Review of Capacity and Terminal Temperature - cells from Shenzhen Luyuan (Amy Wan)

Mike-B

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Reading this with some interest. Was this done using the supplied busbars?

I only ask as these new studs are a few mm too short in my situation. By the time you have a busbar, the thickness of a decent 50mm2 lug and the ring of a BMS lead, the nut only just has enough thread to grab- it’s on, but the stud isnt even flush with the top of the nut. Was hoping to use some thicker busbars but looks like I’ll have to go with the ones from Amy and Co.
 

Hedges

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Maybe you can get longer studs. Or are these laser-welded to terminals?
Milled recess in busbar for nuts?
Is this an issue for single layer busbar, or stacked?
 

Just John

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Maybe you can get longer studs. Or are these laser-welded to terminals?
Milled recess in busbar for nuts?
Is this an issue for single layer busbar, or stacked?
If you want longer studs, ask. At least that's what Amy says on the website.
She tells you the standard stud length, and says you can ask for custom specs. I haven't tried that, my recent 105AH cells purchased from her came with 4mm holes drilled from the factory (and you can tell, compared to drilled aftermarket for the 280AH cells). The larger cells come with blanks from the factory, it's only the smaller capacity cells that come from the factory pre-drilled and tapped.
 

Mike-B

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Maybe you can get longer studs. Or are these laser-welded to terminals?
Milled recess in busbar for nuts?
Is this an issue for single layer busbar, or stacked?
These were the welded studs. Didn’t have the option of custom but it was only a 4S order. The studs on mine are 10mm long FWIW.
 

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Hedges

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Looks like just barely long enough.
Busbar could be wider. Then could be thinner if necessary.

Be sure to prepare and treat bare aluminum for low-resistance contact.

Make sure nothing is going to move cable and cause terminal to rotate nut.
 

Stepandwolf

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The one thing I dislike about the newer welded posts is the reduced surface area of the terminal shoulder. The shoulder should be the main interface for current flow. Reducing the surface contact area just increases the current density over the contact interface.

Cell internal heating begins to show above about 0.5C current. This cell design has thick LiFePO4 cathode, meaning they have a lot of capacity for given weight and volume, but suffer more from ionic layer starvation at higher currents which gives more terminal voltage drop and lower efficiency at the higher C rate currents.

These cells work best (and last longer) below 0.5C so if you need more current consider adding another bank in parallel.
What about the 2mm washers that Lt. Dan made to make the terminal area flat?
 

filippomasoni

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Really valuable work done here, these tests are important!
I ordered 8 280ah cells from Amy, new version just like these ones, a few weeks ago.
I wasn't sure if I had to capacity test all of my cells, but I guess I don't have to now, as everyone seem to speak well about cells from Amy. Maybe I should just capacity test the whole pack once assembled?
 

Gazoo

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Really valuable work done here, these tests are important!
I ordered 8 280ah cells from Amy, new version just like these ones, a few weeks ago.
I wasn't sure if I had to capacity test all of my cells, but I guess I don't have to now, as everyone seem to speak well about cells from Amy. Maybe I should just capacity test the whole pack once assembled?
I didn't bother to capacity test each of my LF280's. I did parallel top balance them, then connected in series with a BMS and capacity tested.

My cells were supposed to be matched but were not. My battery tested at 272ah's or 97% capacity and for what I paid, around $830.00, I am happy with that. Looking at the voltages of the BMS I can see which cell is the weakest and which cell is the strongest when the voltages start to diverge around 3.00 volts.

One of the biggest advantages of having a matched set is the delta will be low towards the end of discharge. In my case my BMS will cut out before my inverter because my cells are not matched. It's supposed to be the other way around. Of course a programmable inverter would solve this problem.
 

filippomasoni

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I didn't bother to capacity test each of my LF280's. I did parallel top balance them, then connected in series with a BMS and capacity tested.

My cells were supposed to be matched but were not. My battery tested at 272ah's or 97% capacity and for what I paid, around $830.00, I am happy with that. Looking at the voltages of the BMS I can see which cell is the weakest and which cell is the strongest when the voltages start to diverge around 3.00 volts.

One of the biggest advantages of having a matched set is the delta will be low towards the end of discharge. In my case my BMS will cut out before my inverter because my cells are not matched. It's supposed to be the other way around. Of course a programmable inverter would solve this problem.
Are your cells from Amy?
I received a capacity test sheet before shipping where they were all around 289-290ah.
I was planning on doing exactly like you and capacity test the whole pack as I don't want to buy 8 separate capacity testers to do everything at the same time. Doing it one by one without compression is not something I would like to do.
 

Just John

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Really valuable work done here, these tests are important!
I ordered 8 280ah cells from Amy, new version just like these ones, a few weeks ago.
I wasn't sure if I had to capacity test all of my cells, but I guess I don't have to now, as everyone seem to speak well about cells from Amy. Maybe I should just capacity test the whole pack once assembled?
Probably easiest to test an entire pack. Most people have an inverter and shunt which is perfectly adequate to test an entire pack. I know mine certainly tested above the stated capacity, and that was in a garage above 90 degrees. If you want to test individual cells, those cheap capacity testers will work fine, they are just slow.
 

filippomasoni

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Probably easiest to test an entire pack. Most people have an inverter and shunt which is perfectly adequate to test an entire pack. I know mine certainly tested above the stated capacity, and that was in a garage above 90 degrees. If you want to test individual cells, those cheap capacity testers will work fine, they are just slow.
Yes exactly. I thought about buying ane of those cheap testers and run one battery at a time, but after I learned about proper compression for longevity of the pack I would need 8 of those to make a proper test, like OP did but that would become too expensive and wasted electronics as I would never need those again
 

Just John

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Yes exactly. I thought about buying ane of those cheap testers and run one battery at a time, but after I learned about proper compression for longevity of the pack I would need 8 of those to make a proper test, like OP did but that would become too expensive and wasted electronics as I would never need those again
In 5 years (when uncompressed cells are supposed to be nearly end of life), I suspect that much better and cheaper batteries will be available. Compression is nice, but I suspect in the next two or three years it will be obsoleted by the 4680 LiFePO4 cells.
 

filippomasoni

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In 5 years (when uncompressed cells are supposed to be nearly end of life), I suspect that much better and cheaper batteries will be available. Compression is nice, but I suspect in the next two or three years it will be obsoleted by the 4680 LiFePO4 cells.
Yes of course, tech is constantly evolving and we've seen great improvement over the last 2-3 years, so it will be even bigger going forward. I might even upgrade before end of life if it could result in a great improved, but if these last me 5 yeas of daily use without issue that's a great win in my mind.
 

Just John

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Yes of course, tech is constantly evolving and we've seen great improvement over the last 2-3 years, so it will be even bigger going forward. I might even upgrade before end of life if it could result in a great improved, but if these last me 5 yeas of daily use without issue that's a great win in my mind.
Uncompressed cells are rated for 2500 cycles. Maybe I am missing something, but 2500 ÷ 365 = 6.8 years.

Certainly 3500 cycles with compression works out to almost 10 years, but if you are happy with 5 years, don't bother.
 

Gazoo

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Are your cells from Amy?
I received a capacity test sheet before shipping where they were all around 289-290ah.
I was planning on doing exactly like you and capacity test the whole pack as I don't want to buy 8 separate capacity testers to do everything at the same time. Doing it one by one without compression is not something I would like to do.
I got my cells from the group buy.

Since the delta of data sheet you received looks good I wouldn't bother parallel top balancing. Most people keep cells between the knees anyways and that helps extend cycle life.

The only BMS I am familiar with are the ones Overkill Solar sells otherwise known as JBD. Mine is accurate enough for testing capacity but the battery has to be cycled fully once to get accurate results. I always recommend testing The HVD and LVD of the BMS before putting it into service. A cheap shunt based coulomb meter would be good to have to compare results but IMO it's not necessary when buying capacity matched cells.
 

RCinFLA

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I missed this thread earlier. Interesting how everyone jumped to the conclusion that terminal temp rise is caused by terminal contact resistance heating.

I have a spreadsheet that I have been developing for a while to predict internal cell temp rise based on cell current demand for a thick electrode (120-150 um) 280 AH cell. I did not have 100% confidence because I had to make a few guessimates on heat transfer properties for cell internal material.

Anyway, the interesting and surprising thing is my spreadsheet predicted 34 deg C cell internal temp from a starting ambient cell temp of 25 degs C for a discharge of 0.6 CA rate. I am sure there will be some extra terminal contact resistance heating that will increase terminal temp some amount above battery internal temp but it appears much of the temp rise shown at the terminal is due to cell internal heating at the 0.6 CA discharge rate. All of the internal cell copper foil layers for anode connections and aluminum foil layers for cathode connections conduct the internal cell heating directly up to the terminals.

The real concern should be the internal heating of the cell which does more permanent damage to the LFP cell.

Just for fyi, the spreadsheet predicts an internal cell temp of 45 degs C at a 1 CA sustained discharge rate. This is why I believe the thick electrode cells should not be used with a sustained discharge or charge current above 0.5 CA. Think about this when cells are tightly packed together like sardines in a can. The irony of contradiction is cell compression does not provide much benefit until current is so high to cause cell internal heating but tightly compressing the cells together makes it harder for cells to dissipate internal heating.

Because of the thick electrode design, which is done to maximize cell AH for its size and weight, to the detriment of maximum CA current handling capability, these 280 AH cells will experience local layer ion starvation onset around 0.5 CA current demand. This causes cell losses to increase at a faster rate above 0.5 CA. At 0.5 CA sustained current there is about 11-12 watts of internal cell heating. At 1.0 C sustained current there is about 35-40 watts of internal cell heating.
 

Just John

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I missed this thread earlier. Interesting how everyone jumped to the conclusion that terminal temp rise is caused by terminal contact resistance heating.

I have a spreadsheet that I have been developing for a while to predict internal cell temp rise based on cell current demand for a thick electrode (120-150 um) 280 AH cell. I did not have 100% confidence because I had to make a few guessimates on heat transfer properties for cell internal material.

Anyway, the interesting and surprising thing is my spreadsheet predicted 34 deg C cell internal temp from a starting ambient cell temp of 25 degs C for a discharge of 0.6 CA rate. I am sure there will be some extra terminal contact resistance heating that will increase terminal temp some amount above battery internal temp but it appears much of the temp rise shown at the terminal is due to cell internal heating at the 0.6 CA discharge rate. All of the internal cell copper foil layers for anode connections and aluminum foil layers for cathode connections conduct the internal cell heating directly up to the terminals.

The real concern should be the internal heating of the cell which does more permanent damage to the LFP cell.

Just for fyi, the spreadsheet predicts an internal cell temp of 45 degs C at a 1 CA sustained discharge rate. This is why I believe the thick electrode cells should not be used with a sustained discharge or charge current above 0.5 CA. Think about this when cells are tightly packed together like sardines in a can. The irony of contradiction is cell compression does not provide much benefit until current is so high to cause cell internal heating but tightly compressing the cells together makes it harder for cells to dissipate internal heating.

Because of the thick electrode design, which is done to maximize cell AH for its size and weight, to the detriment of maximum CA current handling capability, these 280 AH cells will experience local layer ion starvation onset around 0.5 CA current demand. This causes cell losses to increase at a faster rate above 0.5 CA. At 0.5 CA sustained current there is about 11-12 watts of internal cell heating. At 1.0 C sustained current there is about 35-40 watts of internal cell heating.
Indeed, even for very light cell discharge (40 amps) I measure a cell temperature rise of 5 degrees C during the last 30 to 40 ams hours out of a cell.

However, I have found that I must thoroughly clean each cell and busbar connection every single time that I assemble/disassemble a pack. I even bought a thermal camera to be able to see both the cell temperature from discharge, and bad connections. It has turned out to be very useful in diagnosing or preventing any problems.

I am of the opinion the latest flood of bloated cells from vendors is likely due to over discharge, rather than over charging. But that is my opinion, not facts.

I agree with your analysis that it is very much in your own best interest to keep discharge rates low, below 0.5 C, and preferably even lower.
 
Last edited:

RCinFLA

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I am not saying a poor terminal connection doesn't cause heating. Aluminum oxide forms very fast. Clean'um and clamp'um as fast as possible.
 

curiouscarbon

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I missed this thread earlier. Interesting how everyone jumped to the conclusion that terminal temp rise is caused by terminal contact resistance heating.

I have a spreadsheet that I have been developing for a while to predict internal cell temp rise based on cell current demand for a thick electrode (120-150 um) 280 AH cell. I did not have 100% confidence because I had to make a few guessimates on heat transfer properties for cell internal material.

Anyway, the interesting and surprising thing is my spreadsheet predicted 34 deg C cell internal temp from a starting ambient cell temp of 25 degs C for a discharge of 0.6 CA rate. I am sure there will be some extra terminal contact resistance heating that will increase terminal temp some amount above battery internal temp but it appears much of the temp rise shown at the terminal is due to cell internal heating at the 0.6 CA discharge rate. All of the internal cell copper foil layers for anode connections and aluminum foil layers for cathode connections conduct the internal cell heating directly up to the terminals.

The real concern should be the internal heating of the cell which does more permanent damage to the LFP cell.

Just for fyi, the spreadsheet predicts an internal cell temp of 45 degs C at a 1 CA sustained discharge rate. This is why I believe the thick electrode cells should not be used with a sustained discharge or charge current above 0.5 CA. Think about this when cells are tightly packed together like sardines in a can. The irony of contradiction is cell compression does not provide much benefit until current is so high to cause cell internal heating but tightly compressing the cells together makes it harder for cells to dissipate internal heating.

Because of the thick electrode design, which is done to maximize cell AH for its size and weight, to the detriment of maximum CA current handling capability, these 280 AH cells will experience local layer ion starvation onset around 0.5 CA current demand. This causes cell losses to increase at a faster rate above 0.5 CA. At 0.5 CA sustained current there is about 11-12 watts of internal cell heating. At 1.0 C sustained current there is about 35-40 watts of internal cell heating.
thank you taking the time to type this out!

i believe you!


the video references “lipo” but the fundamental thermodynamic phenomenon of tab heating is applicable to LiFePO4 cells in my estimation.

TL;DW the video claims that cooling the tabs/terminals of the cell will more effectively regulate the temperature of the entire cell material.

They say that cooling the cell wall caused the outer layers of jelly roll to operate cold and less efficiently while simultaneously not directly cooling the hottest inner parts.
 
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