Balance Optimization, software?

[falgsc-al]

Greetings,

Has anyone by chance found a program that can match a pack by parameters? I have a lot of cells to choose from, for 2 3S/8P LiPo packs, and I was hoping there was a program to help me choose which 3 to parallel to get a nearest match when I series them together. Optimized matching I would call it but Im not an expert.

Any help is greatly appreciated

 

[sunshine_eggo]

What parameters do you have?

 

[falgsc-al]

mAH and could get internal resistance

 

[OffGridInTheCity]

I've built 98 packs (so far) of 260ah each - nearing 10,000 18650 cells of about 6 different types.

This will work.... (you don't need fancy)
1) Discard cells with abnormally high IR.
2) Set aside cells that test <85% or <90% original capacity - set a cut-off and higher the better based on your source of cells.
3) Arrange the cells by type and 100mah capacity groups based on your testing.
4) Then round-robin, evenly distribute among the packs that will make up your battery. This will give even ah / pack.
Note: IF you can use all the same cell type for your battery, its a little better.

What can happen...
The main thing I found is that even when tested ah are 'perfectly matched' per pack, you can sometimes get a pack in the series that makes up the battery to be 'just a little weaker' for some reason. SO... I leave room (some blank cell holder spots) to add up to add a few cells. Up to 10% of the pack ah size. This has happened on mixed cell packs about 4% of the time. And has happened on same cell type packs 1% of the time.

I don't mean to overblow the pack 'sag'.. I just noticed it because I use Batrium with individual pack longmons - so I can see this easily. If you using a less expensive BMS such as DALY or something like that - it will likely just balance up and you'll not see anything.

 

[sunshine_eggo]

First, 3S8P is confusing. You mean 3P8S for 24V?

Second, if you're capacities don't vary by more than a few %, it's probably not worth the effort.

IMHO, I would go with 3X 2P8S batteries instead of 2X 3P8S for simplicity. 3P complicates matching, and I'm not a huge fan of parallel cells - better to have more redundant batteries than have to break them down when a cell fails. Given the investment, an extra $100-200/battery for its own BMS pays for itself when you have a problem in the future. I wouldn't consider it out of line to simply have 6X 8S batteries in parallel.

IR of your new cells should all be pretty consistent, so that shouldn't be an issue.

You want each of your series elements to be the same capacity. @OffGridInTheCity's process is pretty much it; however, that's more applicable to ~2Ah 18650 cells. With 280000mAh and likely not major variations (would expect no more than 5% total variation if you purchased same type, capacity and brand from the same vendor), you need to sort them from low to high. As you build each parallel cell, pick the first on the list and match it with the last on, e.g., assuming 48 cells:

1+48 = cell 1
2+47 = cell 2
3+46 = cell 3
etc.

 

[falgsc-al]

Yes thank you for the correction I am aiming for 3P/8S for 25.6v pack.

The packs are being split up over 2 places, so thats why Im going with 2 packs.

I also ordered 4 spare cells to use in my 12v camper application which is less pertinent so I intended to send the remainders there.

These are 300AH prismatic cells so I cant exactly leave a spare cell slot to offset any imbalance. Im sure the loss will be minimal if I just went with it, but I am trying to get all the energy I can out of them. They have about +\- 4ah difference between them. Less than 2% which, which I consider good.

 

[sunshine_eggo]

Yes thank you for the correction I am aiming for 3P/8S for 25.6v pack.

The packs are being split up over 2 places, so thats why Im going with 2 packs.

I also ordered 4 spare cells to use in my 12v camper application which is less pertinent so I intended to send the remainders there.

These are 300AH prismatic cells so I cant exactly leave a spare cell slot to offset any imbalance. Im sure the loss will be minimal if I just went with it, but I am trying to get all the energy I can out of them. They have about +\- 4ah difference between them. Less than 2% which, which I consider good.

IMHO, 8Ah total range vs 300Ah, and the 3P complication, not worth "matching".