question

johannes123 avatar image
johannes123 asked

Why same cable lenght to the busbar at Multiplus-II DC in parallel use?

Hello,

I totally understand why the AC-cables at Multiplus II in parallel use must have the diameter / length.
(same resistance for having same amount of energy going through each of them)

But unfortunately I do NOT understand, why this should be at DC-side to the busbar (that directs to the batteries) also.

Said with easy words: The Multiplus II pushes the energy into the cables to the bus bar when charging with the quantity they should push, and sucks the energy out of the busbar / batteries in the quantity they should, why does different cable diameter or lengths make a difference?

Thank you very much

Best regards

Johannes

Multiplus-II
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6 Answers
Alexandra avatar image
Alexandra answered ·

@Johannes123

  1. Current sharing.
  2. Voltage drop. (It is related to resistance)

If voltage is lower on one the amps will be higher, and overload sooner than the other unit.

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1234enough avatar image
1234enough answered ·

For exactly the reason you state for the AC, if one battery bank has a greater resistance it will be using and getting less electricity, plus may go out of balance with other battery banks

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johannes123 avatar image
johannes123 answered ·

@1234enough I totally agree with you for the way between busbar and battery. But my question is for the specific way beween MP-II and Busbar

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dirk-s avatar image dirk-s commented ·
I recommend the guide „wiring unlimited“ von Victron energy. It’s a good handbook and answers your questions.
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Alexandra avatar image Alexandra ♦ commented ·
Path to the current source must be the same

... It is for the same reason. You want them to share the load equally. If they aren't you don't have twice the power. Then there is no point to the system.


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1234enough avatar image
1234enough answered ·

If you are saying all battery banks are connected to a point with equal lengths/ diameter cables and the from there to MP, then all battery banks are indeed equal resistance to MP.

Bit more detail on your system may help all understand / give further guidance

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johannes123 avatar image
johannes123 answered ·

With my installation it will be from MP-II to battery:

2x3 phase MP-II 5.000 in parallel.

70mm² cable from each MP-II

total 6x NH00 160A AC fuse

big compact copper-busbar 25cm x 10cm x 0,5cm

total 7x NH00 160A DC fuses in opposite

35mm² cable

to each 2x Pytes V5 (~ equal to Pylontech 5kWh), 7 strings / 14 pcs in total

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kevgermany avatar image
kevgermany answered ·

The cables from MP II to busbar must be capable of carrying the full current to the MP II. Between busbar and battery, only the current a single battery can/will provide. With the extra details in your last post, makes sense. Your sizes seem ok.

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