Combining solar panels of differing specifications

Okie

I have an off-grid cabin with a 12 volt solar system that’s been installed for about 7 years.  It works fine but I’d like a little more power for charging the battery bank, running lights, fans, and a small coffee maker.  It only gets used about 4 days each month.  Is it okay to add a second solar panel with different specifications?  My thought is to hook 2 panels through a combiner box that would then feed into the existing Outback FLEXmax FM60 MPPT charge controller that charges a 12 V, 610 AH AGM battery bank.  My existing solar panel is a 7 year old Kyocera KD205GX with original specs of 205 W, Vmp of 26.6 V, Voc of 33.2 V, Imp 7.71 A.  I’m thinking of adding a new panel 270 W panel with Vmp of 31.0 V, Voc of 38.3 V, and Imp of 8.71 A.   Is it okay to do this or should I try to find a new panel with similar specs of the existing panel.  Also, when I go through the combiner box, does the voltage add to 57.6 V (26.6 + 31.0) or does it stay at a lower voltage feeding into the charge controller?  Any other thoughts on what to do without spending a bunch of money would be appreciated.

Horsefly

It would be less than ideal, but you can put the new panel and the old panel in series. This would not need a combiner box, and would give you roughly a combined Vmp of 57 volts. The downside is that the Imp of the series would be only 7.71A, so you would be losing the additional current the new panel would otherwise produce, reducing it from a 270W panel to a 239W panel.

The panels have a bit too much difference in Vmp to put them in parallel.

Steve

Okie

Thanks for the quick reply Horsefly.  From your reply it appears that if I want a parallel setup, I should look for a solar panel with similar specs to the existing panel - or even something slightly less since there has surely been some efficiency loss over the past 7 years.  What happens to the voltage coming out of the combiner box - since this is a parallel setup, is it a big deal to have a panel at 26.6 V and another at 31.0 V going into the combiner box?

Horsefly

To answer your question, yes it would be a big deal. The voltages are just too far apart to put them in parallel. 

Not sure why you want to put them in parallel anyway. Can you explain why you want parallel? Remember that you get roughly the same power out of two panels either in parallel or in series.

If you are just going with those two panels, you would be better off with them in series anyway. No combiner box needed (none needed anyway, if you are just wiring two panels in parallel), and the higher voltage / lower current would have less loss going from the panels to your charge controller.  The charge controller you have would easily take the higher voltage of them in series. 

If you do have your heart set on having the panels in parallel, you probably should try and find one that has about the same Vmp. To put to different panels in series, you're best to find ones with about the same Imp. If they are not the same Imp, the actual current you would get would be limited to the lower Imp of the two.

Horsefly

I had to search for it, but I knew I'd seen a video on this topic.

https://www.youtube.com/watch?v=HlsGo9AJn4M

Okie

OK thank you for your reply, I guess I was a little slow to catch on to what you were getting at. I appreciate your help.