More 16s packs in parallel with same BMS

[RikH]

Hello all,

Experimenting with my cells has learned me a lot. One of it being busbars are expensive. I have puchased 100 206Ah cells almost a year ago but I started testing with them a few month ago. What I have learned also is that I don't want the batteries laying flat on their side because I had one leaking. It will be replaced under warranty but still I don't want them laying anymore. So, now I want to rebuild / rethink about how to assemble a new 3p16s pack. What I made before was this:



Here is the cell that went bad in another configuration:



Now I'm thinking of making 3 16s packs and connecting them in parallel. Nothing special so far but I want to do that with only 1 BMS by connecting the cells in parallel on cell level with only balance leads. Being the interconnecting wires bigger than the balance leads to the BMS of course. Has anyone tried this before? What are your thoughts on this?

 

[fafrd]

Hello all,

Experimenting with my cells has learned me a lot. One of it being busbars are expensive. I have puchased 100 206Ah cells almost a year ago but I started testing with them a few month ago. What I have learned also is that I don't want the batteries laying flat on their side because I had one leaking. It will be replaced under warranty but still I don't want them laying anymore. So, now I want to rebuild / rethink about how to assemble a new 3p16s pack. What I made before was this:

View attachment 34654

Here is the cell that went bad in another configuration:

View attachment 34655

Now I'm thinking of making 3 16s packs and connecting them in parallel. Nothing special so far but I want to do that with only 1 BMS by connecting the cells in parallel on cell level with only balance leads. Being the interconnecting wires bigger than the balance leads to the BMS of course. Has anyone tried this before? What are your thoughts on this?

Won’t work unless each set of 3P cells is perfectly balanced (which is pretty much impossible).

if you want to use a single BMS, you need to form 3P sets of cells with high-current busbars so they each essentially behave like 616Ah cells.

Sounds like you know enough to take it from there...

 

[RikH]

if you want to use a single BMS, you need to form 3P sets of cells with high-current busbars

Yes, that is exactly what you see in the first picture.

Won’t work unless each set of 3P cells is perfectly balanced

Well I can top balance the whole pack of 48 cells and than connect them in the way I have in mind. What would be the downside?

 

[fafrd]

Yes, that is exactly what you see in the first picture.

16 sets of 3 cells in parallel like that can be managed by a single 16S BMS.

Well I can top balance the whole pack of 48 cells and than connect them in the way I have in mind. What would be the downside?

If you are thinking of making 3 16S packs and controlling them with a single BMS, there are a few reasons that will not work:

first, if you are thinking to have each sense lead split into 3 connections to each of the 3 16S packs, imbalances that develops between cells at the same position will lead to large currents through the small sense leads that will burn them out.

Second, if you are thinking to connect single sense leads to only one 16S pack and to let the other 2 packs piggy-back on those readings, it will only be a matter of time before the imbalances which accumulate result in an unmonitored cell being discharged below 2V or charged above 3.65V.

Low-resistance connections between 3 cells essentially allows turn to be treated as a single larger cell, so a single sense wire will protect all 3.

If you want 3 16S packs in parallel, you will need a BMS for each so that each individual cell can be monitored and protected by its own sense wire...

 

[RikH]

if you are thinking to connect single sense leads to only one 16S pack and to let the other 2 packs piggy-back on those readings, it will only be a matter of time before the imbalances which accumulate result in an unmonitored cell being discharged below 2V or charged above 3.65V.

No, that would be a stupid idea. What I mean is the connection you mean here:

have each sense lead split into 3 connections to each of the 3 16S packs, imbalances that develops between cells at the same position will lead to large currents through the small sense leads that will burn them out.

Now the question is how big will these currents be. In my understanding it won't be a lot UNTIL one cell goes rogue.

Thanks for your reply, realy appreciated!

 

[fafrd]

No, that would be a stupid idea. What I mean is the connection you mean here:

Now the question is how big will these currents be. In my understanding it won't be a lot UNTIL one cell goes rogue.

Thanks for your reply, realy appreciated!

OK, so I think you are asking about using a single BMS to monitor a 16S3P structure, as opposed to a 3P16S structure.

The obvious answer on sizing the parallel connections is to make them the same size as the serial connections.

There could be more nuance to it than that that might allow you to save an AWG or two on the parallel connections, but is it even worth the trouble?

If charge currents of X Amps are coming in, you want no additional barrier to those charge currents preferring one of those 3 parallel cells over another other than voltage. That is my simple-minded analysis for assuring parallel conductance is at least as high as serial conductance.

If the parallel conductance is high enough, you will never have a single cell going ‘rogue’ - at worst, you will have a single parallel set of 3 cells going rogue together...

 

[RikH]

OK, so I think you are asking about using a single BMS to monitor a 16S3P structure, as opposed to a 3P16S structure.

The obvious answer on sizing the parallel connections is to make them the same size as the serial connections.

There could be more nuance to it than that that might allow you to save an AWG or two on the parallel connections, but is it even worth the trouble?

If charge currents of X Amps are coming in, you want no additional barrier to those charge currents preferring one of those 3 parallel cells over another other than voltage. That is my simple-minded analysis for assuring parallel conductance is at least as high as serial conductance.

If the parallel conductance is high enough, you will never have a single cell going ‘rogue’ - at worst, you will have a single parallel set of 3 cells going rogue together...

Ok, you've convinced me. I'll make some connections to get a view on what will fit best for this install. I'll make pictures and get back. Thanks again fafrd!

 

[zorlig]

You will probably have more cross current than you expect. The design in you image was best. Not having enough bus bars to be able to do it like you did was an error in my build.

My balance wires are 2-3 ft long and with any decent current I can measure voltage differences.

 

[RikH]

You will probably have more cross current than you expect. The design in you image was best. Not having enough bus bars to be able to do it like you did was an error in my build.

My balance wires are 2-3 ft long and with any decent current I can measure voltage differences.

Thanks for your reply zorlig. I have ordered more busbars (being $ 3.00 each ex shipping). ETC don't have them in stock(!?) so I have to wait and now this Chinese new year doesn't expedite it either. We'll see.

 

[FilterGuy]

Hello all,

Experimenting with my cells has learned me a lot. One of it being busbars are expensive. I have puchased 100 206Ah cells almost a year ago but I started testing with them a few month ago. What I have learned also is that I don't want the batteries laying flat on their side because I had one leaking. It will be replaced under warranty but still I don't want them laying anymore. So, now I want to rebuild / rethink about how to assemble a new 3p16s pack. What I made before was this:

View attachment 34654

Here is the cell that went bad in another configuration:

View attachment 34655

Now I'm thinking of making 3 16s packs and connecting them in parallel. Nothing special so far but I want to do that with only 1 BMS by connecting the cells in parallel on cell level with only balance leads. Being the interconnecting wires bigger than the balance leads to the BMS of course. Has anyone tried this before? What are your thoughts on this?

What cells are those with the unique terminals?

 

[RikH]

What cells are those with the unique terminals?

These cells with these busbars. They are ETC cells with the busbars laser welded on them. In hindsight I think this is more for mobile purposes and not necessary for our solar systems. Price for 1 busbar is $ 3.00 which isn't cheap and the welding costed also a little bit extra but I don't recall how much.

 

[FilterGuy]

Thanks for the info!!