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rithym1964 avatar image
rithym1964 Suspended asked

Configure Multi II to charge but continue inverting (no grid pass through) when grid connected?

Question is in the title. Is it possible to grid connect a Multi II so it is charging battery but have it continue inverting from the battery? Essentially supplying the full 230v to the output but using the grid power (lower voltage) to go through the inbuilt charger and supply the loads so the battery does not discharge.

I assume I would need to do this with a separate charger set to supply enough current to more or less cover the average loads and just keep the Multi inverting 24/7. Bit like how a double conversion UPS works always inverting from the battery and using the grid input to supply enough charge to the battery to cover the loads, this results in perfect 230V power 24/7 with no switchover when the grid comes on & off.

Multiplus-II
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bigbadbob76 answered ·

Yes, you would need a separate charger to do this, the Multiplus is a bi-directional charger/inverter so does not have separate charger and inverter built in.

Your efficiency would suffer as you would be constantly down-converting and rectifying the incoming supply and then up-converting and inverting it back to grid voltage.

If you can accept this efficiency cost in order to give you a constant clean supply then it's certainly do-able.

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rithym1964 avatar image rithym1964 commented ·

I can live with the loss in efficiency my 10pm - 8am night tariff is half price, I am more concerned with having 230V clean power 24/7. At the moment when the grid kicks in at 10pm (not enough battery to last all night) the voltage drops to well under 200V which is causing issues with some devices like air con, crypto miners, PC, etc. The good thing is I have a 3 phase grid supply so can get a 48V forklift charger with 3 phase input with a DC output current equal or slightly larger than my nightly loads. I don't want to spend money charging the battery from grid too much as my solar takes care of that most days.

I also have a spare mppt 48V 35A Victron controller I was wondering if it is feasible to run higher voltage from a 3 phase charger (say 120V) through the controller so I can monitor/ change the charge current remotely using VC or VRM like I do with the other controllers. I could fine tune the controller output current to mimic my nightly usage almost exactly.

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bigbadbob76 avatar image bigbadbob76 rithym1964 commented ·
When using a separate charger, make sure it's charge current goes through the shunt (if you have one) and set your GX device (if you have one) to "has DC system"


Don't feed a dc charger into an mppt charge controller, mppt will try and load the charger until the voltage drops.... the charger will try to compensate for this and you'll have a cat fight between the two until one wins and the other smokes. ;-)


better to use a victron charger or another multiplus configured for charge only if you want to fine tune it using VC.

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Alexandra avatar image Alexandra ♦ bigbadbob76 commented ·

Other option is a second inverter - the inverter RS.

It has no AC in and possibly may be cheaper than a charger with the benefit of never synchronising to rubbish grid. Is actually more efficient than the MP2 as well. And th MP2 can just remain to charge the battery.

Or just get a good quality AVR.

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rithym1964 avatar image rithym1964 bigbadbob76 commented ·

I do have a BMV and the shunt is one end of the neg busbar which then goes outside via 90mm2 cable to FLA battery. I also have Venus GX if I activate 'Has DC System' it now shows -950W when the total PV produced at present is 8000W, I have no idea how the GX algorithm calculates that figure the only DC power being produced is from the Victron controllers and there are no DC loads from the busbars except the MPII. This figure of -950W makes no sense. Also, when the MPII is in pass through mode (10pm - 8am) DC Power shows as 0.

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