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Jason avatar image
Jason asked

String re-sizing of existing array

Good Day

Please could I ask for some guidance / advice here.

I'm upgrading my existing system from 2 x 5kw Axpert inverters to a Quatrro 10000va.

My existing array is potentially 7840w - I don't want to have to lose any panels, and or have to add any more panels.

From my existing system, I already have an array of 28 x 280w monocrystalline panels. They are divided into 4 strings of 7 panels each. I've used the Victron mppt calculator, and I'm getting an answer of the SmartSolar MPPT 250/100 MC4.

My panel specs are as follows:

  1. Vmp = 31.4v
  2. Imp = 8.96A
  3. Voc = 39.1V
  4. Isc = 9.50A
  5. Eff = 17.2%
  6. Temp coefficient of Voc = -0.33%/c
  7. Temp coefficient of Isc = 0.059%/c

My questions are:

  1. If I keep my strings as 7 x 39.1Voc = 273.7 (I'm not taking any inefficiencies into account yet), do I potentiely risk running to much voltage into my charge controller? Should I rather re-string my array as follows 7 strings of 4 panels which would be then 4 x 39.1Voc = 156.4v?
  2. Do I need two charge controllers to compensate for the amperage? I'm running 4 x 2.4Kwh pylontech batteries on a 48v system.

Many thanks in advance

charge controller compatibility
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2 Answers
wkirby avatar image
wkirby answered ·

You would damage a 250V controller with a 7 series configuration. You would need to alter your array to 7 strings of 4 series.

Each 100A controller can make a maximum of 100x(battery Voltage) ~ 4.8KW.
So 2x 100A controllers, one controller with 4 strings (~4.5KW) and the other with 3 strings (~3.4KW).

You'd still need to use 250V controllers though since a 4 series configuration can go over 150V which would damage a 150V controller.

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JohnC avatar image JohnC ♦ commented ·

Agreed. I like to save money too though. Rather than broad-brush in 2x 250/100's, this could be done with a 250/85 (16 panels 4S4P), and a 250/60 (12 panels 4S3P).

Not worked out at nominal 48V, but at charge V of (say) 56V (Pylons, dunno?). But would likely suffice.

Just showing my penny-pinching side. :)


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Jason avatar image
Jason answered ·

Many thanks guys,

What do you think of this suggestion below?


But, if you don't want to mess with actual values, (so that you step down an MPPT current size).

28 panels & 4 existing cable pairs, that I am going to assume are 6mm2, because you stated you are looking at an MC4 option.

MC4's are good for 30A. So you don't want more than 3 strings in parallel on a cable pair, and you don't want to be installing more cable if you can help it.

I'd configure 16 panels as two arrays of 4S2P each, with each array brought down on its own cable pair into a 250V 85A MPPT.

That leaves you with 12 panels and two remaining cable pairs.

Two arrays of 3S2P each, with each brought down on its own cable pair into a 150V 70A MPPT.

( In reality the 60A version, would still be adequate).

The 150V option each cheaper than the 250V option for the same power transfer capability.

And you will be running at less than 20A on any cable pair.

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JohnC avatar image JohnC ♦ commented ·

Good work, you're onto it.


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