# BMS for 29s80p Panasonic 18650 battery



## Pawiel (Sep 15, 2013)

hallo 

Few weeks ago I was looking for ready to buy BMS system for 29s80p Battery.
The first idea was to build 29s80p battery, just 80parallel and 29serial.
But one of my friends said, that better solution will be 29s and (4 x 20p), the final effect will be the same, but it will be much easier to controll that battery.

So, here comes the queestion. Does anybody of You know monitoring system, which will monitor each of 20p blocks (connected to 80p block and finally to 29s80p battery)?

I was looking on the Internet for it but I didn't find anything. 

Or is this not to clever idea group these cells into such parallel blocks?

Have a nice day!


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## dougingraham (Jul 26, 2011)

The Tesla Model S pack is 74P of 18650 cells.

It is not clear what you are talking about with 20P blocks. If you make four 20P blocks and then parallel those you have an 80P block. There is no difference. Can you clarify what you mean?

With only 29S this is a pretty low voltage arrangement. An 80P would be somewhere between 176 AH and 232 AH. Giving a pack in the 22kw range. What is your project?


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## Karter2 (Nov 17, 2011)

I suggest you read the "Series and parallel cells" thread just above yours.


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## Pawiel (Sep 15, 2013)

Hallo.
Hmm, I think that there is a diffrence.
I have in my e-bike 10s5p battery block. Few weeks ago one section (5p) suddenly died, the voltage falled down to 2.6V, while other sections were idealy ballanced. The battery company changed whole 5p section, they gave me battery with 5 newcells.
Now, think about much bigger project, where you have parallel section made of 80 cells, each cell, for example for 5$... When one cell dies, whole section beggins work inproperly. After long period of time you will see, that something bad happens with voltage of that section. At the end, you change all cells,to achieve proper section-parameters (capacity, internal resistance, etc.) -> 80 cells x 5$ = 400 $...

When you group cells in smaller groups (for example 20p) and you controll each group (4 x 20p = 80p), when something bad happenes, you will see much faster. And the costs of repair of that "dead 20p-section" will be much smaller. You ucan also cut off bad section (maybe with a FET or something similar).

once again, I want to make 80 cells parallel (4 x 20p), after that I want to connect it to series (29s).


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## dougingraham (Jul 26, 2011)

Pawiel said:


> Hallo.
> Hmm, I think that there is a diffrence.
> I have in my e-bike 10s5p battery block. Few weeks ago one section (5p) suddenly died, the voltage falled down to 2.6V, while other sections were idealy ballanced. The battery company changed whole 5p section, they gave me battery with 5 newcells.
> Now, think about much bigger project, where you have parallel section made of 80 cells, each cell, for example for 5$... When one cell dies, whole section beggins work inproperly. After long period of time you will see, that something bad happens with voltage of that section. At the end, you change all cells,to achieve proper section-parameters (capacity, internal resistance, etc.) -> 80 cells x 5$ = 400 $...
> ...


I believe I understand now. You want to make 4 batteries 29S20P and then parallel those batteries. Effectively you would have a 29S80P pack but the 20P sub modules would not be paralleled. And the reason to do this is because you had a cell go bad in a different pack and it drained all five paralleled cells and eventually ruined them. You wish to avoid the destruction of an 80P section.

What you are proposing will still cause the death of a 20P module if a similar failure occurs in your large pack. Additionally that whole string that has that a sick module will then become overcharged by the neighbor strings and if placed on a charger will be overcharged. With 29S (assuming charging to 4.1V) the normal charge termination voltage would be 121.8 volts. For the series string with the damaged module the effective voltage of the individual cells at charge termination will be 121.8/28 = 4.35 volts. This is overcharged perhaps to the point of causing the electrolyte to break down. In a pouch cell this would puff. If the electrolyte breaks down in all the cells in that string you could lose 580 cells, not just 80 and an overcharge situation like this one could cause a fire.

At this time I feel I must point out that even a rudimentary BMS (half pack comparison) would catch such an issue with a module in the simple 29S80P format and certainly any of the battery monitoring systems would alert you to this kind of problem even telling you which module the issue is in.

With an 80P module you should probably fuse each cell (in the Tesla fashion) so that if you have a cell that goes to a hard short it will blow its fuse instead of blowing up the cell. If one of these cells can do 3C then 79 of them could provide an effective charge current to that one shorted cell of 237C.

I am not a BMS proponent for the prismatic LiFePo4 type cells but when using the more dangerous types of lithium chemistry it is a good idea to at least monitor.


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## Hollie Maea (Dec 9, 2009)

Fuse the individual cells. That's the only reasonable route with these cells. Then it doesn't matter how you moduleize then.


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## sholland (Jan 16, 2012)

Hollie Maea said:


> Fuse the individual cells. That's the only reasonable route with these cells. Then it doesn't matter how you moduleize then.


Doesn't Tesla fuse each individual cell? I seem to remember reading this in a white paper some time ago... maybe it was the roadster pack.


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## Karter2 (Nov 17, 2011)

sholland said:


> Doesn't Tesla fuse each individual cell? .


Yes, on all their packs. Also liquid cooling to all cells with monitoring and shut down systems.


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## Pawiel (Sep 15, 2013)

dougingraham said:


> I believe I understand now. You want to make 4 batteries 29S20P and then parallel those batteries.


Hmm. No. I wrote:



Pawiel said:


> once again, I want to make 80 cells parallel (4 x 20p), after that I want to connect it to series (29s).


I want to make a lot of 20p packs (580).
After that, I will group these packs in blocks , and connect it to parallel (each block will consist of 4 20p packs, this will make 80p block).
So, I will have 29 blocks of 80p.
These blocks I will connect serial, 29s.
So, at the end will be 29s80p.

I am looking for PCB's to have controll over:
- every 20p pack (maybe with possibility of disconnect wrong pack),
- every 80p block (standard BMS function, monitoring voltage).



Hollie Maea said:


> Fuse the individual cells. That's the only reasonable route with these cells. Then it doesn't matter how you moduleize then.


Yes, every cell will be fused.
I will do air cooling. 

sorry, If my english is not good enough.


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## Hollie Maea (Dec 9, 2009)

Pawiel said:


> Hmm. No. I wrote:
> 
> 
> 
> ...


I would forget about the 20p idea. The four blocks in parallel are all the same nodes, so they are functionally identical. There is no extra level of monitor and control that can be done at that level. Better to fuse every cell and just put them straight into 80p blocks.


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## Pawiel (Sep 15, 2013)

Hollie Maea said:


> I would forget about the 20p idea. The four blocks in parallel are all the same nodes, so they are functionally identical. There is no extra level of monitor and control that can be done at that level. Better to fuse every cell and just put them straight into 80p blocks.


The reason why I want to do that is to minimze costs, when one or few cells will be dead. There is no other reason.


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## dougingraham (Jul 26, 2011)

Pawiel said:


> The reason why I want to do that is to minimze costs, when one or few cells will be dead. There is no other reason.


If you have a 1S20P module and you parallel it with another 1S20P module you have a 1S40P module. The fact that they might be in different boxes doesn't enter into it. The modules are in parallel and are not isolated in any way from each other. Think of it this way. If you replace a cell with a resistor can you tell by measuring the voltage at the modules. No you can't. The voltage is the same. Is there a way to tell? There would be current flowing between the modules. And one module would be producing heat. So unless you want to measure the current between the paralleled modules or insulate the hell out of them and watch the temperature change you can't tell.


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