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LiFePO4 capacity testing voltage ranges

mysolarrv

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I have 8 Calb 200 AH cells that I received earlier this year (bought on AliExpress) and I recently purchased a DL24P capacity tester. The plan is to capacity test each cell as I suspect they are actually grade B cells that were sold as grade A cells. I used the batteries this summer in my camper in a 2P4S configuration and they worked good, but I was unable to really capacity test them in that configuration and I didn't have the tester until just recently. Now that the camping season in Michigan is over, I'm ready to run a full capacity test on each cell individually. I'm trying to figure out what the ideal voltage ranges to capacity test them with and at what load. I've seen several different recommendations, but is starting at 3.60V then discharging to 2.5V the best range to test with? Or is it almost as good to just use 2.7V as the cut off voltage and that will be "close enough" to estimate the total capacity? Or does anyone have any specific recommendations before I start my testing? With my cells and tester, I can't reach a .2C discharge rate (that would be 40A load). The tester will max out at 20A (about .1C) and I'd rather not run it at full capacity for 10 hours per cell (total of 80 hours of testing). I'd feel much more comfortable running the tester at say 15A. So a fully charged 200AH cell testing with a 15A load, should take around 13.5 hours to fully discharge.
 
Full capacity is from 3.65V to 2.5V but most capacity is from 3.4V to 3V. In all seriousness why capacity test? You have what you have and they will not be replaced by the vendor if "outside" spec. Assemble the pack and use and then you will learn the usable capacity.
 
Full capacity is from 3.65V to 2.5V but most capacity is from 3.4V to 3V. In all seriousness why capacity test? You have what you have and they will not be replaced by the vendor if "outside" spec. Assemble the pack and use and then you will learn the usable capacity.

I agree full capacity is from 3.65 V to 2.5 V for Lifepo4 cells. At that low of a C rate (15 amps) may not get a true capacity, but will give an idea of the cells ability. Often a higher C rate will give a lower capacity especially with cells that are not grade A. Never did a capacity test on individual cells, but did with the cells in series with BMS connected set for 2.5 V low disconnect and Victron shunt recording AH and WH. The lowest capacity cell limits the battery bank.

If there is time and equipment testing each cell could be interesting. Watch the first test on a cell to be sure not to run below 2.5V. There have been those that didn't configure the tester properly and did not watch closely while the cell went well below 2.5v (not good) since the BMS is not connected to protect the cell.

Attached a spec sheet for 180AH CALB cells that may help.
 

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3.60V then discharging to 2.5V the best range to test with? Or is it almost as good to just use 2.7V as the cut off voltage and that will be "close enough" to estimate the total capacity?
I have been a capacity test denier for a couple years. I have some time with my 280Ah cells (19) and i want to use the best 16. So i bought the tester Will recommended ($50 on Amazon)

I am NOT really interested in the exact capacity but rather knowing which are the strongest 16. I am taking lots of notes and learning a lot. I am charging them up to 3.63V with a hobby charger that tapers fairly well so its a good charge.

What was interesting and i wish i knew beforehand was the discharging. I set my discharge finish voltage to 2.7V after originally using 3.0V in a test run. What i found was that there was a LOT of voltage sag and it cuts out much earlier than i thought it would. My discharge rate was 20.7A which is just below the max (knobs turned all the way) so i could keep it the same for each cell.

What i was seeing is that just moments after hitting 2.7V, and the discharge stopped, the cell voltage was over 3V. Some would bounce back to 3.2V in an hour or two. I am wondering how much capacity I am leaving in the cells and how much percent-wise i emptied the cells.

I am in the middle of testing, roughly 2/3 of the way thru running pretty much around the clock (10 days so far): charge to 3.63V (2.3 hours from mostly charged state), 12-13 hour discharge to 2.7V, then charge back to 3.32V storage charge (3-5h at 15A).
My goal is to get thru all 19 cells using the same methodology so i am not doing any side testing now.

I plan to make a few tests afterwards to see if 2.5V makes a difference with the outlier cells. One of the lower capacity cells was the one that snapped back to 3.2V after a couple hours. Its a head scratcher for sure.
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Full capacity is from 3.65V to 2.5V but most capacity is from 3.4V to 3V. In all seriousness why capacity test? You have what you have and they will not be replaced by the vendor if "outside" spec. Assemble the pack and use and then you will learn the usable capacity.
Yeah, I get your point. It's not like the AliExpress seller is going to offer any sort of warranty replacement if the cells are way below their advertised capacity. Which I guess brings up another question... how do you know which sellers are reputable and which ones are not? Everyone seems to only sell New Grade A cells, go figure.
 
Not easy but searching the forum will find lots of buyer experiences. Number one is don't buy batteries from aliexpress, use alibaba. End of the day it's a bit of a lucky dip but that's the way it is when trying to buy cheap.
 
Yeah, I get your point. It's not like the AliExpress seller is going to offer any sort of warranty replacement if the cells are way below their advertised capacity. Which I guess brings up another question... how do you know which sellers are reputable and which ones are not? Everyone seems to only sell New Grade A cells, go figure.

Amy Wan on Alibaba, her company name is Shenzhen Luyuan. Although I have not received cells from her, I hear there is excellent packaging, can get matched and batched cells and you receive the EVE test reports. You also pay more for grade A but you get what you pay for.

It's my understanding she sells grade A and grade B. She will be on my short list next time I get cells.

 
Not easy but searching the forum will find lots of buyer experiences. Number one is don't buy batteries from aliexpress, use alibaba. End of the day it's a bit of a lucky dip but that's the way it is when trying to buy cheap.
I'm a little embarrassed to admit this, but I didn't realize aliexpress was just an online "discount outlet". Lesson learned. I did read through many of the comments for the seller and almost all were favorable, go figure. And soon after I received my cells I opened a dispute with the seller claiming that the cells were used (they did not appear to be brand new). The seller continued to claim the cells were brand new and not swollen and would not offer any concessions. So I attached pics which clearly showed that a couple of the cells were swollen and aliexpress stepped in to arbitrate the sale. They offered me two options, I could have a $34 rebate or I could ship the cells back for a full refund at my expense. After checking into shipping, cheapest I found was about $450, I decided to take the $34 rebate and keep the batteries. Again, lesson learned. So I guess that's one of my motivations for running a load test on each cell to figure out what I actually have and perhaps pair them up better for my 2P4S battery build.
 
Well, I finally ran capacity tests on each of the 8 cells using a DL24P tester.
Test specifications: 16.0A CC load with a range of 3.60V to 2.70V
Here are my results:
Cell #1: 155 AH
Cell #2: 159 AH
Cell #3: 158 AH
Cell #4: 156 AH
Cell #5: 153 AH
Cell #6: 153 AH
Cell #7: 150 AH
Cell #8: 163 AH
Now these were supposed to be New Grade A 200 AH cells (probably false advertising). I was a little disappointed with the numbers I got, since I used the batteries only a handful of times last year before capacity testing them this year. They did perform okay for most of my needs last year, but when I tried to power my microwave oven with a 1500W pure sine wave inverter, I could only run it for a few short tests before the voltage dropped below the inverter's cut-off voltage (kind of surprised at that). I'm not running the inverter through the BMS as it is only rated at 120A. So obviously that's not interfering with my tests. I did have some connections that were getting too hot, so I may rewire stuff to help eliminate any poor connections or weak points in my design.
 
Now these were supposed to be New Grade A 200 AH cells (probably false advertising). I was a little disappointed with the numbers I got, since I used the batteries only a handful of times last year before capacity testing them this year.

Likely used batteries.

They did perform okay for most of my needs last year, but when I tried to power my microwave oven with a 1500W pure sine wave inverter, I could only run it for a few short tests before the voltage dropped below the inverter's cut-off voltage (kind of surprised at that). I'm not running the inverter through the BMS as it is only rated at 120A. So obviously that's not interfering with my tests. I did have some connections that were getting too hot, so I may rewire stuff to help eliminate any poor connections or weak points in my design.

That's the number one reason for excessive voltage drop - bad connections, failing breakers or thin wires.
 
Full capacity is from 3.65V to 2.5V but most capacity is from 3.4V to 3V. In all seriousness why capacity test? You have what you have and they will not be replaced by the vendor if "outside" spec. Assemble the pack and use and then you will learn the usable capacity.
Here's another reason, so I can enter the capacity values into the app so it can give me better SOC estimates. So I deviated somewhat from my original plan of a 2P4S configuration. The battery bank now has two 4S batteries wired in parallel. Each battery has a BMS (Overkill 120A BMS) and shunt (Victron 500A smart shunt). The shunt is first, then the BMS (it was just easier to wire it this way due to space limitations). During a recent weeklong camping trip, I noticed that the shunts and BMSs showed totally different SOC numbers. I mean, they are not even close. It might be a configuration issue in the app between the 4 devices (quite possible). So one battery has the SE180 cells, and the other has the SE200 cells (yeah long story, certainly not my original plan, but that's what I have now). So after about 4 days of camping, I noticed that the BMSs said I had 0 AH left, but they were both able to still supply power to my inverter and camper?!? While the smart shunt showed I still had capacity (~100AH total). So something is definitely wrong. Hence me wanting to do some capacity testing with each battery, rather than per cell, to get some realistic numbers. Then I can update the BMS and shunts with some realistic values and hopefully get better SOC results.
Anyone else go through this painful process? Did you take a similar approach or use some other method?
 
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