Over-dimensioning a PV array. Why max current input limit for MPPT chargers?

[StokesPen]

I'm going to reference the specifications from the Victron MPPT 150/35 solar charge controller: www.victronenergy.com/solar-charge-controllers/bluesolar-mppt-150-35

This controller can handle a maximum input of 150v with a max PV short circuit of 40amps, so 6kw input. It can handle a maximum output of 2kw with a 48v battery bank. You can also further limit the maximum charge current if you wish.

I understand why the maximum input voltage needs to be adhered to but why does the current matter when the unit is able to limit it anyway. As an extreme example, why can't you have 150v and 100 amps (15kw) of solar input and let the controller plod away outputting just 2kw. I can hook up a cheap DC to DC buck converter to a car battery capable of pushing hundreds of amps but the converter will happily limit the current to just 2 amps without issue.

So why is there a current limit on the input to this and many other MPPT solar charge controllers?

[fourtytwo42]

I think they are just referring to there maximium capacity, the normal maximium ISC of a 60/72 cell panel is 10 amps so that implies they can handle up to 4 strings in parallel.

[DenzilPenberthy]

You can do that, no problem. The controller will only draw up to its maximum current.  Obviously you'll not be getting full power out of the panels when it's sunny but you will get more power when it's overcast than if you put the 'correct' amount of panels in your system.

You might want to consider a fuse in your input though if you have craploads of overcapacity which could deliver loads of power in to a broken MPPT controller input.

[StokesPen]

Thanks, I'm not going crazy then. So the manufacturers specification isn't actually a physical limitation in this case but more a guideline to assist people in sizing their solar array, to prevent excessive over sizing. In my books a specification should be straight up facts with guidelines in the description.

Although it does still say at the bottom of the specification "2) A PV array with a higher short circuit current may damage the controller"

[DenzilPenberthy]

Yeah, I've built a few systems like this where the panels are significantly more powerful than the controller can harness. It's common in off grid battery systems where for example you might install 2kW peak of panels but you're not really interested in getting 2kW out of them on a midsummer day. What you're interested in is getting 150 or 200W out of them for a big percentage of overcast days throughout the year. So there's no point in speccing a 2kW MPPT charge controller.

Somewhere on the Victron website there is a spreadsheet which you can use to select their controllers etc and do the calculations for you. It will warn you if you will be 'wasting' power by using a too small controller but as long as you don't exceed the Voc limit you'll be fine.

[Red Squirrel]

That's good to know that you can do that, I always kind of wondered the same myself.

[David Hess]

It would be a poorly designed charge controller if it could draw enough current to damage itself.