Mixing Solar Panels (again)

[lpdsn]

In the example you quoted where one panel has a Vmp of 19v and the other panel had a Vmp of 18.5v the combined Vmp would be close to 18.5v and this is the voltage the controller would select. It is easy to estimate the loss of this simple model. It would be 0.5/19= 2.6% for the 19v panel. The 18.5v panel will have zero loss. If both panels had an equal output the total loss will be around 1.3%.

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Good MPPT controllers are also very expensive and in general the cheap versions are not worth bothering with.

An MPPT controller can't select the Vmp at the solar panel. It can only lower the input voltage at the input to the controller. The effect at the panels is, I strongly suspect, far more complex than the model you're putting forward.

It's crying out for someone to make careful measurements (GOOD AFTERNOON PBO MAGAZINE). I'm afraid I'm pretty 'time poor' at the moment, so I doubt I'd ever get around to it myself.

I still suspect that two panels with two controllers would give more output than the same two panels with one of the expensive MPPT controllers.

On that point about cheap and expensive controllers, there's a wonderfully archaic understanding prevalent on the forum that you get what you pay for, but in a world where component and manufacturing costs are a pretty small part of the price you pay it doesn't necessarily hold up unless you know why the expensive model is better. I have to admit that my first impression looking at the range of controllers is the expensive ones had clear invested in a case designer and had a far more stylish logo.

So what would you say that the expensive MPPT controllers have that the ones not worth bothering with don't?

Incidentally, my only experiment so far, has been with a very cheap PWM controller, where I got about 85% of a solar panel's rated output into batteries that I'd only just disconnected from shorepower (but were driving a fridge amongst other items). The panel meanwhile was just casually propped up on the side deck (so shadows from guardrails and stanchions) and it was mid-latish afternoon in late March in Ireland. That was way more than I expected under the circumstances.

Re DannyB's question. There would be a voltage drop in the wires between the controller and the battery, so what a second controller would 'see' would depend upon the controller output voltage, battery voltage under charge at that current and the configuration of wiring. It would be very unlikely to 'see' the other controller's output voltage.

 

[William_H]

Yes but it will not matter. A bit like running 2 alternators with regulators into one battery. If one solar controller does get the battery to that point where it judges it is fully charged eg 14.1v it will then cut back charge current. If there is another solar system also contributing then it will cut back current also presumably at 14.1 volts. One is inevitably going to reach it's cut off sooner than the other so will tend to cut back on the basis of the current being provided by the other controller.
You may get unbalanced charge current for a short while. The point is it doesn't matter because the battery will be so close to fully charged anyway.
So it should only happen on a large solar supply and small load and or small battery. You won't be so lucky mostly. olewill

 

[noelex]

An MPPT controller can't select the Vmp at the solar panel.

Yes it can.

This is whole purpose of MPPT. The controller adjusts the input voltage so that it is always at the Vmp of the panel (or Vmp of the solar array if more than one panel is connected).