# Sharing panel voltages

Solar Expert Posts: 225 ✭✭✭
Greetings,

I have a quick question. If my panel array is based on (135watt) 17.7 volts panels do I have to stick with 17.7 volts panels? Since I'm tying them into a combiner box anyway can I just wire up any ol' voltage on its own string and let the Flexmax 80 take care of the conversion to 12 volts (14v). I want to add another panel to get my charging rate up. It's currently around 7.7%, (calculated by Cariboocoot) and I think I can get it to a better 9% with another 135-watt panel.

I would have three strings of 17.7V @ 7.6A plus a single panel of 135watt 17.7 @ 7.6. A panel by itself on it's own string wired to the combiner box.

With three strings of 6 panels I'm currently am at 35.4v @ 22.8 amps. If I add a single panel (135watt, 17.7 @ 7.6) would my new voltage be 35.4 + 17.7 = 53.1? It would be a total of 7 physical panels. How would a solo panel fit in with three strings of 2-in-series?

Actually, that wasn't a quick question at all. Sorry.

Hairfarm

• Banned Posts: 17,615 ✭✭
Re: Sharing panel voltages

Yes, but then again no.

You can wire panels of dissimilar Voltage in series and make the Vmp of the array anything you like.
But when you put dissimilar panels in series the Imp will be whatever the lowest one is.
So you do not want to put one panel with Imp of 7.6 in series with paralleled sets of panels whose combined Imp is 22.8 because effectively you will lose the power of two of those panels as the whole array Imp gets "choked" back to 7.6 Amps.

The FM80 will down-convert any Voltage up to <150 to proper charge Voltage for any system Voltage. However, the greater the difference between array Vmp and system Voltage the lower the conversion so you will lose some power.
• Solar Expert Posts: 3,123 ✭✭✭✭
Re: Sharing panel voltages
Yes, but then again no.
You can wire panels of dissimilar Voltage in series and make the Vmp of the array anything you like.
But when you put dissimilar panels in series the Imp will be whatever the lowest one is.

But what the OP seems to be proposing is to take his two-panel 35 volt strings and add a single panel at 17 volts in parallel.
That will not work.
If he connects the single panel to a totally different MPPT input, then the CC will be able to convert the two voltages independently.

But if all he does is connect the 17 volt panel in parallel with the 34 strings, the 34 volts may be enough to cause reverse breakdown and current flow backwards through the 17 volt panel. But even i f it does not, the MPPT controller will never pull the voltage far enough down for the 17 volt panel to start providing any current, since to do so would cut the output power of the 34 volts strings roughly in half.
SMA SB 3000, old BP panels.
• Banned Posts: 17,615 ✭✭
Re: Sharing panel voltages

Core, inetdog; I sure hope he wasn't planning on putting 35 Vmp @ 22.8 Imp in parallel with 17.7 Vmp @ 7.6 Imp.

At least not without a fuse. Which will go "poof". Hopefully before the panel does.
• Solar Expert Posts: 225 ✭✭✭
Re: Sharing panel voltages
Core, inetdog; I sure hope he wasn't planning on putting 35 Vmp @ 22.8 Imp in parallel with 17.7 Vmp @ 7.6 Imp.

At least not without a fuse. Which will go "poof". Hopefully before the panel does.

Ok. I definitely won't do that then

How about putting all seven in a single series string? That would be 123.9v. However, I recall that if I'm too high over my ideal voltage input range for a 12volt system then I'm losing more power in the conversion than if I just stay put at 35 Vmp @ 22.8 Imp with my original 6 panel, three string arrangement.

Ok. I should rethink this. With a current charging rate of 7.7%, I'll likely need to use the generator to make up the difference to give me a better 10% charging rate, or use less power, or go routinely go beyond a 25% discharge rate and shorten my battery life. Arrgg!

But at least I already own the generator. That will be the cheapest route until I can afford two more panels and the roof rack system to mount them.

thanks,
• Banned Posts: 17,615 ✭✭
Re: Sharing panel voltages

Two things with all panels in series: the Voc adds up and may exceed the input max of the controller (especially in cold weather) and the conversion efficiency drops.

The panels probably have a Voc around 22, so seven in series would be 154. Unless you are running a MidNite Classic 200 or 250 you'd be in trouble. Especially up here where it could hit 200 Volts.

Odd numbers of panels are often difficult to arrange in more than one string.
• Solar Expert Posts: 3,123 ✭✭✭✭
Re: Sharing panel voltages
Odd numbers of panels are often difficult to arrange in more than one string.

Prime numbers make it even harder!!!
SMA SB 3000, old BP panels.
• Solar Expert Posts: 225 ✭✭✭
Re: Sharing panel voltages

Ok. But before I resolve to give up, is there any scenario in which I can add a seventh panel (135 watts, 17.7 v) to my above mentioned array? Or do I have to pair it up with another of the same voltage?

You can wire panels of dissimilar Voltage in series and make the Vmp of the array anything you like.
But when you put dissimilar panels in series the Imp will be whatever the lowest one is.
So you do not want to put one panel with Imp of 7.6 in series with paralleled sets of panels whose combined Imp is 22.8 because effectively you will lose the power of two of those panels as the whole array Imp gets "choked" back to 7.6 Amps.

Let's say someone has three odd panels that don't match. One that is 17.7volts, one that 34volts and another that is 26volts. What way, if any, could those be wired to work together?

Is the problem with the parallelling?
• Banned Posts: 17,615 ✭✭
Re: Sharing panel voltages

Separate charge controllers. You can't parallel panels with mismatched Vmp without losing power.