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?

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 (for another purpose) converting a 10S pack to a 7S, redistributing cells from the 3 disconnected strings back to the 7 active strings with cables.

Thanks in advance.

 

[matthewslack]

I'm intrigued as to why! Is this for huge capacity, or to support very high current/power?

The current ratings are very important for the first and last interconnections, much less so for the intermediate ones as if cells within each pack are properly similar, there will very low currents in them. The end connections take the full current of 18 cells, which depends on what will be powered. Metal sized for 6 cells will not do for 18.

What advantage do you see in one BMS vs simply connecting packs in parallel, having charged to equal voltage beforehand?

 

[theabsurdman]

I'm intrigued as to why! Is this for huge capacity, or to support very high current/power?

The current ratings are very important for the first and last interconnections, much less so for the intermediate ones as if cells within each pack are properly similar, there will very low currents in them. The end connections take the full current of 18 cells, which depends on what will be powered. Metal sized for 6 cells will not do for 18.

What advantage do you see in one BMS vs simply connecting packs in parallel, having charged to equal voltage beforehand?

Hi,
thanks for your reply. It's for extra capacity mainly. I'm still a little confused.

Imagine 3 packs like the one pictured above side-by-side. The right-most (master) one will have the BMS, the pack + and - leads and the balance wires. The other 2 will only be connected to the master by cables between their respective 10 strings, essentially trebling the capacity of each from 6 to 18 cells. How chunky would those connecting cables need to be? And would the serial strip connections in each pack (esp. the master) need to be beefed-up?

I considered giving each pack its own BMS (and still might) but have read that paralleling packs with BMSs attached can be problematic should one low-voltage shut down before the others and thus would require more housekeeping.

 

[matthewslack]

Hi,
thanks for your reply. It's for extra capacity mainly. I'm still a little confused.

Imagine 3 packs like the one pictured above side-by-side. The right-most (master) one will have the BMS, the pack + and - leads and the balance wires. The other 2 will only be connected to the master by cables between their respective 10 strings, essentially trebling the capacity of each from 6 to 18 cells. How chunky would those connecting cables need to be? And would the serial strip connections in each pack (esp. the master) need to be beefed-up?

I considered giving each pack its own BMS (and still might) but have read that paralleling packs with BMSs attached can be problematic should one low-voltage shut down before the others and thus would require more housekeeping.

I prefer multiple batteries over one big one just for resilience. If one fails, it doesn't stop you, if you run one flat, you have a reserve etc..

Your three packs with all those cross connections need careful mechanical assembly so that they can withstand whatever knocks and jolts they may get without any risk of shorting out.

Electrically, the + and - of the ends of the pack need to be beefy enough, especially off your master where you could be taking three times the current the pack was designed for.

I think I would keep separate BMSs, and link externally via online fuses from each pack, and just not run it down to lvc. With 18P, vast capacity, I would regard the housekeeping to stay on top of charge state as fairly minor. Fit a voltmeter if your display does not show voltage, and set yourself a red line 'never go below' voltage.

 

[theabsurdman]

I prefer multiple batteries over one big one just for resilience. If one fails, it doesn't stop you, if you run one flat, you have a reserve etc..

Your three packs with all those cross connections need careful mechanical assembly so that they can withstand whatever knocks and jolts they may get without any risk of shorting out.

Electrically, the + and - of the ends of the pack need to be beefy enough, especially off your master where you could be taking three times the current the pack was designed for.

I think I would keep separate BMSs, and link externally via online fuses from each pack, and just not run it down to lvc. With 18P, vast capacity, I would regard the housekeeping to stay on top of charge state as fairly minor. Fit a voltmeter if your display does not show voltage, and set yourself a red line 'never go below' voltage.

Thanks mate, I'm leaning more and more towards your way of thinking - a BMS on each pack.

Here's a comprehensive post and discussion on another forum re. the issues involved.

Paralleling BMS protected Li-ion packs. - Electricbike.com Ebike Forum

It's getting harder and harder to fit normal packs on to full suspensions bikes with small triangles and with the new small mini cubes that Luna offers are a

electricbike.com

Cheers,
John

 

[StuartsProjects]

I prefer multiple batteries over one big one just for resilience. If one fails, it doesn't stop you, if you run one flat, you have a reserve etc..

Agreed.

I have two batteries for my eBike, to parallel them up would be daft in my view, so many advantages in having two seperate batteries.....................

 

[Nealh]

If one wants to then yes the three seperate packs can be paralled but in there existing state by use of parallel discharge 1T3 wire cable, though it will be best to include a diode on each battery dicharge to prevent reverse current flowing . One can then still have a 18p battery for one large battery and all three will within reason dsicharge much the same , the controller will sip power from all three as one . Any cable will liklely need to be 10 or 12awg .
For charging one would revert to disconnecting the three and charging sigularly, all having their own independant BMS.

 

[Benjahmin]

I've run two paralleled batteries for some time now with no problems.
They give me around 28Ah capacity, but my main reason for doing it is that I live in a very hilly area and the doubled amp delivering capacity means that the batteries suffer less voltage sag up the many steep hills.
Less sag means longer battery life as they are less distressed day to day.
They are simply connected together via a Y lead with Anderson connectors.
HOWEVER, there are some disciplines involved.
They are always charges individually when disconnected from each other.
Each battery has it's own charger but I always finish them off with the same charger to ensure voltage equality.
I ALWAYS measure the output voltage of each battery BEFORE connecting them together, accepting a difference of no more than 0.1v.
Sounds like a faff, but really it's no more bother than charging.

 

[matthewslack]

The Grin Ligo battery system is modular: each block is 10s1p and a shade under 100Wh, which means some airlines will accept a bunch of them, when they would not accept a single battery of the same total capacity.

Each block has it's own BMS.

https://ebikes.ca/36v-ligo-plus-battery.html

 

[theabsurdman]

The Grin Ligo battery system is modular: each block is 10s1p and a shade under 100Wh, which means some airlines will accept a bunch of them, when they would not accept a single battery of the same total capacity.

Each block has it's own BMS.

https://ebikes.ca/36v-ligo-plus-battery.html

Custom BMS designed to be paralleled. And no longer available unfortunately.

 

[matthewslack]

Custom BMS designed to be paralleled. And no longer available unfortunately.

New version next year. I hadn't seen the note about the BMS. Fairly expensive but with Grin you always get top quality.