Could someone read this and explain it as it doesn't make sense to me RE VICTRON Wiring Balancing Series/Parallel

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johnsmith asked Mar 30, '22

Could someone read this and explain it as it doesn't make sense to me RE VICTRON Wiring Balancing Series/Parallel

This paragraph is in the VICTRON Documentation

In case of series-parallel connection, the midpoint
interconnecting cables must be sized to at least carry the current
that arises when one battery becomes open-circuited.
- In case of 2 parallel strings: cross section 50% of the series
interconnecting cables.
- In case of 3 parallel strings: cross section 33% of the series
interconnecting cables, etc.

There can't be a midpoint connection if there is only 1 string so the cable must be sized to at least carry the current is not applicable as their are no batteries in parallel.

The next part about 50% and 33% of the cross section of the Series connecting cables I am struggling to comprehend as the output Load of the bank could be drawn through any combination of the matrix of batteries with unmatching capacities in paralell if a battery has a complete open circuit and therfore a cable 33% of the load carrying capacity of the series connecting cables would possibly glow red before a fuse at the positive on each string could blow.

Or am I missing something?

To keep this post simple lets use some figures, so we are all on the same page when answering.

Batteries 12V 100Ah

Strings 4 x 12V 100Ah = 48V 100Ah per string

Combined parallel String = 3 x 48v 100Ah = 48V 300Ah Bank

Let's use the Wiring Diagram from the Victron Documentation

wiring

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Modifications Space

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Answer 1 · 2022-03-31T03:23:20Z

kevgermany answered · Mar 31, 2022 at 03:23 AM

One battery open circuit is your key. It will kill the 48V feed on the string, but the remaining batteries will contribute to their counterparts in the other strings. Hence the 50% and 33% values. You'll get a huge imbalance, but the emphasis is safety.

The second quote is badly worded. What it means is that the single string midpoint cable must be capable of full load and there aren't any mid point string to string connections so they don't need sizing.

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