How many hours to top balance cells?

[AlexanderKristiansen]

That is a big disparity. But cell 8 wasn't the highest cell when you first posted. Cell 4 had the highest voltage.

Cell 8 has the highest voltage because I charged it the longest

 

[AlexanderKristiansen]

The issue is the voltage of that cell when charging in the pack. Everything is relative. Taking the cell with you will not help you solve the problem. Leave it there. By your next visit it will probably settle close to the others. Have a nice drive home.
I think @smoothJoey and I are almost in complete agreement that the problem is not major.

Too late @Ampster, Im back home with the cell. How do I discharge with a light bulb?

 

[Hedges]

Use a volt meter to keep track of cell voltage, make sure you don't over-discharge.
You are only discharging from up the knee of the curve, not going anywhere near fully discharged, so shouldn't be a problem unless you leave it a long time.

If you've got a lamp with a 100W incandescent bulb or similar, just wire the plug to cell terminals. Make sure you don't short the wires!
A car headlamp would work too. Or tail lamp, just less current.

 

[Ampster]

Too late @Ampster, Im back home with the cell. How do I discharge with a light bulb?

That process only works effectively if the cell is in the pack. You have to do it in increments and charge the pack to see how close the the other cells are. That cell will settle at 3.35 volts if you put a load on it or maybe over time. What is it now?

 

[John Frum]

Cell 8 has the highest voltage because I charged it the longest

I'm think there are misunderstanding afoots here.

 

[Gazoo]

Cell 8 has the highest voltage because I charged it the longest

Doesn't sound to me you charged all cells equally per the guidance given to you.

 

[squowse]

Just a thought - this may be a really bad idea for reasons i can't think of - you don't want to be running a generator for days for the piddly 240W power supply.
Could you use a 12v inverter attached to the highest 4 of the cells. Then use that to power your charger to parallel charge the other 4. Then maybe swap round. You would then be more balanced so you could do more series charging with your generator.
Unless there's a 4S BMS you can use (or is it possible to use an 8S BMS as a 4S?), then you will need to be vigilant and make sure none of the cells drop to 2.5V.

 

[AlexanderKristiansen]

Doesn't sound to me you charged all cells equally per the guidance given to you.

That's correct, and I'll try to explain.

I was too impatient. The problem (for me) was that the voltage of the first cell didn't rise, not on the power supply or when I measured the voltage with a multimeter. I thought I had done something wrong with the power supply and that the cell didn't charge correctly because I couldn't register any rise in voltage.

At that time I had charged the first cell for over 2 hours and the voltage was 3,38V. I charged all the other cells for about 45 minutes each, that's all I could charge with my generator, I was starting to run out of fuel. I connected all the cells and started my generator to charge the pack. Not long after the 8th cell reached 3,63V and the BMS kicks in and stop the charging. The remaining 7 cells are now significantly lower. The cell pack is about 27,2V before it was about 28,0V.

That's why I brought the 8th cell back with me home. If I discharge it to about 3,45 it will be approximately the same voltage as the other cells. I know the pack won't be top-balanced, but at least the 8 cells will be a lot more similar in voltage.

The 8th cell is now 3,51V.

I guess I will have to bring all 8 cells with me back home and have the cells connected for days with the power supply. But I will have to wait until the snow melts and the road is open. Now it's quite a ski trip from my cabin to my car.

 

[Ampster]

The 8th cell is now 3,51V.

It is starting to settle. By the time you get back to the other cells it may settle at 3.35 volts. I realize it must be confusing with three people giving you their own version of how big the problem is and how to fix it.

At the risk of being repititious, I do not think it is a big problem anymore and if you are comfortable only charging to 3.4 volts per cell (27.2 for the pack) the problem will most likely not surface. It has worked for me and my philosophy is do not let perfect be the enemy of good. 3.4 volts is good enough for me.

 

[AlexanderKristiansen]

(sorry for my English, I'm from Norway)

I have to thank @BarkingSpider for this amazing and easy way to dischagre a single cell. Much quicker than connecting a light bulb. And I actually could have done this at my cabin, I had everything I needed to discharge the cell.

I followed his advice and bought an 82 feet cable (25 meters), 15 gauge (if my conversion from mm is correct). On one end of the cable, I stripped it and connected the positive and negative. I just cut the ground of. I tried to measure the Ohm, but it wasn't constant. Sometimes 0, something, other times 8 or 15.

On the other end of the cable, I stripped it and fastened one end of the wires to one of the poles, polarity doesn't matter. Since I had heard that the discharge is pretty fast, I didn't fasten the other cable, I just held it at the other pole.

Boy did that work great! The cell decreased 0,01V every 1-2 minutes. I measured the voltage on the cell every 30 seconds. I needed to go from 3,5V to 3,35. I think I used around 20 minutes to discharge to 3,35.

This is an extremely easy and safe way to discharge a single cell. There is a tiny spark if you look really close but other than that nothing. The only thing to watch out for is to measure the voltage often because the discharge is really fast. Thanks again to @BarkingSpider, you saved my day!

I must say, this is a great community with lots of helpful people, thank you to everyone who tried to help me :-D

 

[Ampster]

Congratulations. It may take a few iterrations with charging cycles to get it closer. There is still the option of just reducing your charging voltage if that runner is persistant.

 

[AlexanderKristiansen]

Congratulations. It may take a few iterrations with charging cycles to get it closer. There is still the option of just reducing your charging voltage if that runner is persistant.

I don't think it's persistent, I know I charged it longer than the others. Someone mentioned to keep the cell connected in series with the others, would that bring the voltage down to a level approximately close to the other cells?

 

[Ampster]

Someone mentioned to keep the cell connected in series with the others, would that bring the voltage down to a level approximately close to the other cells?

No, a serial connection will not bring the voltage down. Lowering the charge voltage of the pack might keep that cell from being a runner.

 

[G8trwood]

Slightly different question, with 16cells, can I group in like voltage groups of 4 and charge with a 12v charger? My marine 3 bank charger is 14.4 volts until current drops below one amp and then it floats at 13.8 for 24 hours then cuts off unless voltage drops below 12.6.
Or just order the bench top power supply
Thanks

 

[Ampster]

Slightly different question, with 16cells, can I group in like voltage groups of 4 and charge with a 12v charger?

That might be a faster way to get them near the top with existing equipment but you would need to watch them carefully so one group of 4 does not get overcharged. I assumed you meant 4P4S? is that correct? I am not sure there is any value in trying to organize in like voltage groups since the voltage curve is pretty flat until you get near 3.4 volts per cell.

 

[G8trwood]

Yes, but I was thinking to charge initially as separate groups of 4s as my charger is 3x10amps

and thank you

 

[Ampster]

Yes, but I was thinking to charge initially as separate groups of 4s as my charger is 3x10amps

It would not be any faster since Amps over time to fill Amphours would be the same time. . You would loose the advantage of putting them in parallel which is that 1/4 of the cells would be balanced to each other. To me that would reduce the odds of one cell being a runner. There is no perfect solution. Most advocates of parallel top balancing believe it is worth the time, expense and effort. It eliminates variations in balance more effectively than anything except taking each cell to 3.65 volts individually.

 

[Hedges]

Yes, but I was thinking to charge initially as separate groups of 4s as my charger is 3x10amps

and thank you

Whatever you do decide to do, if any cells are in series when charging, have a BMS monitoring those cells and interrupting the charge current.
That could be 4s, and it could be 4p4s, but if your only BMS is 16s it would have to be able to work as 4s.
I could come up with ways to use a 12V charger to charge 4s of cells in a 16s pack, having the 16s BMS control the charger.

 

[BretS]

It would not be any faster since Amps over time to fill Amphours would be the same time. . You would loose the advantage of putting them in parallel which is that 1/4 of the cells would be balanced to each other. To me that would reduce the odds of one cell being a runner. There is no perfect solution. Most advocates of parallel top balancing believe it is worth the time, expense and effort. It eliminates variations in balance more effectively than anything except taking each cell to 3.65 volts individually.

@Ampster , I am waiting on my cells now, and learning this whole process while waiting. I am getting 12 cells to put in a 3P4S configuration with a REC active BMS. I have read that you should put the cells under compression from the start. Could I put the cells in my box, under compression, in the configuration that I need them with no busbars, and use the power supply to take them one at a time up to 3.64V? Then put my busbars and BMS leads on and good to go? I am guessing this may take longer? My thinking is to not have to remove them and reconfigure after top balancing...

 

[Ampster]

That would work.
Just a confirmation that you have 16 cells but 3P4S calculates to 12 cells. Either way 12 cells or 16 cells is heavy. I suspect you are not planning on moving that box without some help.