flameskimmer avatar image

Different solar panels for short periods?

On my yacht (Dragonfly Tri) I currently have a Sunware 40w panel that is connected through a FOx-150 PWM controller. This is fine for battery maintenance, however, when sailing and using autopilot, electronics and the family charging every phone/tablet they can lay their hands on, I am consuming the battery (two 55Ah Optima blue tops) and not getting the charge time. This is especially true in the winter months.

I am planning to have a 100w panel that I can connect into the system when sailing to provide additional power. Since I am a tri I have plenty of space to fix on the trampolines and will connect through a socket installed in the coach roof.

I have bought an MPPT 75/10 controller, and ideally, I would like to replace the Fox-150 and wire the panels in parallel. The FAQ said not to mix panels though so want advice on best way to proceed.

I can leave the FOX in place and wire the 100w panel through the Victron, although I think I will get better value of having the MPPT controller connected all the time. Or, as this will not be connected all the time, and only when I am on the boat, can I just connect in parallel through the Victron without the sky falling in.

Thanks, Mike

MPPT Controllers
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All I want to say is thumbs up for trimarans!

flameskimmer avatar image flameskimmer mvader (Victron Energy) ♦♦ ·

It's hard to go on a mono now :-)

Just to add, I do have a separate battery monitor (NASA BM1) so I will be able to see what is going on if using two controllers, it just seems overly complicated...

4 Answers
wkirby avatar image
wkirby answered ·

It really depends upon the type of panels and how many cells each of them have. Ideally they would be of the same type, Polycrystaline or Monocrystaline.

If both panels have the same number of cells then you could possibly connect them in parallel, any difference in chemistry will probably have a minor effect.

If there is a difference in the number of cells between the two panels then you can only connect them in series, but there is a limit to the number of cells you can have in series so as not to exceed the 75 Volt maximum input of the MPPT. I wouldn't want to exceed 100 cells in series as a rough estimate off the top of my head.

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A quick bit of research tells me that the Sunware has 38 cells and is a multi, while the 100 is a 32 cell mono...

So run them separately?

Yep. Since you have two controllers the easy way is what you were thinking of doing. If you don't have the panel yet perhaps look at getting a couple of narrow flex panels and mounting them on the amas? Even the Dragonfly 25 looks to have enough space for a couple of 50W panels (I searched ebay for "flexible solar panel narrow" and found an 80W/350mm one). I would put them in parallel on one controller and just assume that the shaded one will not produce any output... but if they're both in sun that's a win :)

I do like that idea, however just looking for something temporary as properly installing in the amas would mean quite a bit of cabling, sealing and messing about to ensure that nothing is getting caught in the pivot points. Certainly something I'll be looking to do longer term.

JohnC avatar image
JohnC answered ·

You could do this really any way you like.

Providing the two panels have a similar Vmp (eg ~18V) then I'd be inclined to install them in series and leave up full-time on the Victron only. That would optimize charge in lower light, so effectively extend your charging hours to some degree.

Pretty good match too for a 10A cc. But personal preference applies here too, so others may recommend something else.

And oh, encourage the family to charge their kit during the day.. :)

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Surely it's Imp that matters? I am not familiar with dissimilar panels in series, but since the voltages add and all current flows through both panels, isn't it the current you should worry about? So a 12V/40W panel in series with a 6V/20W panel would run ~4A at 18V nameplate, but if you put a 12V/120W(10A) panel in series with a 12V/10W (1A) panel you'd risk burning out traces on the 10W panel by pushing 10A through them.

It'll be less dramatic with a 100W + 50W panel, but still, you're putting twice the current through the 50W one than it was designed for.

Point accepted Moz. Yes, previously understood & respected. But I'd still series em personally. After all, most of the day won't be 'maxing' A.

And that 40W sucka is expensive, like you can even supposedly walk on it.. so maybe a bit of 'I' up it's sleeve? In reality I'd expect it to be able to manage it, and if it couldn't it maybe doesn't deserve to be there.?

It's just me, an opinion.. :)

wkirby avatar image
wkirby answered ·

Armed with additional information about the panels, I'm with JohnC, connect them in series and just use the Victron MPPT. One controller, one array, simple.

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Thanks for all the advice :-)

My only problem with the serial is that I don't have the space on the hull to have the 100w mounted all the time. It was planned just to clip onto the tramp when underway to give the extra oomph. These small Dragonflys don't have a ton of spare space (that isn't right under the boom) to mount things.

I'll get down to her this weekend and see what the wiring would look like as I may be able to install a switch that allows me to cut the 100w panel in and out of the circuit easily.

I'll give an update when I've done some playing.


bluepowershop avatar image
bluepowershop answered ·

Bear in mind that a 30 cell panel alone on the MPPT will not work, or have a very disappointing result.
The voltage of a 30 cell panel is to low for the MPPT to work with!

If you use the panels on a separate controller, connect the 38 cell on the MPPT and the 30 cell on your PWM controller!



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