Disparate Solar Panel Voltages

[wincrasher]

My setup is working very well. I'm tempted to add another panel, but I don't have room for one of equivalent size.

The one that will fit is 100 watts, rated at 17.55 volts. My existing panel is 180 watts rated at 19.25 volts.

What would be the effect of wiring these together in parallel?

[BBQ]

Originally Posted by wincrasher

My setup is working very well. I'm tempted to add another panel, but I don't have room for one of equivalent size.

The one that will fit is 100 watts, rated at 17.55 volts. My existing panel is 180 watts rated at 19.25 volts.

What would be the effect of wiring these together in parallel?

When adding panels,

Parallel connection -- same voltage, any wattage is ok

Series connection -- any voltage, must be similar wattage.


17v and 19v? close enough.

[fratermus]

AltE has a series of youtube vids demonstrating this kind of thing.

[wincrasher]

Interesting, I got the following response from AM Solar:

"Hi Ron,

The panels are different enough to affect current boosting, so yes, it can be a concern if you're trying to squeeze the most charging current out your array. The 180W panel will likely perform like a 160W panel if paired with the SP100.

Not the news you wanted to hear, we know...

Regards,

On Mon, Sep 18, 2017 at 12:18 PM, Merritt, Ron <Ron.Merritt@nrg.com> wrote:
I have your 180 watt Victron kit installed and it’s working great. I’m thinking about adding the SP-100 to this setup. Is the difference in voltage between the 180 and 100 panel of any concern? Would the output on the 180 panel be degraded? Thanks. R"

[markopolo]

I'm assuming you have an MPPT controller.

My guess is that you'd see output gains by feeding that controller a higher input voltage by putting the panels in series.

https://www.victronenergy.com/blog/2...s-or-parallel/

[wincrasher]

Here's the thing though - in series, if either panel gets shading, then the output drops significantly. In parallel, that doesn't happen.

So is it really worth it for the loss of 20 watts as suggested by AM Solar?

Another option is to pry up the 180 panel, move it over and mount a second right beside it. It may be pretty tough to do that, plus the cost of $80 for new mounts.

[markopolo]

I'm not an expert in this for sure, but I would have guessed that the higher voltage permitted by arranging the panels in series would actually allow the MPPT controller to better work its magic.

Victron appears to recommend that in the manual:
Recommended number of cells for highest controller efficiency: 72 (2x 12V panel in series or 1x 24V panel).

So I guess the question to be answered is do you look at output over a short duration (temporary shade, mobile installation) or do you look at output over a long time period like a month (varying conditions)?

I think the best answer would come directly from Victron. They state "Advanced Maximum Power Point Detection in case of partial shading conditions" as a feature so that needs to be fully understood.

[fratermus]

Originally Posted by markopolo

I'm not an expert in this for sure, but I would have guessed that the higher voltage permitted by arranging the panels in series would actually allow the MPPT controller to better work its magic.

I have read that peak MPPT efficiency is attained at ~2x battery voltage. I don't know if it's true but it lines up with the quoted info from Victron, and also with my Genasun's documentation which calls for 23v on a nominal 12v bank.

Mismatched panels can confuse MPPT controllers because there isn't one clear spike in power output. There might be one for the smaller panel, one for the larger, one for both combined, or something weirder. Depending on the algorithm[s], the controller might find a sub-optimal power point and stop, thinking the job is done.

It's a bit of a balancing act; check too little and it could miss the real Vmp. Check too much and the user sees it as slow to find Vmp. Better controllers likely have faster hardware and optimized algorithms to perform exhaustive checks in a reasonable amount of time. The inexpensive Tracer line is infamous for being slow to find MPP (I have two of them).