# Wiring BMS's in parallel



## Zak650 (Sep 20, 2008)

Hi,
You will probably have to unlearn a lot of what you've experienced with lead acid systems and aquaint yourself with lithium cells. The key is to avoid the extremes of the charge and discharge curves. Learn what these curves look like and install hardware to avoid them. An excellent item to include in your system would be a JLD404 meter. It counts amphours, has two relays controls that can be set high and low for either amphours or volts. You can set it up so it disconnect the feed lines to the inverter before it causes a problem for your battery pack. I'm of the non bms, bottom balance school of thought. You can invest in more cells instead of a bms system. There doesn't seem to be evidence of parallel cell problems but capacity differences can be a real problem, so bone up on that aspect of these systems.

My wine bottle analogy:

Think of your battery pack as a series of inverted wine bottles of slightly different volumes. The openings at the bottom are all at the same level(altitude). As you charge the pack each bottle is filled with the same number of drops(electrons) as they approach the fully charged voltage all these different bottles show different psi(voltage) because the tops of the liquid are all different heights due to the volume differences. The charger stops when the sum of the bottles psi(voltage) reaches it's set point. The key is having all the bottles run out at the same instant. As they empty if some bottles still have higher psi as they reach the empty point they overpower their neighbors and drive them to destruction because they have higher psi(voltage) and are able to do so. If they are all at the same psi when they each empty they have no force to cause the other cells to harm each other. 

You can see that in such a system, measuring the voltage near fully charged and making them equal psi or voltage at the top actually ensures that as they reach empty they will all be uneven and cause the death of the cells with less capacity or volume. The closer to empty you can measure and adjust them to equal the better off you will be.


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## Ziggythewiz (May 16, 2010)

I've never heard of parallel strings having that issue. Unless you're charging/discharging at a rate that is hard for the batteries to keep up with they should even out between parallel strings just fine.

What kind of AH capacity are you working with?


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## Siwastaja (Aug 1, 2012)

Maury Markowitz said:


> I want to replace all of this complexity with li-ion packs that directly output 48V. I put 16 cells in a box with a BMS, and presto, I have a system ready to plug directly into my inverter.
> 
> Except what happens when I wire them in parallel?


There should be no reason to do that. Place the _cells_ parellel, then series, not the opposite way; or buy larger cells. If you want to use a third-party battery pack product instead of building your own pack from cells, get a big enough pack so you won't need to put them in parallel. This should be cheaper, unless non-technical market reasons make it otherwise.

This should be the easiest way. At least you save in the BMS module count; you need only one per paralleled cells.


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## Yabert (Feb 7, 2010)

Hi Maury

Parallel cells don't need a BMS. Each cell are in fact an assembly of many cells in parallel. http://liionbms.com/php/prismatic_cells.php
Serie cells need a BMS.


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## Elithion (Oct 6, 2009)

Siwastaja said:


> Place the _cells_ [parallel], then series.


Yes, absolutely. And here's why. Also, the 1st part of this presentation (pdf).


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## Elithion (Oct 6, 2009)

Maury Markowitz said:


> Except what happens when I wire them in parallel?


Because all the batteries are at the same voltage, the batteries are all at the same SOC. Yes, there some dynamic effects (due to internal DC resistance) that result in a temporary variation in SOC, but the batteries become balanced when the current drops.
And because the BMS top balances the individual cells in each pack, the cells are also all at the same SOC when full.

The problem is not when batteries are in parallel, the problem can be when you start disconnecting and reconnecting them, without paying attention to their initial SOCs. See the 2nd part of this presentation.

Still, as I said, cells in parallel are better than string in parallel.

Here is a list of companies that offer ready made battery modules for your application.


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## Maury Markowitz (Jul 6, 2012)

Ziggythewiz said:


> I've never heard of parallel strings having that issue.


This is a serious issue in the lead-acid world. As current flows into the path of least resistance, in a system with parallel strings, one of the strings will tend to have lower resistance and thus receive most of the charge/discharge action.

The claims about this problem sound dubious - like anything more than three strings in parallel will cause one of them to burn out in months. Frankly I find this difficult to believe, but it is widely known and repeated in the solar industry.



Ziggythewiz said:


> What kind of AH capacity are you working with?


I think in Wh. Each battery has 16x100 Ah cells inside it, and I need a total of 48,000 Ah. So ten of these batteries in parallel.


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## Maury Markowitz (Jul 6, 2012)

Elithion said:


> The problem is not when batteries are in parallel, the problem can be when you start disconnecting and reconnecting them, without paying attention to their initial SOCs.


This I *am* aware of. Any suggestions on a simple solution we might recommend to our users?



Elithion said:


> Still, as I said, cells in parallel are better than string in parallel.


Unfortunately I have little flexibility in the packaging. The output must be 48V to match the inverter, so at some point there has to be about 16 cells in a string.

Now you might say "ok, ten in parallel, then 16 of those in series". But every customer wants a different capacity. I would like to have only two products on my shelf, large and small, and mix those to reach any capacity needed. So a user wanting a night lamp might buy a single small one, but one running his fridge during blackouts would buy three of the large ones.

If that were not enough, we need to hit the ~70 cents/Wh point. Flooded cells are selling for 10 cents, and VRLA's between 20 and 35. I've looked at a number of the systems on your site, and they are generally selling in the $1.25 to $1.50 range. You have your own brand, is it significantly less than this?


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## Siwastaja (Aug 1, 2012)

Maury Markowitz said:


> This I *am* aware of. Any suggestions on a simple solution we might recommend to our users?
> 
> 
> Unfortunately I have little flexibility in the packaging. The output must be 48V to match the inverter, so at some point there has to be about 16 cells in a string.
> ...


One possible solution is a smaller "balancing" connector, containing every cell-to-cell connection. This way, the packs are paralleled through the main cables (say 48V), then these smaller connectors connect every cell in parallel, and these smaller-wire balancing connections fix any small misbalance that might happen. This is used in the RC LIPO world and they very often parallel their packs this way. Note that the packs need to be closely at the same SOC (and balanced inside the pack) before connecting as there is no current limit either in the main circuit or the small balancing connectors.

The additional wiring and connectors might not be very nice, but OTOH at 48V pack it's not that much.


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## Elithion (Oct 6, 2009)

Maury Markowitz said:


> This I *am* aware of. Any suggestions on a simple solution we might recommend to our users?


I do offer guidelines on how to safely connect batteries in parallel in that presentation. I distinguish between "at the factory" and "in the field". In your case "at the factory" means "during your installation", and "in the field" means "your client starts connecting and disconnecting batteries willy nilly". 

"At the factory" is no big deal, because you are in control, and you make sure batteries are all at the same SOC before you connect them in parallel.
"In the field" can damage the batteries and the interconnects, unless you add electronics to prevent a switch in each battery from coming on until after the batteries are equalized. That adds a lot of complexity and cost. In your case, a sign that says "Warranty void if user disconnects" or something to that effect, is much cheaper.

This is getting way off topic for this group.



Maury Markowitz said:


> You have your own brand, is it significantly less than this?


Not appropriate discussion for this group; please contact us directly if you wish.



Maury Markowitz said:


> But  every customer wants a different capacity. I would like to have only two products on my shelf, large and small, and mix those to reach any capacity needed. So a user wanting a night lamp might buy a single small one, but one running his fridge during blackouts would buy three of the large ones.?


I totally agree.



Siwastaja said:


> One possible solution is a smaller "balancing" connector, containing every cell-to-cell connection.


Dangerous:


A unbalance in cell resistance with cause high current to flow in that wire, the wire to melt and may be a fire risk.


If you fuse each line, a blown fuse means that the BMS is not able to see all the cells. That too could be a fire risk.


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## Off the grid (Feb 10, 2017)

I have battery packs from a tesla model s 
I have two packs in series. For 48 v and. 7 of those paralled. For 70 kwh. I live off this system currently. 

My question is. About making it one giant 12s pack by paralleling all 12 groups via a larger conductor. Say 6 awg and up 
And then i would need 1 12 channel BMS 
Or split the pack and use 2 BMS slaves and a master instead of 7 which is $4900 

So it would be 518 cells parallel (74x7) 12 series 
Any thoughts 
I would only do this with modules from the same model s battery pack (no mixing)


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