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Discussion Starter #1 (Edited)
Hey there folks!

I've recently come across a really cheap option that I'm thinking about jumping on for some modules to upgrade one of my Ford Ranger EVs to a lithium pack.

The problem is that the modules are 14s, and only 1.77kwh each. To get my voltage I will be running 6 modules in series (84s), which comes out to only about 10.6kWh of capacity. I'm looking to get at least 30kWh, so that leaves me with running 18 modules, as 6s/3p.

This of course means I will need a run a "series/parallel" pack. I've been trying to look into BMS options for this, but I haven't had much luck... It seems like I need something that has "slave" boards for each module, and then a master to control everything else from there.

Thanks for any advice!
 

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I'm not sure what you're describing.

You say you're running 6 modules in series, for a total of 84s?

But then you're going to use 18 modules total, in a 6s3p?

Difficult for me to imagine what your setup is.

If it's as simple as it sounds, all cells in parallel are automatically balanced. So 6s3p is balanced the same way that 6s1p is. If you have a 6s BMS, it'll work just fine.
 

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Discussion Starter #4
I'm not sure what you're describing.

You say you're running 6 modules in series, for a total of 84s?

But then you're going to use 18 modules total, in a 6s3p?

Difficult for me to imagine what your setup is.

If it's as simple as it sounds, all cells in parallel are automatically balanced. So 6s3p is balanced the same way that 6s1p is. If you have a 6s BMS, it'll work just fine.
Yes, 6 14s modules in series for 84s, then 3 of those in parallel. So it would be effectively like 3 84s packs in parallel with one another. So the "3p" doesn't end up being with the cells actually in parallel with each other, just the complete packs in parallel... I've attached an image of what I'm talking about.
 

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3 might not be too bad, but paralleling string like that will cause imbalance issues, uneven wear, shorter lifespan
 

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Yes, 6 14s modules in series for 84s, then 3 of those in parallel. So it would be effectively like 3 84s packs in parallel with one another. So the "3p" doesn't end up being with the cells actually in parallel with each other, just the complete packs in parallel...
Some of us with a mathematical or scientific orientation have resorted to parentheses to clarify this:
3P (6S (14S))

The three strings in parallel do have potential issues. Ideally, you need a BMS for each string, and either three separate chargers or contactors to cut off charging of each string when appropriate. I'm sure lots of people have successfully run EVs with parallel strings of cells and a less-than-ideal charging and monitoring system.
 

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Lots of racers use this setup with the significant change that all the cell taps are paralleled together; example: cell 1 from each subpack is tied to all the other cell 1's via balance wires, and so on. This presumes high quality cells but it can be a successful strategy. It does rely on sufficiently sized balance wires as these have to carry any current mismatch between individual cells on each string. In your situation you'd probably be safer with some way to monitor each cell in each string independently.
 

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Yes, paralleling cell groups at the lowest level across the strings is the solution to this issue
3 might not be too bad, but paralleling string like that will cause imbalance issues, uneven wear, shorter lifespan
I guess then only one "master" balancing device should be used.

That means the sub-pack module BMSs should have protective functionality only.
 
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