2P8S Connect strategy

[bigroadadventure]

I have 16 300AH Sinopoly cells that I'm building into a 2P8S configuration. So far a couple cells are testing weaker than the others. Would you put the weak cells together in a pair so you could keep an eye on them with the 8S BMS or would you pair each with a stronger cell so it can help keep the weak one balanced? I'm seeing variations between 7 and 30mV for the worst cell according to the BattGO (after top balancing and draining).

thanks!

 

[FilterGuy]

There is no 'correct' answer to this. I would split them up. You will get more out of your total bank that way. (If you put them together that pair will be 'double weak' and is going to be the limiting factor of your bank.)

 

[Craig]

I have 16 300AH Sinopoly cells that I'm building into a 2P8S configuration. So far a couple cells are testing weaker than the others. Would you put the weak cells together in a pair so you could keep an eye on them with the 8S BMS or would you pair each with a stronger cell so it can help keep the weak one balanced? I'm seeing variations between 7 and 30mV for the worst cell according to the BattGO (after top balancing and draining).

thanks!

what do you mean by weak ? is the capacity less or do they just sag more under load?

 

[FilterGuy]

It is a capacity thing. Your BMS will probably do it's best to keep them ballanced at the top of the charge curve. However, when the batteries are getting low, those cells will be the first to hit the 'shut down' voltage.

 

[FilterGuy]

It is a capacity thing. Your BMS will probably do it's best to keep them ballanced at the top of the charge curve. However, when the batteries are getting low, those cells will be the first to hit the 'shut down' voltage.

Oops..... Sorry @Craig.... I thought it was the OP asking.

 

[Craig]

It is a capacity thing. Your BMS will probably do it's best to keep them ballanced at the top of the charge curve. However, when the batteries are getting low, those cells will be the first to hit the 'shut down' voltage.

Not necessarily I have tested 100's of cells many can have exact same capacity but store it in different places. so one cell could be .01 volt lower than another at start of discharge but be .01 volt higher later in the cycle. That's why I don't bother worrying about balance within reason until the ends of the curve

 

[Craig]

Oops..... Sorry @Craig.... I thought it was the OP asking.

no worries.

 

[bigroadadventure]

My weakest cell is 60mV under the top cell with the pack at rest after discharging about 70% from top balance.

When under a .2C load (around 65 amps) the difference grows to around 250mV (2.970 vs around 3.220 volts for the other cells). This is the first pack I have ever built/balanced, but this cell seems too bad to keep. Thoughts?

 

[KBWaldron]

If you did do 8P2S and found a similarly stronger cell to match with that weak cell they would average out.

 

[Craig]

In order to do this correctly you need to know capacity and resistance of each cell in the pack. Ideally they would all have same capacity and same resistance. It looks like cells 1 and 2 have a higher resistance which makes them sag a bit. It is possible that it won't matter. I would be more interested in the Can use at 20% SOC if they are close at the top and bottom the middle is really irrelevant.

If you must have it perfect and I understand that things being out if sync are frustrating.
I would set the pack up in 1p8s configuration. Record the cell voltages under load. Take the lowest cell and put that with the highest cell. This way when you parallel the 2 cells together they should help average each other out.

I would test all 16 first and write the sag on each one on some tape or something. That way you can remove later if desired. If you do find one cell ridiculously low it may need to be replaced though. But you won't know until you test them all.

For instance most cells are 2.245 volts

If you have one that is 2.24 and one that is 2.25 together they should average to 2.245 and the pack will be closer balanced.

I individually test every cell in my pack for resistance and capacity before assembly. This way if I see an oddball value in production I can look at cell and decide weather it is expected or not.

 

[bigroadadventure]

I kept pushing the load and that cell dropped out pretty fast. I'm going to put these aside for now and test the other 8 pack and see where I stand with all 16. If this once cell ends up being the outlier I will probably pair it with a strong cell as suggested and keep an eye on it.

Thanks everyone!

 

[Craig]

I kept pushing the load and that cell dropped out pretty fast. I'm going to put these aside for now and test the other 8 pack and see where I stand with all 16. If this once cell ends up being the outlier I will probably pair it with a strong cell as suggested and keep an eye on it.

Thanks everyone!

Yes that is not looking good