Capacity Testing Just Arrived Cells

[roblitespeed]

After waiting a couple of months, I got my first set of 8 100Ah cells. I charged them and top balanced them. I did a capacity test. How are the testers on Youtube getting 100% of the rating out of the batteries? My batteries top balanced at 3.470 (all within .002). Looking at the charts available here and elsewhere, that's essentially 100% (99%+). Since I was using one of those cheap Chinese testers, I limited my discharge to 5A (@ a nominal 24V, so 100W). and stopped the test at 24V. According to most of the charts, that's about 10% capacity and you shouldn't go below that. So, from 27.76 down to 24V I got 88.8Ah. 100% to 10% = 90%. After the test all cells were within 0.06V. Should I be happy with my 88.8Ah using 90% of the charge or should I do a 100% test? I think I'm happy where I am, I'd rather not destroy any cells. How do you do a 100% test?

 

[Gazoo]

How did you balance your cells? Did you parallel top balance? The voltage charts are in the right ball park but they can vary a bit depending on the cells. But fully charged is 3.65 volts per cell and fully discharged is 2.5 volts per cell. When parallel top balanced the pack must be charged to 3.65 volts because cells are not capacity matched unless the supplier states they are. And in many cases the cells will not be capacity matched even though the supplier makes that claim.

If you want to run a full capacity test you must fully charge the battery, then discharge it. You will not destroy your cells providing you are using a BMS, and the parameters of the BMS are set correctly, mainly the HVD and LVD. Running a full capacity test will give you the opportunity to make sure the HVD and LVD of the BMS are working.

The delta at the top and the bottom of the charge will increase. Your deltas of .002 at the top and .06 at the bottom look promising. So the answer is yes. You should be happy with the 88.8ah capacity. Assuming there is 10% left, the battery is at least a 98.8ah battery.

 

[roblitespeed]

Thanks Gazoo. The batteries were charged in series until they were close and then I put them in parallel and set the voltage for 3.65. I charged them in constant current mode until the power supply switched over to constant voltage at 3.65. I then let them charge until the current was 3.33A. This paper indicates that you should stop charging at 0.033c or 1/30c. That's what I did and why the batteries were all resting at 3.47 after the top balance. End voltages on the cells were all between 3.059 and 3.121. I could have charged them a little more and run them down a little further if I wanted a full test. This is my first experience with LifePo4 batteries so I wanted to approach things conservatively.

 

[Lt.Dan]

Sounds like you charged correctly. But I would've taken it further down while capacity testing. Yes its good to be conservative while actually using the battery, but if you want to know the capacity, then use all of the capacity.

 

[Gazoo]

Thanks Gazoo. The batteries were charged in series until they were close and then I put them in parallel and set the voltage for 3.65. I charged them in constant current mode until the power supply switched over to constant voltage at 3.65. I then let them charge until the current was 3.33A. This paper indicates that you should stop charging at 0.033c or 1/30c. That's what I did and why the batteries were all resting at 3.47 after the top balance. End voltages on the cells were all between 3.059 and 3.121. I could have charged them a little more and run them down a little further if I wanted a full test. This is my first experience with LifePo4 batteries so I wanted to approach things conservatively.

I think you are fine. But during the absorption stage it's possible the cells are not properly top balanced until the current drops to zero. And the same author explains in this article.

Personally I don't care for the step method he describes. I used a Riden 12 amp power supply with 12AWG wire and ring connectors. As I recall, I could not get the maximum output of the power supply and it would not go into CC mode until I bumped up the voltage to 3.5 volts or so. So I decided to set the power supply to 3.65 volts and let it do its thing. My charge ended with a tail current of 100ma's.

I used the charging feature of the power supply but the same applies if using a power supply that doesn't have a charging feature. If you parallel top balance to 3.65 volts your delta at the bottom might be bigger. As long as you are going to stay within the knees like most do, I wouldn't worry about it. But if you decide to fully test the capacity of the battery, please post your results including your deltas at the top and bottom.