles. Too much compression is worse than no compression.
Without compression two things go wrong:
1. Internal delamination of the electrodes reducing the lifespan of the cell
2. Structural issues resulting from the rigid busbar. This can be overcome by using flexible busbars. Easy to make your own.

Ty for the super informative info, what voltage would you say is 30% s

…but having said all of the above, if your cells are bad quality they could swell too much and not last long no matter what you do. Even if you compress using springs on threadbars and flexible busbars.

ATM cells came in perfect shape, no bulging at all

 

[RogerIvy]

Below is one of the many voltage curves you will find for LifePO4 batteries (in this case a single cell). It shows the two knees (upper and lower) with the flat part in between. The many curves vary slightly here and there, but the basic idea is that the curve is so flat it’s impossible to use voltage to determine SOC. Many people have tried unsuccessfully to formulate SOC from voltage. This is the reason why the only effective way of determining true SOC is by coulomb counting. Smart shunts etc.

In our case the batches of cells we purchase are really good quality and of very similar SOC when they arrive. When we measure what goes in (when we do a fast top-balance) we know they’re about 30% SOC. In order to comply with shipping laws they can’t be above a certain percentage. I don’t recall exactly what the limit is for land/sea but for air is 30% (So most reputable manufacturers ship them out at 30%).

Some of the cheaper cells we have tested, like the 420ah to-good-to-be-true had huge differences between cells - either they left the manufacturing plant that way and/or self discharge was very high for some cells in transit.

So to answer your question, somewhere around 3.2v

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[Bart]

Just subscribed to your YouTube channel. Linked it for anyone else who's interested - looks good.

Ahh never knew that was Roger from Offgrid Van Life, I've watched like 20 off his and Nigels videos, really really helpful, gonna copy their clamping method of the cells using 18mm plywood and stud bars

 

[Bart]

Below is one of the many voltage curves you will find for LifePO4 batteries (in this case a single cell). It shows the two knees (upper and lower) with the flat part in between. The many curves vary slightly here and there, but the basic idea is that the curve is so flat it’s impossible to use voltage to determine SOC. Many people have tried unsuccessfully to formulate SOC from voltage. This is the reason why the only effective way of determining true SOC is by coulomb counting. Smart shunts etc.

In our case the batches of cells we purchase are really good quality and of very similar SOC when they arrive. When we measure what goes in (when we do a fast top-balance) we know they’re about 30% SOC. In order to comply with shipping laws they can’t be above a certain percentage. I don’t recall exactly what the limit is for land/sea but for air is 30% (So most reputable manufacturers ship them out at 30%).

Some of the cheaper cells we have tested, like the 420ah to-good-to-be-true had huge differences between cells - either they left the manufacturing plant that way and/or self discharge was very high for some cells in transit.

So to answer your question, somewhere around 3.2v

View attachment 548903

All my cells arrived at 3.29v and no visible signs of bloating, do you's usually "clamp" them together when they are at the voltage with which you receive them at? Or do you's usually discharge them to like 2.5v?

 

[RogerIvy]

All my cells arrived at 3.29v and no visible signs of bloating, do you's usually "clamp" them together when they are at the voltage with which you receive them at? Or do you's usually discharge them to like 2.5v?

Yes we clamp them straight away and then do a fast top balance. Once clamped we leave them as is for eternity.

 

[RogerIvy]

These are the threadbars we use:

Easyfix BZP Steel Threaded Rods M6 x 300mm 5 Pack - Screwfix

Order online at Screwfix.com. Steel. BZP. FREE next day delivery available, free collection in 1 minute.

www.screwfix.com

with these nuts:

Joint Connector Nuts M6 x 17mm 50 Pack - Screwfix

Order online at Screwfix.com. Versatile, can be used with joint connector bolts or threaded rod. Suitable for use in a variety of assembly operations. FREE next day delivery available, free collection in 1 minute.

www.screwfix.com

and these tubes:

Amazon product ASIN B008NC7GH2

 

[Bart]

RogerIvy first of all thank you for your's and Nigels youtube videos, super job , 2ndly thanks for the links, I'll prob be starting to install my Basen lifepo4 280ah 4s Bank next week, so I dare say I'll be looking more help^^ and yes the cells are the type with the horrible shallow tapped holes where you screw in the studs, as apposed to lazer welded studs, so I'll need gentle hands, I do plan on using a little red locktite on the studs to ensure they don't move when tightening down the nut, and also to prevent stud movement from vibrations when on the road.

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[Raul]

Hey Bart, you may wanna have a look at this, take your time, you will not be disappointed

 

[RogerIvy]

We’ve tried (and failed) to find a reasonably priced supplier of flexible busbars in the UK. I’d prefer flexible if they were a reasonable price.

 

[Two on Tour]

Here's my new battery pack that I have just built using CATL 271Ah cells
I built it to suit our needs and usage as we will not be drawing big amperage from it, it's the capacity we want as we rarely use ehu.

One bone of contention in the LifePo4 self-build battery fraternity at present is the subject of cell compression.
Personally I'm not convinced, and for the way we are going to be using this new battery pack it is not necessary in my view as the cell manufacturer's aluminum containers are made already to hold the cell contents at 300 kgf as indicated on their spec sheets and by this statement on one manufacturer's specification under cycle life it says "The battery cell is under the action of preset 300kgf force".
There seems to be no definitive answer to compressing cells or not as the cell manufacturer's are not giving clear guidance one way or the other or how to implement cell compression if required leading to everyone having an idea of how to do it.

 

[Ditcha]

Here's my new battery pack that I have just built using CATL 271Ah cells
I built it to suit our needs and usage as we will not be drawing big amperage from it, it's the capacity we want as we rarely use ehu.

View attachment 549351


One bone of contention in the LifePo4 self-build battery fraternity at present is the subject of cell compression.
Personally I'm not convinced, and for the way we are going to be using this new battery pack it is not necessary in my view as the cell manufacturer's aluminum containers are made already to hold the cell contents at 300 kgf as indicated on their spec sheets and by this statement on one manufacturer's specification under cycle life it says "The battery cell is under the action of preset 300kgf force".
There seems to be no definitive answer to compressing cells or not as the cell manufacturer's are not giving clear guidance one way or the other or how to implement cell compression if required leading to everyone having an idea of how to do it.


View attachment 549352


View attachment 549355

What's the black box next to the BMS if you don't mind me asking thanks?

 

[Two on Tour]

What's the black box next to the BMS if you don't mind me asking thanks?

It's an active cell balancer.
The Daly Smart BMS only has a 30ma balancing current which struggles on a larger cell capacity, so I'm using a standalone active cell balancer that I was using on our previous LifePo4 battery.

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[Ditcha]

It's an active cell balancer.
The Daly Smart BMS only has a 30ma balancing current which struggles on a larger cell capacity, so I'm using a standalone active cell balancer that I was using on our previous LifePo4 battery.

View attachment 549363

Great job

 

[EML]

One thing that is always lacking in cross section is the copper tube crimp terminals on the end of your pos and neg cables attached to your battery posts.

True, but they're also very much shorter than the cable run, so have a relatively low resistance.

Very back-of-the-envelope argument: if the terminal and the cable have similar resistivity, and the terminal has a 7mm run to the battery post, and the cable is 100mm, and the terminal has half the cross-sectional area of the cable, then each terminal has a resistance of about 15% that of the cable. So the overall assembly resistance increases by about 30% after crimping.

The quality of the crimping might be an issue, though.

 

[EML]

True, but they're also very much shorter than the cable run, so have a relatively low resistance.

Very back-of-the-envelope argument: if the terminal and the cable have similar resistivity, and the terminal has a 7mm run to the battery post, and the cable is 100mm, and the terminal has half the cross-sectional area of the cable, then each terminal has a resistance of about 15% that of the cable. So the overall assembly resistance increases by about 30% after crimping.

The quality of the crimping might be an issue, though.

Ok - just watched the first 3 minutes of Raul's video, and it looks like they'll go through this anyway. The other 33 minutes will have to wait for a w/end...

 

[Raul]

The interesting comparison is the thermal imaging with the flat bars vs crimped cable jumpers. On the flat bar you can see 4 distinctive hot spots. None on the cable jumpers.

 

[SandraL]

Back in the 1970's I worked in telephone exchanges. The 50v dc power was carried by aluminium busbars.
The joints were draw filed, the joint area smeared in vaseline, and a clamp used to connect 2 overlapping bars.
Then a heavy current passed through joint, 100amps? And a galvanometer used to check the voltage drop.
It sometimes took a few attempts on the wider 6" bars to get a good joint.

I assume the lithium connecting bars are thin enough to mold to the terminals and not require fettling.

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[Ditcha]

Back in the 1970's I worked in telephone exchanges. The 50v dc power was carried by aluminium busbars.
The joints were draw filed, the joint area smeared in vaseline, and a clamp used to connect 2 overlapping bars.
Then a heavy current passed through joint, 100amps? And a galvanometer used to check the voltage drop.
It sometimes took a few attempts on the wider 6" bars to get a good joint.

I assume the lithium connecting bars are thin enough to mold to the terminals and not require fettling.

I suppose its how flat are the busbars? they are most probably lasered out or punched, if punched it could cause some distortion and if lasered limited by quality of raw material.

It would be good to blue the bars and see what contact they have with the battery terminal for piece of mind then if not a bit of fettling like you say to seat them flat.

 

[Two on Tour]

Back in the 1970's I worked in telephone exchanges. The 50v dc power was carried by aluminium busbars.
The joints were draw filed, the joint area smeared in vaseline, and a clamp used to connect 2 overlapping bars.
Then a heavy current passed through joint, 100amps? And a galvanometer used to check the voltage drop.
It sometimes took a few attempts on the wider 6" bars to get a good joint.

I assume the lithium connecting bars are thin enough to mold to the terminals and not require fettling.

With LifePo4 cells, I give the aluminum surface of the terminal posts a light going over with a needle file to remove any oxidisation before applying a smear of dielectric grease to them to prevent further oxidisation.
Each of the crimp terminals on the flexible busbars and balance leads gets a going over with a stainless steel wire brush, then they too get a smear of dielectric grease before being torqued down.