# BMS in a 5s2p design with Tesla Batteries



## Doug183 (Nov 12, 2019)

Background:
I am building out a EV kit from EvWest, with 10 Tesla Battery packs. I am going to series 5 bundles of 2 parallel Tesla packs making it ~114Volts (5*22.8volts= 114v). If I am using the naming scheme correctly, this is a 5s2p. 

The kit did not come with a BMS and I think I want to add one, maybe an Orion2 and EV Stealth Battery adapter boards, but reading Orion’s literature makes me concerned (because I am clearly a bit of a newbie), plus I have a few questions of my own.



Main Concern:
1) Orion talks about not using parallel strings. In my 5s2p setup (Figure 1), have I done this? Or are they talking about two separate strings of batteries (creating higher voltage), then those strings paralleled together? (Figure 2) 

1a) So is Figure 2 also 5s2p but in 'parallel strings'? CORRRECT or INCORRECT?


BMS questions:
2) I want to monitor the batteries as best as I can, but from my reading, it looks like if I parallel two packs of Tesla batteries, then the BMS no longer monitors the 6 cells inside the pack? CORRRECT or INCORRECT?
2a) For clarity, does this mean I can ONLY monitor the voltage of a whole "parallel 2 pack"? CORRRECT or INCORRECT?
2b) This means I need a BMS that does a minimum of 5 contacts? CORRRECT or INCORRECT?
2b) If the correct answer is to monitor the five “parallel 2 packs,” where do I attach the BMS into this “2 pack”, because, there are so many points…. See the diagram and perhaps the community can advise me what numbers I should attach the leads to. (I am guessing points 4,8,12,16,20)
3) With the Orion2, EV Stealth Battery Boards and my 5s2p setup, can I still tap into the Tesla 6 cells just to see them? (I am guessing not)
4) Can I still tap into the Tesla thermistors or do I now have to rig up 5 thermistors, one for each 2p pack?
4a) Can I tap into the thermistors in each individual Tesla Battery pack or only one of the packsin the 2 pack (giving me maybe an approximation of the other unmonitored sibling) or only my home made thermistors for the entire 2 pack?


5) Any other important safety things or obvious things I am missing?

6) Should I go with another BMS, like Thunderstruck Dilithium BMS?

Thanks
Doug


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## kennybobby (Aug 10, 2012)

Both figures are using modules or strings in parallel. The problem with parallel strings is controlling a runaway overcurrent situation in one parallel leg in the event of a Cell failure in the other leg. It could be done but will require separate contactors for each leg. In figure 1, a fail-over might require you to shut down the whole pack; in figure 2 you could theoretically shut down one leg and still use the other. It is not a trivial solution to design the system for either configuration, but figure 2 might be easier.

Tomdb of this forum has developed a board which can communicate with the original Tesla modules boards to provide Cell monitoring and balancer drive.

You will need a high level overall BMS to control all the parallel paths plus lower Cell-level monitoring.

What vehicle are you building?


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## sfeldma (Aug 21, 2019)

Hi, 

I'm considering using the same 10 modules for a dual AC 34 motor setup, so I'll be following this thread; thanks for posting the questions. 

Is your figure 1) a 2p5s configuration and your figure 2) a 5s2p configuration? I like to be sure I understand the naming convention.


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## kennybobby (Aug 10, 2012)

Maybe somebody else will chime in here, but i've never seen or heard about a standard convention for this. As long as you have a diagram or post a description of the configuration then it should be possible for readers to follow along.

i refer to the Tesla Modules as having 6 Cells, where a Cell has 76 or 86 or xx? cells (Panasonic or Tesla 18650) in parallel. Each one of those 6 Cells would need to be monitored and balanced by a Cell Monitoring Board, which really ought to be sold and included with each module since Tesla put it there.

So a module consist of 6s??p of cells depending upon what kind of pack it came from.

i would refer to figure 1 above as 5s2p of Modules. Figure 2 is 2p strings of 5s Modules.

The Battery Management System or BMS must oversee the Pack control at both the Module and Cell level to be effective and provide for safe operation.

With dual motors you will need 2 motor drive inverters anyway, so maybe just have 2 Packs, one for each motor?


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## brian_ (Feb 7, 2017)

kennybobby said:


> i refer to the Tesla Modules as having 6 Cells, where a Cell has 76 or 86 or xx? cells (Panasonic or Tesla 18650) in parallel. Each one of those 6 Cells would need to be monitored and balanced by a Cell Monitoring Board, which really ought to be sold and included with each module since Tesla put it there.


I understand this scheme, but capitalizing "cell" to get "Cell" with a different meaning seems confusing to me. One alternative is to refer to the group of 76 or 86 or whatever cells which are paralleled together as a "cell group". A Tesla Model S or X module has 6 cell groups in series.


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## Doug183 (Nov 12, 2019)

Its a 1972 Porsche 911T with 1974 930 Turbo Fenders. Dual Motors.


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## Doug183 (Nov 12, 2019)

These are questions and what I think. Please correct me:

What a BMS will and will not do:

Will do, I think:
1) Monitor Battery voltage?
2) Monitor Battery temperature?
3) If charging, will shut down the charger via CAN if batteries get to a certain high voltage?
4) If charging in a series, the BMS internally will modulate resistance to cell packs in serial to balance the the serial packs?
5) When driving and batteries get too low,......? will send a message to the controller via CAN? I have no idea if that is even true. Then maybe the Controller goes into limp mode or..... shuts off. Again, all a guess.
6) In hot thermal events, can send a signal to turn on a water pump to liquid cool batteries if so equipped?


Will NOT do:
1) Will not directly shut down one or more cell packs?
2) Will not directly shot down the entire battery back in a 'runaway' situation? 
i.e disconnect a contractor. This is the job of the controller.?

Thanks
Doug


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## brian_ (Feb 7, 2017)

Doug183 said:


> What a BMS will and will not do:
> 
> Will do, I think:
> 1) ...
> ...


No, I've never heard of a BMS which can do this.

Do you mean balancing packs which are in series with each other? To do that the BMS would need to bypass current around the pack which needs less charging, which would mean dissipating an unreasonable amount of power in a bypass shunt.

All a BMS can typically do about balance is:

monitor each cell level individually, so charging can be stopped when any of them are fully charged (instead of waiting for the overall pack average to be fully charged, meaning that some levels are overcharged), and
balance cell levels before charging starts (bottom balance) or after charging is complete (top balance) by discharging the most-charged cells though a resistor to bring them closer to the other cells.


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## Doug183 (Nov 12, 2019)

@brian_ Thanks. That is more clear.

I guess I am a little hung up the concern about a "runaway battery event" especially if you parallel, but it seems a BMS doesn't really help with a runaway event even in series. So, I imagine a BMS might help avoid that event completely by top or bottom balancing the pack based on the weak cells, so those never get too stressed. Then over time, you will see your capacity (i.e. mileage) drop and then you would go an investigate to identify the weak cell. Thus avoiding the situation that you might overcharge a cell and have it.... catch fire? (Is there anything else a "runaway battery event does"?)


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## zanisail (1 mo ago)

Doug183 - I am very curious where you landed on this. I am electrifying a 1977 VW Bus using 10 Tesla Model S modules and want to use your Figure #1. I wanted to find out where you landed with the Dilithium BMS? My best current understanding is that I would measure each parallel (2 module couplet) and double wire both batteries into the cell taps in positions c1 through c6. I think this way I loose some resolution (ie. monitoring 12 cells with only 6 measurement points). let me know if you are open to jumping on a call. Best - Mike


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## vvoltage (Aug 1, 2014)

zanisail - It would be safer to not tap into the module cells and parallel them. You would be adding a lot of interconnects that would be subject to failure as noted above. If you are needing two MS modules in parallel then I'd recommend making 2 separate strings as in the second figure above. I have not personally used it yet but Toms BMS seems like a great fit for you here. According to the manual, it natively supports the concept of parallel strings of modules. You should check out his simpBMS or latest EVS-BMS. He's had some supply issues but I think those are getting worked out in the new product.


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