wiring panels in series or parallel?

System

Hello,
I am in the process of installing my off-grid solar system for my home and would love to hear some of your opinions from you good folks. I am still relatively new to all this but I have been browsing this forum for sometime.

The question that I have is whether should I wire my panels in series or parallel? But first, let me give you my system specs:

4 X Siliken 230 watt max, vmp 29.5 v, imp 7.7 amp voc 36.9 (24 volts panels?)
Blue sky solar boost 6024h 60amp MPPT controller
6 interstate 95 ah marine/deep-cycle batteries wired in parallel (12 volt system) - cheap but probably not the best idea!
Xantrex 1600 prosine sinewave inverter 12v
about 60 feet of 8 awg wiring

The solar boost 6024H states :
60 amp maximum output current
nominal pv voltage: 36v/ 48 v (3 or 4 series modules).... not sure what that means
pv open circuit voltage: 140v max.

Would wiring my panels in series (with total of 147 volts open circuit voltage )work ok with my controller or perhaps 2 series strings of 2 panels wired in parallel work better?

Would my wiring be sufficient without much significant losses. What fuses would have to be employed in such configuration, their rating and placement.

Cariboocoot

Re: wiring panels in series or parallel?

Welcome to the forum.

The Blue Sky's 140 V input limit means the maximum Voc of the array. All four panels in series would exceed this, even before you factor in any increase due to cold temps.

I'd suggest the best thing to do here is to make two strings of two panels in series and then parallel the strings. That way your array specs would be:
Voc 73.8, Vmp 59, and Imp 15.4

With that and your 6 AWG wire your V-drop should be under 3% even at 60 feet. With this configuration no panel fuses are needed because there are only two parallel connections.

System2

Re: wiring panels in series or parallel?

Thanks for some great info! I assume that the voltage drop using 6 gauge wire would be similar to using 8 gauge in my system.
I hope that my 60 amp controller ( 720 watt max, if i'm correct) will be able to handle my 920 W max power array, which practically, I doubt I would ever be able to reach that level of current, in which case the controller would shut of, right?

Cariboocoot

Re: wiring panels in series or parallel?

Yes, charge controllers' rating is their maximum current out. You are probably aware that your 920 Watt array will not actually produce 920 Watts except on a few occasions. Most of the time it will average some percentage under that, typically 77%. So it effectively becomes a 708 Watt array and the peak current is likely to be 708/12 or 59 Amps. Essentially you are at the maximum for that controller on a 12 Volt system.

I mistakenly put "6 AWG" where I meant "8 AWG". Double-checked it now and get 2.4% V drop based on 48 Volts nominal and 60 feet of 8 AWG.

Getting six batteries in parallel to share current evenly is tricky. Take a look at the Smart Gauge wiring diagrams here: http://www.smartgauge.co.uk/batt_con.html You're looking at using method 3 for best results (each battery having equal length wires going to common connection points for both positive and negative). Ideally each of those batteries should have a fuse on it too, as should the (+) wire from the charge controller.

Unless I've missed something else, it should all work.

System2

Re: wiring panels in series or parallel?

This is all really great stuff and now I'll be able to sleep assured that my system will be in top shape and long lived.
I wish I had come across this info earlier of how to design my battery bank properly.
The battery bank I have came from my older system. It's about 1 year old, it's been cycled to about 50% DOD, fully recharged often, but right now it's already show signs of weakness with a higher load.

So, what would be good gauge size of wire if I was going to run each battery independently to the bus bar, and what rating of the fuse would have to be used in each branch?
...guessing that about 200-250 amp per leg would be good protection in case any battery would short out internally.

Cariboocoot

Re: wiring panels in series or parallel?

The sizing of battery wire is dependent on the current draw of the inverter. In this case 1600 Watts, with some surge rating perhaps? (Can't remember the specs of every inverter in the world, for some reason. Must be getting old.) Technically it should be rated for maximum output power at minimum input Voltage, which would be 1600 / 10.5 (probably) or 152 Amps. So using wire capable of 200 Amps is a good idea. That means 2/0 (aka '00') or better. Ideally each battery (+) would have its own fuse, just in case something goes wrong with the interconnections. Blue Sea makes some nice battery terminal fuse block units that you should be able to get at a marine or RV dealer. The fuses: http://bluesea.com/productline/overview/379
NAWS has this nice selection of inverter fuses: http://www.solar-electric.com/infubr.html