Combining dissimilar solar panel strings on same controller

Susido

Rather than buying another controller (which would be a Classic 150 Lite), I'm considering using my existing Midnite Classic 150 to control a new string of (3) 260 watt panels -VMP 30.4V / IMP 8.56A / VOC 37.5V / ISC 9.12A along with my existing array of 24 x 130W 12V nominal panels. These are themselves a mixture of different makes of panels approximately VMP 17.2-17.6V / IMP 7.3-7.4A / VOC 21-21.6V / ISC 7.75-8.25A. 

I've read that the general rule is that it is OK to use the same controller if the arrays can be configured within 10% of the VMP. If so, I can reconfigure by existing 130W panels to 5 series of 5 (I realize I'll need one more panel) to give a combined VMP of 86 - 88V compared to a combined VMP of the 3 new panels of 91.2. So well within 10%. 

The Classic 150 sizing tool indicates my existing array at about 70% of capacity @ 48V and the new array should add another 20%. I don't feel the need for an additional controller for future expansion (I'm done I hope). I realize there's still efficiency advantages to using 2 controllers but I don't think it's worth it to me. Am I overlooking something?

vtmaps

Your plan makes sense.  

I haven't played with the classic sizing tool in awhile, but something doesn't add up...  you claim that you will be moving from 70% to 90% capacity of the classic 150.   Your current array is 130 watts X 24 = 3120 watts.  The three additional panels will add 780 watts, bringing the array to 3900 watts.

The Classic's capacity is a function of the input voltage... the higher the input voltage the lower the capacity.  The Classic power chart shows that the Classic's capacity (with a 48 volt battery) with an input of 90 volts is 4847 watts (83 amps).   Your proposed array would be at 80% of the Classic's power limit.

btw, the Classic Power Limits chart is about thermal limits.  The Classic limits the power it transmits to protect itself from melting.  Good, conservative design is to NOT run them for many hours a day, every day, when they are on the verge of melting. 

Maybe the Classic sizing tool is enforcing good design by telling you that your array is at a higher percent of the Classic's capacity than it really is.

The whole business of matching an array to a controller depends, to some degree, on the climate.  In the winter my array will often put out its nameplate power for sustained periods.    I could double my array, but that would mean operating at 100% of my controllers capacity for sustained periods.   I would not do that.

In the summer, the panels are hot and I typically see a 25% reduction in their power.   If I lived in a year-round hot climate, I would have no qualms about doubling my array because I would only be operating the controller for sustained periods at 75% capacity... there might be moments (edge of cloud effect) where I reached or exceeded capacity, but they would be brief.

--vtMaps