The Victron  Multi relies on DC coupled MPPT to charge the batteries and in turn used the Battery DC to make AC.

One of the limitations of this set up is you are subject to the voltage limits imposed by these kind of MPPTs. There is a "Xantrex/Scneider/Conext" model that can do 600v, but it is by no means cheap.

And it appears that 3rd party MPPT's are not optimised by the Victron ESS.

The Victron options are 150 and 250V, but a safety margin has to factored in because it is allegedly an absolute voltage ceiling.

The Multi is also functional bi-directional charger, so it can also charge from AC.

So whats the point?

The point is a PV inverter can be used to charge the batteries.

Caveat: There is a ratio of 1:1 charger to PV inverter (together with a reasonably sized battery) that must be complied with.

The inbuilt MPPTs on a PV inverter  accept far higher input voltages. ( around 400V -1000V typical).

So the PV panels can be considerably further away, without the cable losses of higher current.

 

So let's take this one step further,  let's do away with DC MPTTs altogether and just charge from the AC side.

Not in a ESS system, with a grid connection, but in a bare-bones offgrid set up. (Even just using a Phoenix inverter).

Now the system becomes capable of delivering 2 x rated continuously when the sun is shining.  Both inverters can serve the load simultaneously.

DC MPPTs would limit the deliverable AC to just the Multi.

Most of these PV inverters of any sort of size have two inbuilt MPPTs.  So capital cost-wise they already fairly offset.

Over-paneling a PV inverter is the order of the day, whereas over-paneling a DC MPPT is frowned upon.

Another distinction which I think might be important, is that these inbuilt MPPTs work flat out all day and do not have to conform to a charging profile. The PV inverter only backs-off if there isn't the load, (due to frequency-shifting). I have a gut-feel that DC MPPT charge controllers leave something back on the roof at some  charging stages without ESS. It also seems that 3rd party DC MPPTs also do not get optimized functionally.

If all the charging is through a Multi then even the BMV may be redundant.

We have to weigh up the losses of converting AC to DC and DC back to AC again at night,  but feeding loads directly from the PV inverter during the day and negating cabling losses might  completely or substantially offset these losses. We'd also have to consider the aggregate of our load because it would still be the PV inverter doing all the work, and the Multi just allowing it to be spread evenly.

The optimum probably lays somewhere in between the two of AC and DC coupling depending on the load profile throughout the day.

Not everyone can afford everything at once when they go solar, maybe this a viable set up whilst transitioning to the end goal.

 

 

 

 

 

 

Edited January 16, 20197 yr by phil.g00

4 hours ago, phil.g00 said:

And it appears that 3rd party MPPT's are not optimised by the Victron ESS.

Well... first off, how can it? (many MPPTs don't even have external control facilities), and second... every manufacturer wants you to buy their stuff, it's downright silly to hand business to someone else, other than in cases where it helps you to sell more stuff :-)

4 hours ago, phil.g00 said:

Now the system becomes capable of delivering 2 x rated continuously when the sun is shining.  Both inverters can serve the load simultaneously.

This isn't necessarily always true, or in short: Don't rely on it. Very often the PV-inverter will sense an islanding event at that moment when the grid fails. After a minute or so it will reconnect to the output of the Multi and work as you say, but for that whole minute the Multi (and your battery bank) must carry the full load. So really you don't have twice the capacity... unfortunately.

21 minutes ago, plonkster said:

Well... first off, how can it? (many MPPTs don't even have external control facilities), and second... every manufacturer wants you to buy their stuff, it's downright silly to hand business to someone else, other than in cases where it helps you to sell more stuff :-)

Agreed, just mentioned that because Victron has a  250V input limit. - you might want higher to avoid cable losses and then there isn't a optimal DC MPPT solution available.

And the system I described optimizes the inbuilt 3rd party MPPT's without ESS.

26 minutes ago, plonkster said:

Very often the PV-inverter will sense an islanding event at that moment when the grid fails.

Sorry, maybe it didn't come across well, I was talking about a possible transitional stage of a system being still off-grid.

I think most people test the water with a small offgrid solution? 

Maybe the PV inverter would react the same to a sudden load as a grid loss, though you might be right.  I'd have thought that it would be the PV inverter that was already going full tilt and the hit would be on the Multi to pick up the pace. I don't know though.

However, I'd counter that with the Multi being able to go 2X for short time and the PV inverter (possibly) delivering you are in a no worse position to ride through bumps.

But the big thing is that you will have more capacity for steady state loads without imposing heating limitations, and that's not to be scoffed at.

1 minute ago, phil.g00 said:

However, I'd counter that with the Multi being able to go 2X for short time and the PV inverter (possibly) delivering you are in a no worse position to ride through bumps.

Agreed. That is true. I am just pointing out that you should not rely on it, ie don't design the system with too small a Multi because you figure the slack will be picked up by the PV-inverter.

And you are absolutely right: Using a PV-inverter is a good alternative to a high voltage MPPT. Just watch out for a low battery deadlock situation: The battery is low so the Multi is off, but because the Multi is off there is no AC to tie with so you cannot charge. So usually we advise adding a few panels and a small MPPT to pull it out of such a deadlock situation. That is if you want to do it automatically... if you are physically present and have a backup AC generator... then of course it's not essential.

 Good point about the deadlock.

A 600Vdc MPPT wouldn't go amiss in Victron's product range, the more people expand their set up the further away the roofs are from the batteries.

3 hours ago, phil.g00 said:

 Good point about the deadlock.

A 600Vdc MPPT wouldn't go amiss in Victron's product range, the more people expand their set up the further away the roofs are from the batteries.

could not agree more.... i am already looking at my flat's roof for expansion  in july