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Unread 02-11-2019, 01:58 PM   #11
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Default Re: Want 7 more Leaf Modules...knew this would happen

I don’t think the zeva will see them as individual cells, it still sees the pack the way you had it.
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Unread 02-11-2019, 02:54 PM   #12
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Default Re: Want 7 more Leaf Modules...knew this would happen

DaveTM, You need to connect the center taps of the modules that are in parallel.

Each Zeva channel will be balancing 4 cells (2 modules in parallel) instead of 2 cells (1 module) like before.

Once you put all of the buss-bars together, You will connect a single BMS wire to each buss-bar, it will make more sense when you draw it out.
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Unread 02-11-2019, 04:32 PM   #13
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Default Re: Want 7 more Leaf Modules...knew this would happen

Thanks Gents!! I wasn't even thinking of the center tap. I "upgraded" my little diagram to reflect how the center taps would have buss-bars and then be wired to the ZEVA. All totaled up, there will be 15 connections to the ZEVA. (If my diagram is correct....which I believe it is.)

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Unread 02-11-2019, 05:50 PM   #14
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Default Re: Want 7 more Leaf Modules...knew this would happen

Not how I would do it. You should parallel the old and new so it would go in your diagram
OLD NEW OLD NEW OLD NEW etc Otherwise the pack will not stay in balance if the old and new cells are different capacities (which they probably will be)

Your connection diagram is correct for a 14 module pack.
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Unread 02-12-2019, 08:34 AM   #15
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Default Re: Want 7 more Leaf Modules...knew this would happen

Quote:
Originally Posted by DaveTM View Post
So, if I have the "add-on" 7 module's thought out correctly, the setup would look like the connections in the photo below. My bus bars would now connect 4 terminals & two on each end, where my 7 module pack now connects two and there is a single one on each end.

However, how would the ZEVA "view" the cells now that there are 28 individual cells?

Maybe I need to look at the ZEVA documentation again.

The string
would be Pack+ +new/old parallel- +new/old parallel- +new/old parallel- +new/old parallel- +new/old parallel- +new/old parallel- +new/old parallel- Pack-

Seven new/old pairs in series!
Each parallel pair would have all 3 terminals paired together.

Bob
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Unread 02-12-2019, 08:55 AM   #16
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Default Re: Want 7 more Leaf Modules...knew this would happen

At least we all agree that you should not wire strings of batteries in parallel.

The reason I suggested keeping the cells in parallel matched by their age/capacity is that there are studies that show that placing batteries of different capacities in parallel causes a reduction of their useful life.

It also causes the parallel cell with the lowest internal resistance to carry the brunt of the load, which is why I suggested keeping the temperature probe near the mismatch pair.
https://www.sciencedirect.com/scienc...78775316300015
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Unread 02-12-2019, 10:06 AM   #17
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Default Re: Want 7 more Leaf Modules...knew this would happen

But you also cannot connect cells of much differing capacities in series. That causes much more issues with overcharge attempt of lesser capacity cells and chronic undercharge of higher capacity cells.

The BMS will be working overtime trying to deal with that much imbalance.
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Unread 02-12-2019, 10:19 AM   #18
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Default Re: Want 7 more Leaf Modules...knew this would happen

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Originally Posted by BobBoyce View Post
But you also cannot connect cells of much differing capacities in series. That causes much more issues with overcharge attempt of lesser capacity cells and chronic undercharge of higher capacity cells.
He is using a good BMS with Cell monitoring so neither one of those issues exist.
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Unread 02-12-2019, 10:59 AM   #19
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Default Re: Want 7 more Leaf Modules...knew this would happen

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He is using a good BMS with Cell monitoring so neither one of those issues exist.
As a former OEM of EVs and aerospace engineer, we would measure cells and match them up in even A/H parallel sets so that when we strung those parallel sets were placed in series, they were all less than 1 A/H capacity of one another with packs of hundreds of A/H capacity. This becomes even more critical in aerospace applications where reliability is a more important consideration than system cost. If your power system fails while on-mission, you cannot simply make a service call. LOL

A good BMS is one that does not waste energy by dissipating it in resistors. It has isolated DC-DC converters with a common source/sink bus that allows excess energy to be redistributed amongst the packs cells until balance is achieved. For golf carts, it is not cost effective to design such a BMS, so we tend to use more commonly available resistor balance BMS systems. But a resistor balance BMS would waste much more energy when faced with the task of constantly having to dissipate power from the lesser capacity cells, just to keep them in voltage balance with higher capacity cells in the pack. It is far better to parallel the old and new cells, as the alternative would.greatly diminish usability capacity during each and every charge cycle to that of the rebalanced weakest cell. That would result in a much lower effective cycle life of the pack.
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Unread 02-12-2019, 11:41 AM   #20
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Default Re: Want 7 more Leaf Modules...knew this would happen

Well, You are kind of changing this debate and creating other discussion topics.

We are not talking about "Aerospace Applications", but a golf cart.

We are not talking about the "BMS efficiency", but is ability to disconnect the pack when any cell goes below or above the preset limits.

The only energy "wasted" is the delta between the "Max Cell Voltage" and the "Balancing Voltage" settings since the charging will stop as soon as the first cell(s) reach the "Max Cell Voltage" setting.

I do agree that pairing the old and new cells will likely yield a slight higher SOC for the pack, and it may not make much difference either way if the cells are close in capacity, but it adds another point to consider when deciding how to proceed.
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