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A forum member "Yabert" in my other thread suggested I could use a single quality BMS to monitor more than one string of cells.


I'm using a Chevy Volt battery that will have a total of 8 individual battery packs in a 3p12s configuration. Each pack is 2kw, 12 cells in series and I have 8 of these that I want to run in parallel.



I was told that the BMS leads could be set up so the cells are paralleled.



I'm totally confused on this.



Can I only parallel two cells or can all eight be paralleled?



And if the cells are in parallel, how does the bms correct an imbalance in a single cell? Wouldn't bleeding energy from a high cell also bleed energy from the paralleled cells?


Are there safety drawbacks or limitations to this method?



Thanks,
 

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When you paralel cells you are effectively creating one big cell

When you charge the BMS tells you if one of the "groups of cells" is hitting the top of it's charge early

Or if one of the "groups" is still low when the others are charged the BMS gives that "group" a wee bit more charge


If a cell fails - one of the 12 in the group dies OPEN CIRCUIT - then the 11 remaining cells in that are now the group and as a result that group has less capacity

When you discharge it will reach the "warning voltage" when the other groups are fine

When you charge it will reach the "warning voltage" when the other groups are fine

If the cell dies "SHORT CIRCUIT" then it simply takes its brothers down with it and the entire group fails
 

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Discussion Starter #3
When you paralel cells you are effectively creating one big cell

When you charge the BMS tells you if one of the "groups of cells" is hitting the top of it's charge early

Or if one of the "groups" is still low when the others are charged the BMS gives that "group" a wee bit more charge


If a cell fails - one of the 12 in the group dies OPEN CIRCUIT - then the 11 remaining cells in that are now the group and as a result that group has less capacity

When you discharge it will reach the "warning voltage" when the other groups are fine

When you charge it will reach the "warning voltage" when the other groups are fine

If the cell dies "SHORT CIRCUIT" then it simply takes its brothers down with it and the entire group fails

I think you gave the right answer to the wrong question. I'm probably not making myself clear or I have such a complete misunderstanding of what the other guy said that I'm incapable of asking the right question.

The Chevy Volt battery has welded cell anodes, they can not be separated and have to be used in their 12 series module configuration. Each module is 2kw and 44.4 volts nominal and there are 8 of them. So its basically 8 individual battery packs, each 44.4 volts with 2kw of energy.

The idea is to parallel the 44.4 volt modules. Until I got the advice in this forum, I thought I was going to need one BMS controller for each 12 series module, but then the other guy said I could use just one BMS to monitor multiple modules by "paralleling the individual cells" through the BMS. Maybe someone should draw me a damn picture because I'm feeling stupid.
 

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NO
I am using Chevy Volt packs in my car

Each module has the main current paths welded together - leave them alone
Each module ALSO has a wire going to each set or three paralleled cells - these wires go to the Chevy BMS

You first get the "modules" to the same voltage
Then connect them together so they are exactly the same voltage
THEN connect their "BMS wires" together

You now have the cells paralleled - the BMS wires will keep them all at the same voltage - in other word the same state of charge

You will also need to connect the BMS wires to your BMS


The modules have the bolt on connectors - one each end
On the top of the module you will find a socket - the BMS connector goes into this socket

The other end of the BMS connector goes into the chevy BMS - - cut the cable so that you can access the individual wires
 

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Discussion Starter #5
NO
I am using Chevy Volt packs in my car

Each module has the main current paths welded together - leave them alone
Each module ALSO has a wire going to each set or three paralleled cells - these wires go to the Chevy BMS

You first get the "modules" to the same voltage
Then connect them together so they are exactly the same voltage
THEN connect their "BMS wires" together

You now have the cells paralleled - the BMS wires will keep them all at the same voltage - in other word the same state of charge

You will also need to connect the BMS wires to your BMS


The modules have the bolt on connectors - one each end
On the top of the module you will find a socket - the BMS connector goes into this socket

The other end of the BMS connector goes into the chevy BMS - - cut the cable so that you can access the individual wires
Thank you for the explanation. I'm not having any problem with locating things or the mechanics, I'm confused on how the bms is connected to 8 different packs.



So I have 8 packs each with 12 cells (really 36 but we treat them as 12). So each pack has cell #1, cell #2, cell #3 and so on. Are you saying that all Cell #1's get connected to the same bms terminal? That's 8 individual cells, in 8 different packs, all connected to the same #1 bms port? Then all the #2 cells connected to the bms port for that number, and so on?

I can not find anything with google that references this way of doing it.


What stops large currents from flowing through the BMS wires when a load is put on the pack?



I'm also considering just purchasing a single bms and connecting it to one 48 volt bank at a time. Leave it there for a day or two, then connect it to the second pack and so on. This way, the packs all get balanced on a regular schedule. Not sure if this is wise or not.

Since I'm using this for solar and charge and discharge rates are measured in 6 to 12 hour time frames, I'm guessing the packs should stay fairly well balanced all on their own. Thought maybe I could just get away with a "part time" bms on each pack.



Any google links where they discuss the paralleled cells technique?


Thanks a bunch, I'm learning!
 

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Discussion Starter #6
Is this what you're talking about?

I drew a simple schematic for a 24 volt system using 4 individual paralleled batteries with 6 cells in each battery.


Green lines are BMS leads.
 

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Yep that right


The only time that significant current could flow would be if something had gone very very wrong - the normal route for the current will be the main wires

If you feel paranoid then put fuses in the BMS lines between the modules - simple fuses so that the fuses blow before the wires melt
That would make it pretty bullet proof
 

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Yep that right


The only time that significant current could flow would be if something had gone very very wrong - the normal route for the current will be the main wires

If you feel paranoid then put fuses in the BMS lines between the modules - simple fuses so that the fuses blow before the wires melt
That would make it pretty bullet proof

Thanks! User Yabert posted this picture of a Zema BMS. I can't figure out what that resistor board is. Is that a custom job or something Zema makes or what?
And what is its purpose?
 

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Why do you need so much current for a residential solar storage application?
Well, say you want to run a refrigerator. A little one can draw less than 1 amp @ 120V, but a large one can draw more than 10 amps. Mine says 11.6 amps on the label, or 1392 watts.

Ignoring losses through an inverter you would need to have 116 amps DC @ 12V or 31.6 DC amps @ 44V in order to supply 11.6 amps @ 120V AC.

This also ignores the motor start current, which can be 2-3 times the running current. So real world startup current could be as much as 90-100 amps @ 44V for a few seconds.
 

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Why do you need so much current for a residential solar storage application?
Current isn't the only reason to parallel cells/modules. Parallel configurations offer more system capacity without changing the voltage. And as Murphy pointed out in the initial thread, parallel configurations only need a single BMS across one module instead of having a BMS that has to manage cells across all the modules.

Say for example the system capacity needs to be 12 kWh. A single Volt module is 2 kWh. So 6 modules are needed. A series configuration of 3P72S, noting that internally a module has 3 cells welded together, would have a nominal system voltage of 288 volts and would require a BMS system to manage 72 cells. That means a much larger BMS and an inverter that can take up to 300V input. This is a much tougher job than organizing in 18P12S configuration which parallels the 6 modules together and connects the 13 individual cell terminals of the 6 modules to each other. The system voltage is 48V nominal, 50.4V maximum, and only requires a single 12S BMS, which is an easier find, to manage the bank. Both give the 12 kWh of capacity, but have completely different management structures. BTW the 48V 12S system would by definition have to offer more current in order for the load power to remain the same. In fact the advantage of the 6S1P module configuration is that the system current is only 1/6 the current of the 1S6P module system.

If you think about it a bit, the inverter more than anything else is going to define the correct configuration. If it's high voltage/low current, then use series battery configuration. If it's low voltage/high current, then parallel the modules.

ga2500ev
 

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Why do you need so much current for a residential solar storage application?
Well, say you want to run a refrigerator. A little one can draw less than 1 amp @ 120V, but a large one can draw more than 10 amps. Mine says 11.6 amps on the label, or 1392 watts.

Ignoring losses through an inverter you would need to have 116 amps DC @ 12V or 31.6 DC amps @ 44V in order to supply 11.6 amps @ 120V AC.

This also ignores the motor start current, which can be 2-3 times the running current. So real world startup current could be as much as 90-100 amps @ 44V for a few seconds.
Thanks.

Look at the question again.

I did not ask, "Why is so much current needed for a residential application?", in which case you could get the opportunity to spread your peacock feathers and not derail the *thinking process* I was trying to provoke. Spoonfeeding teaches nothing.

The question still stands to OP.

Because of that derail, the prior post to this one got triggered to correctly explain the consequences and tradeoffs ahead of the answer I was seeking in the spirit of getting OP steered towards understanding the problem first, versus assuming OP did understand, given his noob questions on here. Spoiler alert, lol.
 
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