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Storing Top balanced cells?

JoeyJibJab

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Tasmania, Australia
Hey guys, I bought 16 200ah CALB cells about 7 months ago. The seller assured me they were Top balanced before shipped. If they were Top balanced, will I need to perform another balance when I assemble the pack? I probably won't assemble for another 2-3 months, so it could be 10-12 months since I got them.

This of course is assuming they were Top balanced in the first place.
 
Your question has an underlying assumption that the vendor's top balancing also means they were shipped to you fully charged. Most vendors ship cells at approximately 50% state of charge. They could have been top balanced then controlled discharged to 50%.

Spec on self discharge is 3% capacity per month but typically they are in 1-2% per month range when new.

Three or four months storage, even if received at 50% state of charge would not be a problem. Just try not to store them in a hot shed.
 
Your question has an underlying assumption that the vendor's top balancing also means they were shipped to you fully charged. Most vendors ship cells at approximately 50% state of charge. They could have been top balanced then controlled discharged to 50%.

Spec on self discharge is 3% capacity per month but typically they are in 1-2% per month range when new.

Three or four months storage, even if received at 50% state of charge would not be a problem. Just try not to store them in a hot shed.
Okay thanks. The cells were at 3.4v when received. They are now 3.2v. I suppose I'll give them a go and if they are way out from each other I'll have to perform a top balance. Would rather not if it can be helped though.
 
If you received them at 3.4v then they were at full charge.

LiFePO4 require a fairly accurate DVM (better than +/- 0.05%, +/-2 mV tolerance) to get an unloaded, rested, state of charge estimate from cell voltage.

After top balancing to 3.65v they will drop to about 3.400v in a short period of time then gradually drop to about 3.360v.

80% SOC open circuit is about 3.332v, 50% SOC open circuit is about 3.302v. 20% SOC open circuit is about 3.234v. You can see the voltage spread is not very wide, thus the requirement to have an accurate DVM to read voltage.

Self discharge leakage current goes up when cells stored at higher temps (>30 to 35 degs C)

If your DVM is accurate, droping from 3.40v initially to 3.20v in seven months is a bit concerning. That would mean a very high 10%+ per month self leakage rate if voltage readings are accurate.

Reasonably accurate cheapo DVM I can recommend are Unt-T UT61E and ANENG AN870
 
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If you received them at 3.4v then they were at full charge.

LiFePO4 require a fairly accurate DVM (better than +/- 0.05%, +/-2 mV tolerance) to get an unloaded, rested, state of charge estimate from cell voltage.

After top balancing to 3.65v they will drop to about 3.400v in a short period of time then gradually drop to about 3.360v.

80% SOC open circuit is about 3.332v, 50% SOC open circuit is about 3.302v. 20% SOC open circuit is about 3.234v. You can see the voltage spread is not very wide, thus the requirement to have an accurate DVM to read voltage.

Self discharge leakage current goes up when cells stored at higher temps (>30 to 35 degs C)

If your DVM is accurate, droping from 3.40v initially to 3.20v in seven months is a bit concerning. That would mean a very high 10%+ per month self leakage rate if voltage readings are accurate.

Reasonably accurate cheapo DVM I can recommend are Unt-T UT61E and ANENG AN870
Sorry I was wrong! They came to me at 3.34v and are flicking between 3.29 and 3.30v. My DVM only goes two decimal places so I will need to get a more accurate one. Is there a clamp meter that does accurate voltage readings as well as current readings? Would be great If I could get just the one meter, as opposed to buying a multimeter and a clamp meter. Thanks!
 
I have not seen a clamp on ampmeter that has real good voltage accuracy. Right tool for the job applies.

Clip-on amp meters are not that accurate for current but generally you don't need better then few percent accuracy. I have several clip-on amp meters. One does well for AC surge peak reading for things like starting motor current and air conditioner turn on surge current. One is good for moderate to high currents, one is good for lower currrents. Some are only AC current. Some do both AC & DC. Better ones use hall effect sensors vs. just current transformer.

I do like the UNI-T 210E. It has three scale positions for AC/DC current for 2 amp, 20 amp, and 100 amps, but it is only capable of 100 amps max so may not be high enough amperage measurement, particularly for lower voltage inverter systems. Many clip on amp meters only have two amperage scale positions, like 20A/200A or 40A/400A. 210E is about 3% accuracy on clamp on amperage. Voltage readings are +/- 1.0%, not so good for battery voltage measurements but okay for A.C. grid voltages.
 
Sorry I was wrong! They came to me at 3.34v and are flicking between 3.29 and 3.30v. My DVM only goes two decimal places so I will need to get a more accurate one. Is there a clamp meter that does accurate voltage readings as well as current readings? Would be great If I could get just the one meter, as opposed to buying a multimeter and a clamp meter. Thanks!
Again assuming accuracy of your DVM, 3.34v is approx. 85% SOC, 3.30v is approx. 50% SOC. 85% to 50% range is the flattest part of curve needing the highest DVM accuracy and resolution to estimate SOC.

35% drop in 7 months is still pretty high cell leakage rate of around 5% per month. But you are still a long way from over discharging cells.
 

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Is there a clamp meter that does accurate voltage readings as well as current readings?
sadly the answer is no....the clamps are not accurate, linear, or precise; but they sure are convenient!!!
"its not the right answer, but its the easy answer" should be the moto of the multimeter clamp hehe
I have one I use on balance leads; I know that I can tell IF anything significant is flowing and what direction...and I know the numbers are mostly meaningless for anything beyond a go/nogo type reading for low currents.

you can get accurate current reading from shunts but not from clamps- hall effect(ac-dc) or transformers(ac only).
Now that said, you can get "relative" readings if you are looking for current direction and the flow is more than say 30ma as I mentioned.
 
sadly the answer is no....the clamps are not accurate, linear, or precise; but they sure are convenient!!!
Ah bummer. I suppose I should invest in both types. Cheers :)
Again assuming accuracy of your DVM, 3.34v is approx. 85% SOC, 3.30v is approx. 50% SOC. 85% to 50% range is the flattest part of curve needing the highest DVM accuracy and resolution to estimate SOC.

35% drop in 7 months is still pretty high cell leakage rate of around 5% per month. But you are still a long way from over discharging cells.
Okay thanks, hopefully the cells are okay quality. They are definitely not "brand new" as they were advertised unfortunately. I'm hoping to get the pack assembled within the next couple of months, so I'll keep a close eye on them in the mean time and record the readings with a more accurate DVM. Thanks for your help :)
 
Hey yall. I just top balanced my 12v LifeP04 cells (4s configuration) and to be honest was pretty proud of the results. I started out in series with the BMS hooked up and got them charged up to 3.45v then reconnected in parallel and charged up to 3.6v and they all matched perfectly with less than .005 difference according to the BMS.

According to one video I watched they said it wasn't a bad idea to leave them in parallel for a day to let everything balance out even more.

So here is my issue...I've left them i parallel and the V has slowly decreased from 3.6 down to 3.49 over the course of 48hrs. Is this normal for them not to hold a full"100% SOC"? Or is that to be expected and I should be happy that they are still pretty much topped off and around 95% capacity. OR lol... does this have anything to do with me leaving them in parallel?

Thanks so much for the help in advance. I felt like this was a relevant thread but if anyone knows a better one please let me know (I'm new here)
 
It looks pretty normal to me ... and 3.49 is still in the beginning of the knee .... so, I think you are good to go.
 
Hey yall. I just top balanced my 12v LifeP04 cells (4s configuration) and to be honest was pretty proud of the results. I started out in series with the BMS hooked up and got them charged up to 3.45v then reconnected in parallel and charged up to 3.6v and they all matched perfectly with less than .005 difference according to the BMS.

According to one video I watched they said it wasn't a bad idea to leave them in parallel for a day to let everything balance out even more.

So here is my issue...I've left them i parallel and the V has slowly decreased from 3.6 down to 3.49 over the course of 48hrs. Is this normal for them not to hold a full"100% SOC"? Or is that to be expected and I should be happy that they are still pretty much topped off and around 95% capacity. OR lol... does this have anything to do with me leaving them in parallel?

Thanks so much for the help in advance. I felt like this was a relevant thread but if anyone knows a better one please let me know (I'm new here)
Hey mate, I'm pretty sure that's normal behaviour for the voltage to settle that amount :)
 
Hey yall. I just top balanced my 12v LifeP04 cells (4s configuration) and to be honest was pretty proud of the results. I started out in series with the BMS hooked up and got them charged up to 3.45v then reconnected in parallel and charged up to 3.6v and they all matched perfectly with less than .005 difference according to the BMS.

According to one video I watched they said it wasn't a bad idea to leave them in parallel for a day to let everything balance out even more.

So here is my issue...I've left them i parallel and the V has slowly decreased from 3.6 down to 3.49 over the course of 48hrs. Is this normal for them not to hold a full"100% SOC"? Or is that to be expected and I should be happy that they are still pretty much topped off and around 95% capacity. OR lol... does this have anything to do with me leaving them in parallel?

Thanks so much for the help in advance. I felt like this was a relevant thread but if anyone knows a better one please let me know (I'm new here)
It's normal for the voltage to settle.
 
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