[Guest]

Don't top balance your cells. Just buy them and charge them and go. You do need to watch them for a few cycles to be sure they all fall within a normal range. They should with no intervention. Don't discharge your new pack first. You need to charge it up first. NOT top balance. There will always be some variances but they should be very very very closely matched from the factory.

I fully agree that cells should be well balanced and if your truly concerned then you need to contact the company and have them log every cell capacity and IR and SOC from the factory and to be sure that they all match. Expect to pay more because it will take some effort to IR and Capacity match a set of cells for you.

 

[Guest]

Smaller cells will age faster than larger ones, which will in turn reduce their capacity more, accelerating the difference.

No, the smaller cell will last the normal time and the others will just last longer. The higher one can't affect the lower on at all. The life of the low cell is the life of the group. It sounds like your saying that the cell with more capacity will cause the lower capacity cell to actually age faster than normal. If you set your pack up to operate within the factory specs then it will last as the factory states. Might even last longer. But the one with more capacity can't make that low cell actually age faster.

Pete

 

[Guest]

Larger cells age slower than the smaller ones that age normally.

 

[JRP3]

Don't top balance your cells. Just buy them and charge them and go. You do need to watch them for a few cycles to be sure they all fall within a normal range. They should with no intervention. Don't discharge your new pack first. You need to charge it up first. NOT top balance. There will always be some variances but they should be very very very closely matched from the factory.

I never said you should top balance, or bottom balance, I was just using them as examples to clearly point out the differences of the effects in cell capacity and relative SOC.

I fully agree that cells should be well balanced and if your truly concerned then you need to contact the company and have them log every cell capacity and IR and SOC from the factory and to be sure that they all match. Expect to pay more because it will take some effort to IR and Capacity match a set of cells for you.

My point is it should be common practice to sort cells in that manner at the factory. CALB, and I suspect TS, already measure capacity and resistance of each cell as part of QC so it's a simple matter to group like cells together.

 

[JRP3]

No, the smaller cell will last the normal time and the others will just last longer. The higher one can't affect the lower on at all. The life of the low cell is the life of the group. It sounds like your saying that the cell with more capacity will cause the lower capacity cell to actually age faster than normal. If you set your pack up to operate within the factory specs then it will last as the factory states. Might even last longer. But the one with more capacity can't make that low cell actually age faster.

Pete

Never said the larger cells would do anything to the smaller cells, I said the differences will be exaggerated as time goes on. These differences will probably lead to an imbalanced pack, we just don't know the effect over time.

 

[Guest]

My point is it should be common practice to sort cells in that manner at the factory. CALB, and I suspect TS, already measure capacity and resistance of each cell as part of QC so it's a simple matter to group like cells together.

Well it does cost money to group and measure for EACH cell. I'd expect if you wanted that you'd have to pay for that service. I would not expect any different. I think the manufacturing process is pretty darn close and that there should be some checking of batches but not always every single cell. That is very time consuming and can become very expensive to pay to have that level of perfection.

If you want it I'd hope that they would charge a nominal amount for that service so you can have a well matched pack.

I know you never said to top or bottom balance. I was just trying to be more or less generic and not say YOU in particular on that. That was not the way it was meant to come across.

I'd like to see how close new batches really are. I'd bet they come pretty darn close. Now if you were talking Hi-Power I'd say be up front on what you want. I'd say those guys are less interested in QC than CALB or Winston.

Pete

 

[MN Driver]

No, the smaller cell will last the normal time and the others will just last longer. The higher one can't affect the lower on at all. The life of the low cell is the life of the group. It sounds like your saying that the cell with more capacity will cause the lower capacity cell to actually age faster than normal. If you set your pack up to operate within the factory specs then it will last as the factory states. Might even last longer. But the one with more capacity can't make that low cell actually age faster.

Pete

"The life of the low cell is the life of the group."

No, it's not. If the first cell in the pack reaches end of life for whatever reason, remove it or bypass the cell, adjust the charging voltage and keep driving because the rest of the group is going to be fine once the first cell fails. You will lose the capacity of that one cell and have a slightly lower pack voltage but if it was your lowest capacity cell and the next lowest capacity cell is a bit better than this one was, it might not be so bad. 100Ah cell is only going to be 320wh anyway, so a mile or mile and a half or so depending on the efficiency of the conversion.

 

[JRP3]

Well it does cost money to group and measure for EACH cell. I'd expect if you wanted that you'd have to pay for that service. I would not expect any different. I think the manufacturing process is pretty darn close and that there should be some checking of batches but not always every single cell. That is very time consuming and can become very expensive to pay to have that level of perfection.

Every cell is already tested for capacity and resistance. It has to be in order for them to know if they have a functioning cell and to see if it falls within specs. As Jack says it's an automated process. Each cell has a serial number on it. The only added effort is to include a copy of the data sheet with the cells. CALB cells are only slightly more expensive than TS last I checked, which probably has more to do with their flatter discharge curve and better density than the data sheet that comes with them.

 

[Guest]

"The life of the low cell is the life of the group."

No, it's not. If the first cell in the pack reaches end of life for whatever reason, remove it or bypass the cell, adjust the charging voltage and keep driving because the rest of the group is going to be fine once the first cell fails. You will lose the capacity of that one cell and have a slightly lower pack voltage but if it was your lowest capacity cell and the next lowest capacity cell is a bit better than this one was, it might not be so bad. 100Ah cell is only going to be 320wh anyway, so a mile or mile and a half or so depending on the efficiency of the conversion.

Well yes a cell can crap out for what ever reason and yes the rest of the cells are still good and you can marry in another to keep it all going. I do think you know what I meant. The low cell is the limiting factor until it craps out or a cell craps out and you have to marry in another. I am not in any disagreement with that.

 

[Guest]

Every cell is already tested for capacity and resistance. It has to be in order for them to know if they have a functioning cell and to see if it falls within specs. As Jack says it's an automated process. Each cell has a serial number on it. The only added effort is to include a copy of the data sheet with the cells. CALB cells are only slightly more expensive than TS last I checked, which probably has more to do with their flatter discharge curve and better density than the data sheet that comes with them.

Yes that is true but there is still work to do to gather and go through all the information to sort out the closed matched cells. It is not fully automated when it comes to that. Yes testing is done in the manufacturing process to the degree that the machines need to do. The final sorting will need to be done by hand and it will cost and I would charge for that level of balance. Even if you ask you may get that level of service and maybe they will do that for you for no extra. All you have to do is ask. Gotta remember that they don't just build a few cells at a time.

Pete

 

[jeremyjs]

Do any of the manufacturers, available to the public, make their cells close enough to matter? From what I've read if all the cells in a pack are within 1% or less of each other in IR and Capacity you can bypass the whole cell level bms/monitoring issue and simply treat the pack as one big HV battery. Only worrying about over charging/discharging on the pack level and not the cell level. After an initial balance, either top or bottom, of course.

 

[JRP3]

As mentioned the latest shipments of CALB cells have come in under 1%.

 

[jeremyjs]

As mentioned the latest shipments of CALB cells have come in under 1%.

Impressive

 

[Guest]

After an initial balance, either top or bottom, of course.

Not needed unless the cells are so out of balance in SOC that you have no choice. Load them up, charge them up and go. They do not need a pre top or bottom balance if they are within those close tolerances. If you pick up a badly managed batch of lithium cells like I did you have no choice and then bottom balancing is best. But new fresh from the box and if you got them as one single shipment I would not do anything but charge them up then use them.

Pete

At 1% or less your batteries are pretty good. My SOC for my cells were so out of whack that it has taken quite some time to find matching cells. I am hunting for matching in capacity as well as balancing them for voltage at the lower end. So far it is working. Just for me it's a slow process. I do work a full time job and have other interests too.