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Wiring a VMIII/King to the Grid input of a VMIII for extra solar energy


DrLoLCat
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I have an off grid solar system with VMIII:

  • 16 x 340W solar panels (North-West) - 2 strings of 8 panels each
  • 4 x US3000 Pylontech

I am missing out on some good sun in the early morning, which I want to capture as follows:

  • Add another US3000 to the existing VMIII
  • Add another VMIII or King with solar panels connected from the rooftop which gets son in the early morning.
  • This new VMIII or King would then have its AC output permanently wired to the grid input of the existing VMIII which would be set to charge from utility and solar.
  • I can have the new VMIII or King run from an old lead acid or no battery, I only want it to send solar power to the downstream inverter which charges the 5x US3000 Pylontechs

Will this work?

 

Of course this would not have been required if the VMIII could parallel. 

 

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18 hours ago, DrLoLCat said:

This new VMIII or King would then have its AC output permanently wired to the grid input of the existing VMIII which would be set to charge from utility and solar.

I don't see what you achieve by this. You are still limited to 5 kW output, but you've paid for 10 kW.

Why not just get an external MPPT to charge the 48 V battery directly? With 5 x US3000, you would not be worrying too much about charging with too much current (they can take 185 A between them). Most of the time, the loads will be using at least 10 A as well. So with 80 A from your VM III, you could afford at least 100 A of external charger (at least, from a charge current perspective, maybe not a hip pocked currency perspective).

You can probably get quite a large external MPPT for less money than a second VM III.

I'm also not a fan of feeding inverter outputs to other inverter inputs; they are expecting sine wave inputs, and the outputs from inverters may have high frequency noise that doesn't bother appliances, but might upset the downstream inverter.

Edited by Coulomb
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Thank you very much for the advice.

I have a Bluesolar MPPT 150/70 that is currently not being used. I'll be very happy if I can use it in parallel with VMIII BMS charging.

My concern with this was only about non-BMS charging in parallel with BMS charging from the VMIII.

As you pointed out I won't have to worry about charging with too much current at least. If there is nothing else to be concerned about I will follow this solution and program the Victron Bluesolar MPPT with Absorption 53.2V and Float 51V.

Victron equipment (in BMS mode) however seems to charge Pylons at 52.4V even if the BMS requests 53.2V. 

I will probably take a connection from every 2nd or 3rd Pylon to the busbar when I add this additional charging capacity.

The limit on Pylon cables is 120A.

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44 minutes ago, DrLoLCat said:

Victron equipment (in BMS mode) however seems to charge Pylons at 52.4V even if the BMS requests 53.2V. 

Oh. So the BlueSolar will talk to the BMS of the battery as well as the VM III inverter? I suspect you can only run one with the BMS.

I would connect the BMS to the charger with the highest power, and set the other one to fixed voltages a little lower than what the BMS is going to ask for. For example, 53.0 and 51.6 V. So then the BMS gets to control the last stage of the charge. You may have to play around with the settings to get it right.

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  • 1 month later...

The BMS of the Pylon stack will only be talking to to the VM III inverter.

Thanks for the advice about letting BMS control last stage of charging, I will set up the voltages accordingly.

I understand 5x Pylons can take 185A but what about the 25mm2 Pylon cables? I believe the current carrying capacity is 120A.

So then, does that mean the BMS in a 5x Pylon stack will limit to 120A? Or self-protect to 120A to avoid cables burning? 

If the BMS actually allows the full 185A, that would mean I have to do something like the wiring shown below to avoid cables burning? (assuming I add more solar panels to enable 185A)

Please review my wiring. Or is this actually unnecessary? 

image.thumb.png.bc67e9b77c7320669a942682862ef72f.png

 

 

 

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  • 2 months later...
On 2020/12/29 at 1:18 PM, Coulomb said:

Oh. So the BlueSolar will talk to the BMS of the battery as well as the VM III inverter? I suspect you can only run one with the BMS.

I would connect the BMS to the charger with the highest power, and set the other one to fixed voltages a little lower than what the BMS is going to ask for. For example, 53.0 and 51.6 V. So then the BMS gets to control the last stage of the charge. You may have to play around with the settings to get it right.

This worked perfectly with the first iteration of settings by the way. The installation has been running without any intervention for 2.5 months.

I also did the wiring as per the above drawing to keep the current through any individual piece of cable as low as possible.

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