25R low lifespan?

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rg12

100 kW
Joined
Jul 26, 2014
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1,591
Just tested a 10 month old pack of a customer of mine and the pack started sagging really bad lately, it was about 5V sag under 110A load when it was healthy and about a month ago the sag started increasing to even 15-20V.
I tested the sag while bypassing the BMS and it was the same.
The pack never heated up, was always room temp, it looks physically brand new.
The pack was built with original fresh stock (tested the codes on the internet) Samsung 25R cells.
It's a 72V 20Ah built with 0.15mm 8mm wide pure nickel.
The first and last cell rows have each cell connect to a copper busbar for even current distribution.
Pack is perfectly balanced.
The guy was riding it everyday and 110A is usually bursts, even if on the highway on full throttle it never pulls constant 110A.
Even if it was constant 110A, thats 13.75A (burst) from each row of cells which is below the 20A rating of those cells.
I'm clueless, could it be that this is what I should expect as the longevity of packs under that amount of stress on a daily basis for 10 month?
 
Ever day for 10 months = 305 cycles.
How deep of a discharge cycle each day ????
If it's close to 14A per cell and 305 full cycles, than yes I would expect that kind of lifespan, but that's just my opinion...
Matador
 
25R go to 80% capacity pretty quickly but that kind of sag sounds like he may have a wiring issue, like cells in one or more parallel group are not fully connected and those groups are dropping more volts than expected under load.

In my experience 25R are reliable cell and stand up to abuse, but if this guy wants minimal sag look into Turnigy Graphene.
 
That sounds pretty normal for a 25R pack, from what I've read. Once you are certain you are happy with a specific voltage, one option is to build up a pack from high-amp LiFePO4 flat foil packs, and use side-panel compression to hold them flat. LiFePO4 is known to last many times longer than NCA/NCM.

It might not have quite as much Amp-hours as NCA/NCM, but you can get high amps and long life from LiFePO4

the voltage is lower too, so you will need 16 cells in series for 48V, instead of the common 13S
 
All welds are in place, physically brand new.
Will test each row of cells under load to see if there is a row that sags more than the others and update here.
Could it be maybe loss of conductivity from the nickels? Is there such thing? each cell is connected with 8mm wide 0.15mm thick pure nickel and 20Ah is 8 cells in parallel meaning that the bursts of 110A went through 8pcs 8*0.15 nickel strips.
 
rg12 said:
All welds are in place, physically brand new.
Will test each row of cells under load to see if there is a row that sags more than the others and update here.
Could it be maybe loss of conductivity from the nickels? Is there such thing? each cell is connected with 8mm wide 0.15mm thick pure nickel and 20Ah is 8 cells in parallel meaning that the bursts of 110A went through 8pcs 8*0.15 nickel strips.
Click to expand...

I'm afraid that your customer needs a new bigger more powerful battery pack and he will have it for about two years if he uses it hard everyday. I use my ebike every day and 30Q cells has started to sag quiet a lot after a year. 25R cells have worst cycle life so it seems to be normal to me.

He can give his battery pack to another guy to use it with maximum 40-50 amps and you can build at least a new 20s - 14p battery pack for him.
 
Nickle has only 22% the conductivity of copper and with a tiny spot welds , it's like drinking water with a coffee stirrer .

It is using a 24 gauge copper wire (copper equivalence ) with a 30% drop in voltage at 110 amps on the last connection in parallel .

Cycle life is determined by how hard they are used, They rate them by 0.5C or less to get @1000 cycles.

I agree with Matador about his assessment

Jim
 
I just talked to the guy, it's 10 month old and only about two month ago it went from 5V sag to 20V in a matter of a few weeks.
It doesn't seem right that less than a year and your pack is toast since 20V sag is unusable.
13.75A per cell is not a lot for a 25R cell and even that is for bursts.
The pack with no load is 4.11 per cell and all perfectly balanced and just now tested balance under load of about 70A and the voltages range from 3.75 to 3.55

Do you think adding a second layer of nickel will do any good?
 
rg12 said:
I just talked to the guy, it's 10 month old and only about two month ago it went from 5V sag to 20V in a matter of a few weeks.
It doesn't seem right that less than a year and your pack is toast since 20V sag is unusable.
13.75A per cell is not a lot for a 25R cell and even that is for bursts.
The pack with no load is 4.11 per cell and all perfectly balanced and just now tested balance under load of about 70A and the voltages range from 3.75 to 3.55

Do you think adding a second layer of nickel will do any good?
Click to expand...

maybe then there are some bad cells that ruin all the battery pack. by the way you have 0,2V difference under load, my pack after 16 months of use doesn't have more that 0.055V voltage difference under load.

There are several ways to find the problematic cells. Balance all the cells, leave them for 5-7 days to rest and the ones that loses voltage cause you the problem or another way under load you check which parallel group sags more and there you are probably there is a dead cell there.
 
rg12 said:
All welds are in place, physically brand new.
Could it be maybe loss of conductivity from the nickels? Is there such thing? each cell is connected with 8mm wide 0.15mm thick pure nickel and 20Ah is 8 cells in parallel
Click to expand...

No. Nickel conductivity will not change over time. The conductivity (which is the inverse of the resistivity mesured in ohm x meters) is a constant value that does not change over time. The few micrometer thickness of nickel oxide that form over time on the surface actually has a protective effect on furter oxidation, on the contrary to rust (iron oxide) wich spreads like cancer once it attact iron or carbon steel.

https://endless-sphere.com/forums/download/file.php?id=223364&mode=view (in the case of thes 8mm x 0,15mm strips we're talking 58,3mOhm per meter (or ampacity of around 3,6A being very conservative)
 
Also, pulling 110A through 8 of these is pushing it too far... 14Amp throug each 8mm x 0.15mm nickel strips is very likely to generate much more heat than the discharging cells themselves from their intrinsect internal resistance. The nickel strips can take 3.6amps .... maybe 6 to 8 amps for short burst... not much more. Iam inclined to think that at these amps, the nickel strips are acting as "heater elements" and are likely to have contributed to cooking the cells "well done". Basically, just like applying a warm to hot soldering iron for 10 months on both the positive and the negative sides of each cells. Lithium cells don't like to much heat... Either double up the nickle, go copper or put more cells in parallel.

Personally, I would not ask more than 8A, max 10A per 25R cells to reach a good sweetspot between lifespan and performance per weight ratio. That's just my gut feeling though. Setup for 6A per 25R cell and your pack will last 4 years or more.
 
look at Samsung's datasheet for the 25R.
~125 cycles to 80%, at probably a 1C draw.
The pack was pulling a max of about 5.5C, which is kind of pushing it.

We don't know what the average C was during a full discharge, but i'd imagine it's what.. 3C?
So you're probably talking about a faster rate of capacity loss than the spec sheet.

So you've got two things going on..
1) This was a short life cell to begin with.
2) The cell has lost significant capacity over what i assume is ~300 cycles if used daily, and the internal resistance has risen significantly.

What was an advertised 8C cell is now more like a 4-5C cell and you're still trying to draw 5-6C peak.. of course it sags like crazy. Cold weather might be part of the culprit too, in addition to the loss of capacity.

A 4-5C cell at 3C will cook itself in a pack where all the cells are sealed up and sharing heat. So if the average draw is around 3C, we can expect the degradation to go exponential from here on out, unless the current is turned down significantly... or your customer is riding in near freezing temps.. but next summer, he's gonna have a bad time!

This is why we need lots of headroom in our designs. I always say that you should always wildly overspec your C rating relative to what cells are rated for. Consider that the maximum continuous C rating is rated based on whether the cell explodes or not in a temperature cooled lab with a single cell. Your battery pack is a bunch of these cells with no air passing through them. You can take this optimistic maximum C lab rating and cut it in half.. and your average current draw, ideally, would be a 1/4th of the max.
 
Looking for a new pack. The Samsung 25r were the best compromise ? The problem I see is the connections. How to get a rated buss for this high discharge cell. I was thinking of a nickel with a layer of copper and holes over the cells to spot weld. Plus I'm Spoiled with 5yr old. A123 20ah pouches @ 90amps.
 
Check out Headway or AHR32113 orAHR32157, They are bolt connections and in a 72v ,1.4kw combination puts out 200amp continuous
 
there is no high capacity cell that cen deal with these loads for any lifespan at those current levels.
headways will also die under those loads.

you need a bigger battery. more cells in P and crank the voltage (more S) if possible. but you need physcially more cells to spread the load.
preferaby until you get a maximum of 1C discharge at speed. so the battery needs to be 4~5 times as big.
 
neptronix said:
look at Samsung's datasheet for the 25R.
~125 cycles to 80%, at probably a 1C draw.
The pack was pulling a max of about 5.5C, which is kind of pushing it.

We don't know what the average C was during a full discharge, but i'd imagine it's what.. 3C?
So you're probably talking about a faster rate of capacity loss than the spec sheet.

So you've got two things going on..
1) This was a short life cell to begin with.
2) The cell has lost significant capacity over what i assume is ~300 cycles if used daily, and the internal resistance has risen significantly.

What was an advertised 8C cell is now more like a 4-5C cell and you're still trying to draw 5-6C peak.. of course it sags like crazy. Cold weather might be part of the culprit too, in addition to the loss of capacity.

A 4-5C cell at 3C will cook itself in a pack where all the cells are sealed up and sharing heat. So if the average draw is around 3C, we can expect the degradation to go exponential from here on out, unless the current is turned down significantly... or your customer is riding in near freezing temps.. but next summer, he's gonna have a bad time!

This is why we need lots of headroom in our designs. I always say that you should always wildly overspec your C rating relative to what cells are rated for. Consider that the maximum continuous C rating is rated based on whether the cell explodes or not in a temperature cooled lab with a single cell. Your battery pack is a bunch of these cells with no air passing through them. You can take this optimistic maximum C lab rating and cut it in half.. and your average current draw, ideally, would be a 1/4th of the max.
Click to expand...

Whats weird is that the pack was always room temp, if it was heating issues from the nickels or the cells then I can understand it.
 
as I said before it is very simple. Check for dead cells, if there aren't any it means the pack is at the end of it's useful life for this setup.
 
rg12 said:
Whats weird is that the pack was always room temp, if it was heating issues from the nickels or the cells then I can understand it.
Click to expand...

Just cycles then, if that's the case.
I googled the datasheet. See page 8.
https://www.powerstream.com/p/INR18650-25R-datasheet.pdf

If the user put a 300 cycles on it, each cell is now ~1.6AH and probably performs more like a 4C cell.
If it always ran cool when new, it probably runs warm/hot now.

For a pack like this, i would have used 3AH cells and a hell of a lot of them ( 30AH or more ), but with lower peak current.

https://eu.nkon.nl/sk/k/30q.pdf

Think about it.. a full cycle on a 30AH pack lasts 50% longer than a 20AH pack. In addition to this, a cell like a Samsung 30Q hits 80% at ~225 cycles whereas the 25R hits 80% at ~125 cycles..

The larger 30Q pack would produce about 2.5x more miles before it's toast..

You could also combine this with the trick of balance charging to 4.15v and make it last 3.0x longer, at the expense of only using ~27.5AH of the 30AH.

So at best.. the battery can last around 3 years with daily use on a high power / high speed build like that.. it gets better if you have more battery space and can accept more weight.. with one of those tank-sized enduro frames, you might be able to get away with 3.5AH cells and a 50AH 72V pack or something crazy like that. :)
 
Ok talked to my customer again, something isn't right here...
Out of 10 month he used them about once a week for the first 7 month and then for 3 month daily.
The last 1.5 month out of the 3 month of daily use the voltage started sagging more and more from 5V to about 20V today.
I weighed the cells which came from my trusted supplier that I worked with from day one and they weigh 43.5g compared to the stock I have right now which also came from them that weighs 44.8-45.2g

Any ideas?
 
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