Discharge Rate .2C or higher for matching cells?

Shale MacGregor

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I understand that .2C is the general rule of thumb for discharge rate in testing capacity, my question is if my primary purpose is matching cells for capacity (especially when they might be used in a situation with higher than .2C discharge), is can I discharge at a higher C to get through the test more quickly? I've tried some cheaper testers and some series discharge options and everything is getting more complicated and or taking longer.

I have a very nice siglent load tester that I can generate a pretty graph for and it can easily handle 30 amps but I have been sticking with 11 amps for .2C. 30 amps would be closer to .6C

Thanks for any information!
 

FilterGuy

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If you are only trying to check relative capacity between cells your are testing, I see no reason why you could not use a higher C rate for the test.
 

fafrd

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If you are only trying to check relative capacity between cells your are testing, I see no reason why you could not use a higher C rate for the test.
And testing at a C-rate as close as possible to your max discharge rate is the best way to see whether the IR of your cells is well-matched…
 

Shale MacGregor

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And testing at a C-rate as close as possible to your max discharge rate is the best way to see whether the IR of your cells is well-matched…
I do have an IR tester for that but I agree that makes sense as well. Thanks for the feedback, much appreciated!
 

fafrd

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I do have an IR tester for that but I agree that makes sense as well. Thanks for the feedback, much appreciated!
I don’t have an IR tester but did use the ‘delta_V / delta_A’ method between 1A and 10A to measure internal cell resistance.

Actually pumping 80A through the cells (0.14C in my case) exposed issues in some cells (greater voltage drop than expected).

So I’d be interested in knowing whether your IR tester measures resistance similar to what voltage drops you experience under load…
 

chrisski

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I’m torn about capacity testing my sixteen 280 ah cells after they arrive. I may purchase a $75 IR tester and only top balance the cells.

This will probably take me three weeks. I have a 30 volt 10 amp power supply and a can discharge at about the same rate. Between things like needing to go to work and sleep, I don’t see that being done any quicker than that. Top Balance, Capacity TEst, top balance again and then organize and put in service.

I wonder how different the results will be between IR testing and the full three week process. In my case, I can closely monitor the individual cells for several days if changes need to be made.
 

FilterGuy

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I’m torn about capacity testing my sixteen 280 ah cells after they arrive. I may purchase a $75 IR tester and only top balance the cells.

This will probably take me three weeks. I have a 30 volt 10 amp power supply and a can discharge at about the same rate. Between things like needing to go to work and sleep, I don’t see that being done any quicker than that. Top Balance, Capacity TEst, top balance again and then organize and put in service.

I wonder how different the results will be between IR testing and the full three week process. In my case, I can closely monitor the individual cells for several days if changes need to be made.
I don't capacity test my cells any more. What I realized is that I never really do anything different based on the results. Consequently, I just top ballance and then build the battery. I have even deployed the finished battery without capacity testing....
 

fafrd

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I’m torn about capacity testing my sixteen 280 ah cells after they arrive. I may purchase a $75 IR tester and only top balance the cells.

This will probably take me three weeks. I have a 30 volt 10 amp power supply and a can discharge at about the same rate. Between things like needing to go to work and sleep, I don’t see that being done any quicker than that. Top Balance, Capacity TEst, top balance again and then organize and put in service.

I wonder how different the results will be between IR testing and the full three week process. In my case, I can closely monitor the individual cells for several days if changes need to be made.
The only real reason to perform IR testing is to uncover defective cells early/quickly.

Several members have successfully gotten defective cells replaced because they documented IR way outside of spec soon after getting them.
 

fafrd

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I don't capacity test my cells any more. What I realized is that I never really do anything different based on the results. Consequently, I just top ballance and then build the battery. I have even deployed the finished battery without capacity testing....
And the same is pretty much true of capacity testing as well.

I used a quick way to capacity test in parallel:

-Top balance all cells to full (this takes a long time but you can speed up the process by first precharging in series connected through a BMS).

-Connect all cells in series.

-Connect the max-voltage 1P battery (48V in my case) to an AC heating element (mine was ~7 ohms, meaning ~7A discharge).

-discharge in parallel and characterize / monitor cell voltage on the frequency that makes sense (it’ll take a total of over 41 hours to discharge 16 280Ah cells through a ~7 Ohm heating element).

This process allows you to spend no more than a weekend discharging a full pack of cells and you can pretty easily identify whether you have any weak / defective cells (because they drop below 3V long before the others).

I found my two weakest cells, pulled them from the pack, and ran individual-cell capacity tests on them after recharging to document for the vendor that these two cells were defective (also did the same with the best two cells to document they were in spec).

Once we have settled on true Grade-A vendors, this early confirmation testing may not be needed, but any bargain / cheap cells will always likely need to be characterized to avoid a nasty surprise…
 

Shale MacGregor

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Yeah, these are supposedly new cells pulled from EV packs but the variance in capacity is enough that I need to characterize the batteries. I May settle on the series approach with my quick disconnects since that would get me through them in 24 sections.
 

Shale MacGregor

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So I attempted to discharge at .54C on batteries rated at 55AH and rated for 1C discharge rate.... the voltage dropped below 2.5 before I even hit 1AH of discharge..... this was repeated with freshly fully charged batteries from different cases three times.
 

fafrd

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So I attempted to discharge at .54C on batteries rated at 55AH and rated for 1C discharge rate.... the voltage dropped below 2.5 before I even hit 1AH of discharge..... this was repeated with freshly fully charged batteries from different cases three times.
Bummer, but can’t say I’m terribly surprised.

Do you get longer lifetime when you reduce discharge rate to 0.1C or below?

Have you attempted to use the IR = Delta_V / Delta_I technique to estimate your internal resistance?
 

Shale MacGregor

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Bummer, but can’t say I’m terribly surprised.

Do you get longer lifetime when you reduce discharge rate to 0.1C or below?

Have you attempted to use the IR = Delta_V / Delta_I technique to estimate your internal resistance?
Yeah, using IR = Delta V / Delta I I was getting somewhere in the 19 to 20 range, using the IR tester I am getting .5-.7 range

If I discharge at .2C I get full capacity, but what is the point of them being rated at 2C if I cant discharge more than 2% of their capacity at .54C
 

fafrd

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Yeah, using IR = Delta V / Delta I I was getting somewhere in the 19 to 20 range, using the IR tester I am getting .5-.7 range

If I discharge at .2C I get full capacity, but what is the point of them being rated at 2C if I cant discharge more than 2% of their capacity at .54C
Wait, your reading 0.5 to 0.7 mOhms using an IR tester but the delta_V / delta_I technique is giving you 19 to 20 mOhms (27 to 40 times higher???).

I assume you’ve done everything correctly as far as your inter-cell wire / busbar size and the quality of the connections you’ve made to the cell terminals, right?

If the 19-20 mOhms your calculating have anything to do with the reality of your cells internal resistance, it means these IR testers are utterly worthless…

And yeah, if you need to discharge at 0.54C and capacity drops to only 2% at that rate, those cells are next to worthless…

If you are getting full capacity at 0.2C you could build a higher-capacity battery with 3 of those cells in parallel for each series ‘cell’ but who to say that after going to all of that effort, degradation won’t continue so that you need to discharge under 0.1C to get close to full capacity…
 

Shale MacGregor

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Wait, your reading 0.5 to 0.7 mOhms using an IR tester but the delta_V / delta_I technique is giving you 19 to 20 mOhms (27 to 40 times higher???).

I assume you’ve done everything correctly as far as your inter-cell wire / busbar size and the quality of the connections you’ve made to the cell terminals, right?

If the 19-20 mOhms your calculating have anything to do with the reality of your cells internal resistance, it means these IR testers are utterly worthless…

And yeah, if you need to discharge at 0.54C and capacity drops to only 2% at that rate, those cells are next to worthless…

If you are getting full capacity at 0.2C you could build a higher-capacity battery with 3 of those cells in parallel for each series ‘cell’ but who to say that after going to all of that effort, degradation won’t continue so that you need to discharge under 0.1C to get close to full capacity…
The reality is that I sized this pack based on 2C max draw, 1C being overkill and likely needing to hit .4 to .6 C a few times a day, so .2C really doesn't help me unless I want to go up to 80 or 100kWh size..

I dug around online and think this is probably the most accurate spec sheet for this battery.... I guess I need to learn how to read these things. (even though when I bought the cell the only specs were 2C max discharge, not the details found below)
1632965564478.png
 

fafrd

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The reality is that I sized this pack based on 2C max draw, 1C being overkill and likely needing to hit .4 to .6 C a few times a day, so .2C really doesn't help me unless I want to go up to 80 or 100kWh size..

I dug around online and think this is probably the most accurate spec sheet for this battery.... I guess I need to learn how to read these things. (even though when I bought the cell the only specs were 2C max discharge, not the details found below)
View attachment 66939
Yeah, it looks like these cells are only rated for 0.2C discharge…

So you’ll need to make 10p ‘cells’ to discharge at 2.0C (or at least 3p ‘cells’ if 0.6C is the actual max discharge rate you’ll actually hit…).
 

ken morgan

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am I missing something here? I discharge at 35-40 amps on a 400 amp hour battery bank, so is that roughly a .1c discharge? i rarely see more than 35-40 amps at 48 volts. thats AC, 2 fridge/freezer, and associated other devices.
 

Shale MacGregor

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am I missing something here? I discharge at 35-40 amps on a 400 amp hour battery bank, so is that roughly a .1c discharge? i rarely see more than 35-40 amps at 48 volts. thats AC, 2 fridge/freezer, and associated other devices.
My last peak draw per my current monitoring at my house was 28,786 watts
 

Shale MacGregor

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Is that with bogart tri-metric? Or something else. Magnum only does amps draw at least I have not found a menu that would change to watts.
Sorry not familiar with those, I was just referring to my readings from my Sense monitor that tells me how much power I am drawing and tries to guess what is drawing it. With a 48v system I will need to upwards of 600 amps from the system, and that is at current energy usage, not accounting for some additional loads I may be adding in the coming months, so I am hoping for access to all 1320 amps that my planned packs would supposedly be able to deliver.
 
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