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AC Coupling with Grid Tie System Retrofit
#11
I think the Batrium kits are 15s. But you can you add more longmons to increase the series count. You can even have 1 Watchmon with 28 longsmons monitoring 2 strings of 14s ( I think this is how Daromer monitors two of his strings, not sure tho)

Also, going with LiFePO4's, your top voltage at 16s would be 57.6V (3.6V per cell), and Batrium can go higher than that as I've seen readouts over 60V.
16s would also give you a bottom voltage of 44.8V (2.8V per cell). Should fit the voltage range of your inverter just fine.

BTW, LiFePO4 cells fit "extremely" well with pretty much "any" inverter, as they are basically drop in replacements from 12V to 96V and beyond.
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#12
It is possible to double up with shunts and BMS, I personally wouldn’t, especially if you’re using new cells.
Your first string will be 1P??S (I’m not familiar with LiPo).
That being 1 cell in parallel and ?? Cells in series.
When you’re first setting it up I’d decide how many packs you may want to end up with in parallel and make the bus bars that long and lay all your cells out in 1 line, either horizontal bus bars with a single vertical stack of cells or vertical bus bars with a horizontal line of cells, the shunt would go onto the 0V bus bar on the cable running to the inverter/charger. The light weight BMS balancing cables then just go to each individual cell.

Commission your system, settle it down, tweak it and become comfortable with its peculiarities and how it serves your usage pattern.

When/if you decide to add more capacity it’s a simple matter of putting another line of cells into your system, tying into the feed bus bars and putting lught jump leads between each parallel cell for the BMS. The only software change is then to change the overall capacity of your battery.

If at some stage you have reason to think that a cell is starting to fail (say at that time you are 3P,??S) mark the 3 cells that are questionable, shuffle them so that those 3 are in different Ps and monitor. The new P that is showing signs of failing has the suspect cell in it, you’ve already marked which ones were suspect the first time, there’ll only be one marked cell in that P, it’s a fair bet that’s your suspect cell.

You can run totally independent strings and monitor everything with very high accuracy if that’s your thing, I’m inherently lazy and only like to capture data that will allow me to monitor and maintain at a glance.

I was hanging out for an active balance system rather than a passive balance, I also initially liked the idea of having everything back on the BMS board. I then ended up buying a WM4 on the logic if anything is going to fail over time it’s likely the unit that is converting electrons to heat, having this as a discrete unit lets me troubleshoot and replace easily.
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#13
For clarity on this forum, pack/string construction is listed as XsYp, always listing the series first, the parallel. Other forums may be different, but the vast majority of ppl here use the XsYp nomenclature.

If a pack isn't performing as expected, then you may need to desolder all cells and find the ones that are not the same voltage as the others. Then remove them completely. Don't add them to another pack to test if they are positive suspect as you'll be stressing those cells in that pack, and doubling or tripling your work. Test the cells in a separate charger/tester, plus it will be faster doing it this way.

If it isn't clear, when Jon says the shunt goes on the 0V rail, he means the Negative line.
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#14
thx to all for your detailed answer.
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#15
Thanks Korishan

Tobias is using large format cells, that is why I suggested swapping cells to troubleshoot.
I would also recommend doing the same with packs where people have packs paralleled to find out which pack has suspect performance.
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#16
As the new Watchmon Plus has only 15s capability i would rather use a battery bank setup of 15s3p
so the 3 parallel cells are acting as 1 cell so i only need one watchmon for the complete setup.

i need to take care to link just cells in parallel which are identical in voltage and capacity and Resistance ?
Is there anything else i need to take care of the grouping of cells ?
Is there any specific problem with these large LiFePo cells to put in parallel ?
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#17
Hi
There’s no issue with large format cells going in parallel.

All cells in the system obviously need to be the same chemistry for voltages etc.
From there it’s a bit more flexible.
Think of your pack as a matrix of Series and Parrallel cells, say like a spreadsheet of columns and rows. If your bus bars are horizontal each Column represents a Series of 16 cells and the main current flow is vertical. When you add another string in parallel you’re adding another column of 16 cells, the current flow horizontally is negligible, just balancing really.
The cells in one column can be a different capacity to the other column, but you want all the cells in a column to be the same. Eg column A can be 1.5 times the capacity of the cells in column B with no issues, but you want all the cells in column A to be the same capacity and you want all the cells in Column B to be the same capacity.
IR (IMHO) is important if you lean on you cells hard by charging and discharging at a high rate, the kinder you are to your cells the less important IR is.

If you’re buying new large format cells as long as each series group is the same spec I think you’ll end up with a nice setup.
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