Wiring-Array to MPPT Charge Controller

[sluggo]

I've inherited an off-grid PV system my father put together in the 80's. I've been doing a lot of research but can't find much info on wiring. This is my situation: The present array is made up of four old Arco panels outputting 140 w at 12v. I found a great deal on 3 BP 70w 24v panels. I'm estimating it's 140 feet from the panels to the power house where the charge controller & batteries are (It's at a remote location so an estimate is all I have right now). I would like to install an MPPT charge Controller and output all the panels at 24V. to the Charge controller. The present array can be wired to output at 24v and the 3 new panels each output at 24v. so I would have four 24v arrays wired in parallel. I gather I need a combiner box at the base of the tower where the panels will be. I don't particularly want to dig new trenches and lay new wiring to the power house though. The present wiring is large enough to carry the load as it was wired for 12v. Here's the question: Can I combine all the arrays onto the present wire to the charge controller? If so, I would assume that the MPPT controller would not be able to then discern that there are four separate arrays and it would lose some of its effectiveness? Any ideas, questions, thoughts or pointers to wiring info are appreciated.

-Mark

[Flux]

Are you intending to keep it a 12v system or are you changing to 24v?

You seem to have little choice but run the panels in parallel at 24v. If you stay at 12v then a mppt converter will be necessary. If you upgrade to 24v then the mppt may bring you a small gain but I suspect not a lot over direct connection.

Mppt will optimise the average panel output and generally the characteristics are similar at 36 cells per 12v. Normally it is better to use cells in a high voltage set up and that is where mppt comes into its own but your panels don't lend themselves to series connection unless you use 2 separate mppt controllers.

Flux

[sluggo]

Thanks Flux. I was intending to keep it at 12v Because that's what it is set up for, although I guess I should research what's involved with a 24v system before I decide.

-Mark

[sluggo]

Here's a new thought: It's possible for me to get more of the same 70w panels. I could get another 70w and take down the current 140w array. With four 70w @24v I could wire 2 pairs in series and have 48w feeding into the charge controller, then down to 12v in the batteries. I would lose 70w by ditching the old array but I could probably make up some (how much I don't know) of that by sending 48v over the 140 foot line from the array to the Charge Controller and make up some from the fact all the panels would be the same and the MPPT controller would be happier. I then could probably sell the the old 140w array for more than the cost of the 70w panel.

Any thoughts as to how much more efficient this setup (140w@48v +140w@48v) would be than the 140w@24v + 70w@24v + 70w@24v + 70w@24v traveling a line of 140 feet?

Thanks - Mark

[Flux]

This is getting a bit too involved for me. I don't want to make decisions for you that may or may not suit.

Using 4 of the new panels at 48v with mppt would be fine, if you can sell the old ones at a reasonable price it has something going for it ( how long are they going to last?)

I have no idea of your cable run, 140 ft is not that frightening if the cable is sensible. I think this cable loss issue is not as bad as often thought and sometimes the extremely heavy cable people use doesn't make much economic sense, the issues are different with a current source compared with a voltage source.

If you stick at 12v with the mppt then it will certainly be more efficient at 48v than 24v but whether you will be better off with 4 70W panels or 3 and the existing ones then I really don't know, I wouldn't expect there to be much difference. Depends on the relative cost of another 70W panel compared with what you could get for the old ones.

If you go the new panel route you might consider 3 70W connected in series as nominal 72v rather than in parallel for 24v. If you use 4 new ones then series again should be within the rating of the better mppt controllers and would be better than series parallel at 48v.

Flux