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Started my top balancing

BretS

BretS

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Started my top balancing, 3 cells at a time since my bank is a 3P4S in 2 sections. These are 310Ah cells from Exliporc Energy on Alibaba. Couple lessons I learned...

1/4" bus bar is too thick for the little studs - had to counter bore the bottom a bit to set them down some.

Make sure you get a power supply with a fine adjustment and 2 decimal places on the readout... this one is extremely touchy, and I had to set it with my DMM...

I know it will take a while, but I don't have another means of charging them because I have a contactor BMS that has to be assembled in place in my system...
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Using your DMM was correct thing to do, You always want to use a good meter to set voltage levels, its very rare for a "cheap" power supply to be accurate within 50mv. It can certainly take a while to finish; consider putting some simple wide painters tape over at least one side of the battery "just in case" something drops across there ;-)
 
You should also put something big and red on the + side of the cells. I see the little + but when working with them, its super easy to equate black to -.

Also, you should label your cells. Hopefully you've recorded initial voltages somehow. For labeling, i recommend using letters (A, B, C...) because the cell position is a number. When your notes are "cell 3 is cell 5 ..." its easy to come out with gibberish for notes. Don't ask me how i know.
 
Here they are with the lexan covers to protect from the "just in case"... I have them arranged like my diagram, and figured I can't screw up 3 at a time since I haven't made the rest of the bus bars yet... I was just trying to get the first set charging as soon as I could. Can I let these charge overnight, or is there too much concern of something going wrong?
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BretS said:
Can I let these charge overnight, or is there too much concern of something going wrong?
Click to expand...
Personally I would want to be watching at least for the first run. Once they get over 3.4v it won't take long.

Let me correct myself, I would want to be watching every run.
 
MisterSandals said:
You should also put something big and red on the + side of the cells. I see the little + but when working with them, its super easy to equate black to -.

Also, you should label your cells. Hopefully you've recorded initial voltages somehow. For labeling, i recommend using letters (A, B, C...) because the cell position is a number. When your notes are "cell 3 is cell 5 ..." its easy to come out with gibberish for notes. Don't ask me how i know.
Click to expand...
I checked them all, and they were 3.295-3.299. Once each set is balanced, I will check them and record voltage, then check again after 24 hour rest period. If there aren't any really bad apples, they are going to work... I don't need a second job or science project capacity testing or any other stuff.
 
Plan looks good. Please post how long this takes. The 10 amps looks good but still going to take some time on each set.
 
BretS said:
Started my top balancing, 3 cells at a time since my bank is a 3P4S in 2 sections. These are 310Ah cells from Exliporc Energy on Alibaba. Couple lessons I learned...

1/4" bus bar is too thick for the little studs - had to counter bore the bottom a bit to set them down some.

Make sure you get a power supply with a fine adjustment and 2 decimal places on the readout... this one is extremely touchy, and I had to set it with my DMM...

I know it will take a while, but I don't have another means of charging them because I have a contactor BMS that has to be assembled in place in my system...
View attachment 49121View attachment 49122View attachment 49123View attachment 49125
Click to expand...
Since you are only doing 3 cells at a time, be sure that you top balance each set of 3 to as close to the same voltage as possible..... otherwise they are not balanced.
BretS said:
1/4" bus bar is too thick for the little studs - had to counter bore the bottom a bit to set them down some.
Click to expand...
Interesting. How much did you have to counter bore?

BTW: I would be tempted to flip the bars over and put the non-bored side against the cell terminal pad. If you have any depth difference on the bores or if there are other flaws in the bore it will affect the contact between the bus-bar and the terminal pad.

Finally: The bus bars look like the silver plated rounded edge bar stock sold by on-line metals. If so, make sure to 1) polish the silver before connecting and 2) use a diaelectric grease designed for electrical connections. (Silver oxide is an insulator and you don't want to get bad connections)
 
BretS said:
Excessive self discharge over 24 hours was my plan. Am I correct in that?
Click to expand...
LiFePO4 cells are not known to self discharge very much at all. What you'd need to do would be to either do a capacity test (i was not up for the effort this required) or to build up your battery and see if any cells discharge or charge quicker than the others. These will be the lower capacity cells, aka the bad apples.

If you get any that go rogue near high and low state of charge, you can narrow your operational voltage range to where all the cells stay similar and within limits. Or some have used balancers.
 
BretS said:
Excessive self discharge over 24 hours was my plan. Am I correct in that?
Click to expand...
sadly the only way to tell what their capacity really is, well, is to do a capacity test...

self discharge or measure voltage does not tell much at all.
I just use one of the 20amp constant battery current testers that run $30-$40; and yes, it will take about 15hrs per battery once they are charged.
(and then you have to charge them up again hehe)
 
This has been going really slow... almost done with the 3rd set of 3.

After they rested 24 hours, all of them are coming in at 3.541 - 3.549, except for one that is 3.515... does that seem like a problem? I don't have any spares, so not much choice but to use it. I am going to be using a REC active BMS, so it will help to some degree.
 
BretS said:
This has been going really slow... almost done with the 3rd set of 3.

After they rested 24 hours, all of them are coming in at 3.541 - 3.549, except for one that is 3.515... does that seem like a problem? I don't have any spares, so not much choice but to use it. I am going to be using a REC active BMS, so it will help to some degree.
Click to expand...
It will take a substantial amount of time. The voltage difference after some settling time does not seem like a problem to me. I would not worry about it.

If you had a different BMS, it is much faster to charge in series and then top balance. But what you are doing (especially setting the voltage using your DMM) is correct, if a bit slow.
 
Cells normally drop and settle after a full charge to 3.38 to 3.4v after 12 hours rest.

Yours that are holding at 3.5v after 24 hours rest worries me about overcharge cause by using too low of a charge current and too high a CV voltage for balance. Then again, those are huge cells.

The general rule is that if you are charging at 0.05C or lower, then the CV limit should only be at most 3.45v. So you are kinda knife-edging it. Not talking solar irregularities here, but ac charging.

(propeller-head notes about 0.05C or lower: since you are already charging at what would be considered the customary tail-current absorb stopping point, when you hit 3.45v you are already fully charged and saturated)

When you are done and put your bms on it, don't wait too long to discharge them a little bit to ensure they are out of the top of the charge knee.

A friend of mine did nearly the same as you but with a wall-wart. "Hey, check it out - I charged them, went on vacation, and these are still holding at 3.6v!" Eyes glazed when I told him that they should have settled to 3.38 to 3.4v max at rest. I heard his wallet cry as we did the first capacity test.

You'll be cool - just put a bit of a discharge on them as soon as you are done.
 
Last edited:
Substrate said:
Cells normally drop and settle after a full charge to 3.38 to 3.4v after 12 hours rest.

Yours that are holding at 3.5v after 24 hours rest worries me about overcharge cause by using too low of a charge current and too high a CV voltage for balance. Then again, those are huge cells.

The general rule is that if you are charging at 0.05C or lower, then the CV limit should only be at most 3.45v. So you are kinda knife-edging it. Not talking solar irregularities here, but ac charging.

(propeller-head notes about 0.05C or lower: since you are already charging at what would be considered the customary tail-current absorb stopping point, when you hit 3.45v you are already fully charged and saturated)

When you are done and put your bms on it, don't wait too long to discharge them a little bit to ensure they are out of the top of the charge knee.

A friend of mine did nearly the same as you but with a wall-wart. "Hey, check it out - I charged them, went on vacation, and these are still holding at 3.6v!" Eyes glazed when I told him that they should have settled to 3.38 to 3.4v max at rest. I heard his wallet cry as we did the first capacity test.

You'll be cool - just put a bit of a discharge on them as soon as you are done.
Click to expand...
Why does everything on here, even Will's video, say 3.65 for top balancing?? I was thinking I was being conservative with going to 3.585...
 
BretS said:
Why does everything on here, even Will's video, say 3.65 for top balancing?? I was thinking I was being conservative with going to 3.585...
Click to expand...
1. Don't worry about it, 3.65v is 100% full. 3.4v is 98% (or greater) full. The objective of a top balance is to get all cells 100% full. The difference between 3.4 and 3.65 is about 2 amp hours.
2. A top balance done properly will last a long time, in normal operation you don't want to charge above 3.4v per cell.

The settling voltage difference has to do with adsorption, i.e. letting it charge until it can accept no more. For Eve cells, the manufacturer states to charge at .5C (140 amps) until the amps input drop to 14 amps (.05C). They also say full capacity for a true grade A cell is measured at .33C charge and discharge rates (a little over 92 amps). For top balancing, I personally let it absorb at 3.65v, since I want them all to be 100%. From my testing, the difference between charging to 3.65 and letting it absorb and charging to 3.65 and stopping when the amps on drop to 14 amps is approximately 1.5 to 2 amp hours measured, settling voltage will be approximately 3.45 and 3.5ish (after an hour).

As far as I can tell, you need to top balance at most once a year, probably less often. I am not sure yet, I have only had the cells a little over 6 months.

Just don't go over 3.65v and you will be fine. You will get a larger variation in capacity by a 15 degrees Celsius change in temperature.
 
Just John said:
1. Don't worry about it, 3.65v is 100% full. 3.4v is 98% (or greater) full. The objective of a top balance is to get all cells 100% full. The difference between 3.4 and 3.65 is about 2 amp hours.
2. A top balance done properly will last a long time, in normal operation you don't want to charge above 3.4v per cell.

The settling voltage difference has to do with adsorption, i.e. letting it charge until it can accept no more. For Eve cells, the manufacturer states to charge at .5C (140 amps) until the amps input drop to 14 amps (.05C). They also say full capacity for a true grade A cell is measured at .33C charge and discharge rates (a little over 92 amps). For top balancing, I personally let it absorb at 3.65v, since I want them all to be 100%. From my testing, the difference between charging to 3.65 and letting it absorb and charging to 3.65 and stopping when the amps on drop to 14 amps is approximately 1.5 to 2 amp hours measured, settling voltage will be approximately 3.45 and 3.5ish (after an hour).

As far as I can tell, you need to top balance at most once a year, probably less often. I am not sure yet, I have only had the cells a little over 6 months.

Just don't go over 3.65v and you will be fine. You will get a larger variation in capacity by a 15 degrees Celsius change in temperature.
Click to expand...
Thank you for the explanation! I have not even gone to 3.6 (3.585), so I feel better... this can all be overwhelming for a newbie... I understand why most people just buy off the shelf, but I couldn't afford that and my system too... At least I am learning, and challenge is good... just don't want to damage anything, or my 5th wheel after this is installed.!
 
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