question

Depends on what is the battery voltage.

A 75/10 MPPT can supply to the battery and/or loads at most 10A.

Simplified (because the actual voltages are not the nominal 12V/24V, but slightly higher):

For a 12V battery, that means about 12Vx10A=120W (actual maximum limit is 145W).

For a 24V battery, that means about 24Vx10A=240W (actual maximum limit is 290W) .

Even if the PV panels can generate more than those limits, you won't get more, because the MPPT is the limiting factor.

On the other hand, depending on location, weather, panel orientation, the PV panel might rarely be able to produce the rated power for long periods of a day.

So if you are using a 12V battery, I would say that only for the 270W panel the actual power gain might justify the purchase of a larger MPPT (with more than 10A - preferably 20A).

If you are using a 24V battery, there is no reason to change any MPPT.

I have 3x200Ah 12V Victron Smart Lithium batteries.

Do I get it right that both 75/10 are too small?

What size of mppt shoul I install if replacing both with only one?

Do I get it right that both 75/10 are too small?

Theoretically, yes, but as I explained only one (the one for the larger PV) might be worth replacing.

As a starting point, you should check how often and for how long each charger reaches its maximum power (145W).

What size of mppt shoul I install if replacing both with only one?

If the PV panels are different, you should keep them on separate MPPTs.

Also, if the PV panels are oriented differently (angle of inclination and azimuth), you should keep them on separate MPPTs.

PV configuration is:

1: 2x100W

2: 100W+170W

would it the be better to change to

1: 3x100W

2: 170W

Those three 100W PV modules are all exactly the same model?

List Voc and Isc for all your panels.

How are the panels placed?

Are all installed flat (horizontal) on the roof?

Any shadows on them from some other objects on the roof?

Two of the 100W panels are original from the factory, the other two monokristallin instlled later.
I’ll check the data for the monokristallin panels.

The are all 4 flat on the roof, sometimes under trees in shade.

100W : Voc 20.29V, Isc 6.19A

170W : Voc 22.91V, Isc 9.32A

these are connected to one of the 75/10 mppt

So you have 3 types of panels (2x 100W, 1x 100W and 1x 170W).

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How you have them connected now? In series or in parallel?

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In order for the SmartSolar MPPT to start the battery charging, the panel array voltage must be at least 5V above the battery voltage. So that 170W panel (with Voc=22.91V) is only slightly above this limit if you put it alone on a 75/10 MPPT, but it should work alone. Still, better try it and see.

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Check if the two factory installed 100W panels have close values of Voc and Isc as that third 100W panel.

if Isc is the same or close, while not ideal, you could put those three 100W panels in series and get a bigger MPPT for them.

If all 3 panels are in series, the string Voc will be about 60V. I would recommend to get either a 100/xx or (better) a 150/xx SmartSolar MPPT charger for this string. As a cheaper option, a 75/xx might be OK, but this need to be checked based on actual string Voc and minimum temperatures you can have. Still, if the price difference is small, go for the higher voltage options (100/xx or 150/xx).

For a 300W PV array, with your 12V battery, the xx (current to the battery the MPPT can output) needs to be 20A-25A (or higher).

Check the local prices for various MPPT options. If the price difference is small, go for the higher current and/or and higher voltage version.

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Before purchasing anything, another factor to keep in mind is if you will need to upgrade again soon.

Maybe consider replacing all those small PV modules with two larger ones (if they fit on the roof), for instance. And then one (even) bigger MPPT.

This would be worth considering if you can sell or repurpose what you have now.

The 2 x 100W original PV are connected in series to one of the 75/10 mppt.

The 2 new 170W+100W are also in series to the other 75/10 mppt

1. optimal setup would me to have the:

- 3 x 100W in series to a 150/35

- 170W to 75/10

If difficult to rewire on the roof:

- old 2 x 100W in series to 75/10

- new 100W + 170W in series to 150/35?

In your first post you said you have 200W PV on one charger and 270W PV on the other charger.

But having those 270W from different panels in series (100W+170W) changes things.

The smaller 100W PV panel will limit the string curent to its current. So the 170W panel, when in series to that 100W panel, can't actually produce 170W. It's more like you have two 100W panels in series.

As I said, observe the actual production and see how often each 75/10 charger gets to its maximum of 145W.

I think you would be better off by replacing all those small and unmatched panels with two ~300W big panels (if you have room on the roof) and wire each to one 75/10 charger.

In good weather you will not get the maximum from those panels, but in bad weather you will see a big improvement over the current setup.

"Best bang for the buck" in your case (as I said, as long as you can fit two big panels on the roof).

Or replace just the 100W+170W PV array with one big ~300W PV panel.

Or just leave all as it is.