# low voltage (tesla) battery pack (48 volt), Which BMS? (ZEVA?)



## boekel (Nov 10, 2010)

Hi,

For a 2s2p setup of 4 Tesla modules, I'm contemplating using a ZEVA bms system / soc display:

http://zeva.com.au/index.php?product=112
http://zeva.com.au/index.php?product=128
http://zeva.com.au/index.php?product=119

Does anyone have experience with these systems?

Or other good off the shelf systems with a good price point for a 12s system?
Requirements:
-charge / discharge disconnects
-soc monitoring
-balancing

Extra's:
-temperature monitoring
-logging

This battery pack is going to be used in a small boat, normal cruising is max c/4 load, charging c/8


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## skooler (Mar 26, 2011)

Orion?

Sent from my SM-G935F using Tapatalk


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## GoElectric (Nov 15, 2015)

Hi. That is a big can of worms, but there has been some discussion of BMS reqmts on the thread I started "Working With Tesla Packs". As Jack points out in his latest video, this is not trivial. 

I don't think an Orion can be used to monitor packs in parallel, but I think it can be expanded to accommodate 72 'cells' in a 12S (modules) pack. I am using the Orion, and it has features like monitoring the difference between cell voltages, which should be on your list of requirements.


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## Ampster (Oct 6, 2012)

The Zeva looks to be a good price point for a 12s system. I have an Orion Jr which is a 16s size and about the same price point per cell but more expensive if used for a 12s system. They all seem to have the same functionality.


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## SWF (Nov 23, 2007)

I spent a lot of time considering a parallel/series setup with the tesla modules I am using in my conversion and ended up deciding to go with a simple series string. Although I think that using these modules in parallel may be possible, managing them becomes more complicated. I had several communications with three different BMS suppliers and only ZEVA is set up to handle parallel/series cell configurations. However, it does not completely eliminate the issues of this configuration, which you can read more about here and here. The modules themselves are made up of paralleled 18650 cells (74p6s configuration), but the paralleled cells are permanently connected and are individually fused. To parallel at the module level, you would need to make sure the modules are first perfectly balanced and then permanently connect them in parallel (i.e. no disconnect switch or contactor between the paralleled modules). Then you would need to use a BMS that monitors and balances each set of paralleled cells within the modules.


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## boekel (Nov 10, 2010)

So I purchased the ZEVA bms a while ago...never used id for the Tesla packs...

Using it now for a test setup (self consumption solar battery..thing)

Turns out you can only set a 'balancing voltage' above where the cells above this voltage are 'balanced'... so not bringing the highest cell back to the average cell voltage...if all cells are above this voltage they're all 'balanced'...

This could sort of work by switching charger on/off everytime a cell hits the balance level...but you also can only set the level per 0.05v so not a lot of choice..

So went to the ZEVA website...and yes...a new version comes out where these things seem to be fixed... not compatible with old hardware 

So I spent 445 (australian) dollars on a BMS that's not doing what I hoped it would...


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## SWF (Nov 23, 2007)

boekel said:


> So I purchased the ZEVA bms a while ago...never used id for the Tesla packs...
> 
> Using it now for a test setup (self consumption solar battery..thing)
> 
> ...


I assume you have version 2 like I do? It appears the main difference with their new version 3 is the shunt voltage is set dynamically to the average of the cells in the pack rather than a fixed value. So with version 2 you only get balancing when a cell voltage hit this fixed value as the cells reach full charge, rather than at any point while charging. Not sure this is too big of an issue unless you are only partially charging and never getting cells to the shunt voltage when charging.

I can't recall if the other BMS systems I looked at have fixed or dynamic shunt thresholds.

I agree that it would have been nice to have slightly better resolution than 0.05V, but this is probably still enough for most purposes.


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## nebster (Aug 15, 2017)

SWF said:


> To parallel at the module level, you would need to make sure the modules are first perfectly balanced and then permanently connect them in parallel (i.e. no disconnect switch or contactor between the paralleled modules). Then you would need to use a BMS that monitors and balances each set of paralleled cells within the modules.


I am paralleling a set of strings (not Tesla cells) and am using contactors for emergency disconnect. I am curious why you write that they should be "permanently connected." One of the big advantages of paralleling is that a portion of the pack can be jettisoned if there is a problem or simply taken offline for maintenance.

Can you elaborate?


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## SWF (Nov 23, 2007)

nebster said:


> I am paralleling a set of strings (not Tesla cells) and am using contactors for emergency disconnect. I am curious why you write that they should be "permanently connected." One of the big advantages of paralleling is that a portion of the pack can be jettisoned if there is a problem or simply taken offline for maintenance.
> 
> Can you elaborate?


The links I included in the post where I made that statement go over the issues in detail. The main issue is that once paralleled cells are disconnected, they can become unbalanced relative to one another (one has higher voltage). So if they are unbalanced and reconnected in parallel there can be very high current flow from one cell to the other.


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## nebster (Aug 15, 2017)

SWF said:


> The links I included in the post where I made that statement go over the issues in detail. The main issue is that once paralleled cells are disconnected, they can become unbalanced relative to one another (one has higher voltage). So if they are unbalanced and reconnected in parallel there can be very high current flow from one cell to the other.


I've read the linked articles prior to my post before, thanks. No where in them does it stipulate that paralleled strings have to be "permanently connected."

I certainly agree that putting a new string in parallel at a different voltage is asking for trouble! There are plenty of environments where that scenario is (a) rare and (b) completely manageable.

There are advantages to paralleling large strings, some of which are glossed over in the article (e.g., in Reliability, for some reason David appears to assume the system has no string-level monitoring and disconnect capability) and some of which are missed entirely (e.g., more flexibility in physical wiring topology and a much-simplified fusing strategy).

There is no free lunch in serial vs parallel; rather, tradeoffs abound.


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