Connecting identical packs in parallel with a single BMS

[theabsurdman]

Hi,
I want to connect 3 identical 6P packs' parallel strings together side-by-side to operate as a larger pack off a single BMS - 2 questions:

1/ would it be sufficient to make the nickel strip connections between packs, illustrated by red lines in the pic below, to just 4 cells out of each 6? Am I right in assuming that the current requirement would be max pack amps divided by the number of cells in each parallel group?

2/ what would be the implications of recruiting cojoined cells into a parallel group using a single cable connection rather than using nickel strips?

Thanks in advance.

 

[theabsurdman]

Anybody?

To clarify what I'm trying to achieve. I have 3x 10S6P li-ion packs that I want to make into 1x 10S18P pack running off a single BMS. So I have to connect the 10 parallel groups of each pack together. The problem is how to achieve that efficiently.

I was also considering converting the 10S pack to a 7S, redistributing cells from the 3 disconnected strings back to the 7 active strings with cables.

 

[The Battery Doctor]

I just did this with 4 10S5P packs, i used large 14Awg wires which is over kill. Power will only flow through in series, any imballance will be extremlly small so 24 AWG would work fine if the cells are new or of the packs have very similar capacity.

 

[ZeroEm]

Will the second battery be connected to the controller and be charged with the first or will all the power flow thru the first battery? IF so the bridging will need to handle the current.

Need to bridge the batteries on both sides of the group not just one.

by theabsurdman » Nov 24 2022 6:48am

Hi,
I want to connect 3 identical 6P packs' parallel strings together side-by-side to operate as a larger pack off a single BMS - 2 questions:

1/ would it be sufficient to make the nickel strip connections between packs, illustrated by red lines in the pic below, to just 4 cells out of each 6? Am I right in assuming that the current requirement would be max pack amps divided by the number of cells in each parallel group?

2/ what would be the implications of recruiting cojoined cells into a parallel group using a single cable connection rather than using nickel strips?

Thanks in advance.

 

[docw009]

I hope you realize that you will be connecting both the top and bottom of these batteries? The double reds are redundant. That nickel is all at the same voltage. Just a single connect from the positive/negative of each cell group is enough.

Looks like you already have a BMS on the one in the picture. Replace it with a common port model on all three packs, and I believe you could run all three in parallel without having to join them.

 

[lnanek]

Looks like you already have a BMS on the one in the picture. Replace it with a common port model on all three packs, and I believe you could run all three in parallel without having to join them.

Isn't that how fires start, like https://youtu.be/JCaDz_2YcGQ ? Joining the batteries at the p-groups and using a single BMS seems much safer. The power is in one place with one BMS that can cut all charging and discharging and have pack current rated fuses around it if desired. The p-groups across all the batteries are guaranteed to be the same voltage if directly wired together. You also don't have to wire ideal diodes in front of each battery to prevent one from ever feeding the other if they lose balance due to a cell group in one having an almost dead cell, or the discharge/charging wiring being different lengths, or user stupidity like in the video where the guy had separate manual switches for the two batteries which pretty much guarantees he'll eventually leave one switch off over the years and unbalance the pair.

 

[Diggs]

I tend to agree with @docw009 and use common port BMSs on each of the packs and run them in parallel. I do it with two packs 4p14s and 5p14s and have had no issues for 4 years now and as I have stated in other threads I only charge to 90% when they are together as the cells are not all the same. There are also combiners for multiple packs most notable Bolten/Area13 has one for two packs but there are others for combining more packs. My thought is that multiple packs on a single BMS could lead to problems.

 

[theabsurdman]

Thanks for all your comments guys. I took my eye off this thread for a while but the input has been appreciated.