Third charge controller with higher voltage panels?

Bigwooo

I currently have a 12 volt battery setup with three individual strings of panels. Two strings each consist of 6 -(24v) 175 watt solarworld panels in parallel, the third string is currently 4 (12v) - 85 watt Shell panels in series/parallel to 24v. Each of the three strings is controlled by an FM80.

I plan to replace the 4-85 watt panels with three 250 watt solarworld 24 volt panels. The wire from the combiner box to the charge controller for this string is #6 and it's 20 feet long (my other strings are #2 wire, but that does me no good for this string). If I wire the panels in parallel, the distance from the combiner box to the charge controller is right about at the edge of the 1.5% voltage drop table.

Since this charge controller is only going to control the three 250 watt panels, is there any reason why I should not wire them in Series to 72 volts (even though my other two strings are 24 volts), so I don't have to worry about the resistance in the #6 wire? If I understand the Outback FM80 instructions correctly, as long as all the panels are the same voltage, on the same controller, in this case series 3x24v to = 72v, the charge controller will step it down to the battery voltage (12v), same as the other two 24 volt strings. Right???

Anyone see a problem with wiring them in series?

Thanks

Cariboocoot

Re: Third charge controller with higher voltage panels?

You are correct that the charge controller will not care what Voltage the array is as long as it is above the battery charge minimum and below the controller's input maximum. The output will take care of itself. There will be a slight reduction in efficiency, but not much.

The Solarworld 250 panels are 31.1 Vmp 37.8 Voc so three would be 93.3 Vmp 113.4 Voc so unless you get some really cold temps it will work.

However, your run is 20 feet of 6 AWG? According to my calc that would work in parallel (24 VDC @ 32 Amps over 20' on #6 wire).

Maybe I'm confused. It's been one of those days.

Bigwooo

Re: Third charge controller with higher voltage panels?

You're correct about the voltage drop. I calculated wrong. We don't have real cold temps. 65 degrees is considered pretty cold 8). I had considered series because of my mis-calculated voltage drop. I think I'll stick with parallel.

Thank you.