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Top Balancing...Do I need to go to 3.650V?

LivingTheDream

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Feb 15, 2021
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Hello All,
I am finally putting together my batteries for my full time RV set-up.
  1. I am setting this up as an 8S2P system with a total of 16 280AH Eve Cells
  2. Using two Overkill BMS's
  3. I've charged my first set of eight cells in series with one of the BMS's to 280Ah and the total charge resting is at 27.16V and the cells range from 3.374 - 3.493
  4. I just pulled off the BMS and reconfigured to parallel and hooked up my power supply
Being full time RV, means I have to run my generator to perform this charge, which has been quite long already. I see that the cells are a bit off balance, but with the multimeter, that's not necessarily the case.

My question is this, do I have to take these to 3.650V in order to call them "top balanced"?

TYIA,
Brian
 
It depends on what you want and how far the cells are apart.

Personally, I would wire them up and try to hit at least 27.6v for a full charge. If none of the cells trip hvd on the bms I would just run it as is. If you have a high cell, you can just discharge it manually, until the pack hits full.

The bms will slowly top balance over time as long as you can charge to whatever you are setting as your cut off voltage.
 
Is time the primary concern? Or just prefer not to max out the cells?

I would call 3.500 minimum for the effort but it will not take but a few more minutes to get 3.650
 
It depends on what you want and how far the cells are apart.

Personally, I would wire them up and try to hit at least 27.6v for a full charge. If none of the cells trip hvd on the bms I would just run it as is. If you have a high cell, you can just discharge it manually, until the pack hits full.

The bms will slowly top balance over time as long as you can charge to whatever you are setting as your cut off voltage.
Ok, I just hit 27.8V and one of the cells tripped for over voltage. The rest were over 3.4V, so it appears that only two of the cells were a bit high. How would you recommend to discharge the two high cells? Or would it be best to just take them off the PS and hook them up in parallel for a day or two to balance out amongst each other?
 
I would just go down to 27.7v and call it good.

But to discharge you can hook up a load, like a DC bulb, to the cell while it's still connected. It shouldn't take very long, maybe 30 seconds to a few minutes at the most.
 
Is time the primary concern? Or just prefer not to max out the cells?

I would call 3.500 minimum for the effort but it will not take but a few more minutes to get 3.650
I guess time is more of the concern as I have to keep my generator running to do the charge. I was wondering if I had the cells at 3.500 and then balanced them in parallel, doesn't that essentially balance them? I know once my system is all up and running and I haven't blown anything up, it'll be worth it. However, I feel incredibly ignorant at times even though I have a basic understanding of electrical (albeit from the USAF back in 1991). LOL!
 
The rest were over 3.4V, so it appears that only two of the cells were a bit high. How would you recommend to discharge the two high cells?
Hopefully your high cells are only 3.65V (or less). Now is the time to parallel them and finish your top balance (i like just below 3.65V but at least 3.60V).
 
Yes, I have them hooked up with a BMS and it triggered at 3.650V.
Great, if you want to top balance, put them in parallel and top balance. This is as high as you can go with a BMS.

Although some will say you can just charge the lower voltage cells (while still in series) and bring them all up to 3.65V. I have not tried this but some swear by it. I'm an "all in parallel top balance" kind of guy. Others aren't. But you're most of the way there so might as well "get 'er done" one way or the other.
 
I guess time is more of the concern as I have to keep my generator running to do the charge. I was wondering if I had the cells at 3.500 and then balanced them in parallel, doesn't that essentially balance them? I know once my system is all up and running and I haven't blown anything up, it'll be worth it. However, I feel incredibly ignorant at times even though I have a basic understanding of electrical (albeit from the USAF back in 1991). LOL!
Just connecting them has been shown to be of little help. Need to move some power in the same direction on all of them.
I don't think it will take long to get at least 3.500 on the set.

Another thought...
assemble the second set in series and charge until the high cell is 3.4
Use the inverter to drive the power supply connected to the parallel set to complete the parallel process without the generator.

Assemble the balanced cells in series and put in service. Complete the top balance on the second battery without generator.
 
All top balancing means is filling each cell full.
Full is indicated when the cell reaches 3.65 volts.
No magic occurs when cells are put in parallel.
Of course each cell should be charged at the same current to the same voltage.
 
Great, if you want to top balance, put them in parallel and top balance. This is as high as you can go with a BMS.

Although some will say you can just charge the lower voltage cells (while still in series) and bring them all up to 3.65V. I have not tried this but some swear by it. I'm an "all in parallel top balance" kind of guy. Others aren't. But you're most of the way there so might as well "get 'er done" one way or the other.
Ok, I went with this approach as it seems the easiest. What's a few more hours on the generator, once these are done and I have 560Ah of battery coupled with 1620W of solar, I shouldn't need to run it again. LOL! That's if I did my math correct and I can avoid places that need A/C!
 
You can top balance down to 3.45v cell voltage just need to ensure current in each cell drops to a very low level and it may take a longer time depending on how much charge current is applied. With a 10 amp power supply parallel charging 16 cells the current is so low it will be done when all cells reach 3.45v.

The current taper off time takes longer the greater the charge current..

3.5-3.55v is a good target to use.

These numbers are actual cell terminal voltage. If you have too much wire resistance voltage drop this can greatly extend the time to make the final little bit of charge %. Try to keep less than 50mV voltage drop from power supply terminals and battery terminals. This requires #10 or #8 wire cables. Keep them short in length, less than 3 feet. Don't use alligator clips, use screwed down lugs. Throw away the cheap #16 cables with alligator clips that come with the power supply and get larger wire cables.

Cheap switching power supplies have a soft transition between CC mode and CV mode and can begin to cut back on CC amperage when cell voltage gets within 100 mV of power supply CV limit. This extends the top off time. Linear power supplies typically have a sharper CC to CV transition so if you have one it may be better to switch over to linear supply when cheap switching supply current starts to cut back.
 
I got to 3.615V and took off the power supply. I'll let this sit in parallel over night and then work on the second set of eight. I saw a video where someone did the same, charging in different sets of eight, and then he went in and pulled a cell from one set and then one from the other set to make his final groupings. Would you recommend this and what does it accomplish if anything?

All eight cells are reading 3.577 while at rest now.
 
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8S2P system with a total of 16 280AH Eve Cells
then note the voltages tomorrow
My guess would be to group the lowest 8 voltage cells (lower capacity?) together and the 8 highest (highest capacity?) together. The idea is that each 8S battery will perform better with "more similar" cells. Then, should you find some from the lower capacity cells are too high you might try swapping with the higher capacity cells if you find some unusually low for that group. Good notes will be invaluable.

I recommend labeling cells with letters (A, B, C...) because when you note the cell position in the battery, that is a number. Its easy to create garbage notes when both cell name and cell position are numbers.
 
So is it normal for one of the cells to spike once everything gets to the 3.400 mark? As things are charging in series, I'm within a .004 differential between cells. Then all of a sudden one spikes to 3.650 with a .149 differential.
 
So is it normal for one of the cells to spike once everything gets to the 3.400 mark?
That is normal, it means they are not perfectly top balanced.
3.65 - .149 = 3.501 so they are darn close.
With a conservative charge profile your BMS can probably manage that.
 
I ruined 2 out of 96 cells in my 35kwh bank while taking my time and now I aim for 3.5 to 3.55.....3.65 is to close to danger and not worth it.
 
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