If he has a 100W briefcase he's got 2x50W solar panels,
Aye.
he could match those same panels but its usually better to fit less bigger panels than more little panels..
There are a couple of problems with that statement.
First of all, he doesn't need to match those same panels.
Second, bigger panels offer some advantages, but smaller panels also offer some advantages, so really it just depends. Smaller panels offer redundancy in case of a panel failure, and often offer more bypass diodes. Larger panels offer more square inches of solar cells facing the sun per square foot, which can be important on small roofs where every square inch matters, and larger panels can save quite a lot on the expense of copper wire in large installations.
Also, if he did match those same panels, then he would have mounting flexibility that a larger panel couldn't give him. For instance, he could mount one 50w panel to the left of a roof vent, and the other to the right of the vent.
and with that MPPT he dont have to use 12v panels, most tend to perfer house type panels that run at ~36v so they can even provide a bit of absorption charge while shaded..
I run a 300w panel into a Victron 100/30. I do that because I got the panel cheap, and since it's 36v and I want to charge a 12v battery, I pretty much have to use MPPT or accept the losses from running a 36v Vmp panel at battery voltage instead of Vmp, which would happen with a PWM.
As for shading, my 300w panel has 6 bypass diodes (one per every 12 cells, 6v drop per bypass), so it can produce at 36v (no diode bypass), or 30v (one bypass), 24v (two bypasses), or 18v (three bypasses). If there is so much shade that it gets 4 bypasses and drops to 12v, then that's not enough to charge a 12v battery so it's essentially a 100% loss.
But I run what I brung. There is no reason to suppose that that is what I'd "prefer". I wouldn't. I would prefer 3 x 100w panels, which would give me greater redundancy and rigged in series would still give me the 6 bypass diodes for shade tolerance. The reason I'm not running what I'd prefer, is that I got a good deal on the 300w, so what the hell.
But you're wrong that he doesn't have to use 12v (12v nominal, which is normaly 16v-22v Vmp) panels. He does because he's already got one. So if he wanted to get a 36v panel, then he would have to run separate charge controllers. By sticking with 18v Vmp, he can use just one controller.
If I were running 4x50W 12v panels with that Victron I'd have em in a series for higher voltage, less current and lower losses.. which would make hooking up a briefcase a bit of a challenge wiring wise (a jumper cable would have to sit in place of stored portable panels).. or he could install 1 200W+ panel thats much simpler wiring and lighter and makes less holes in the roof.. and then wire up a 2nd, smaller controller like a Genasun GV10 near battery w/a quick disconnect for his portable panels.
You're missing the point. This happens a lot on ExPo. Someone asks a question about how to use *what they already have*, and some guys jump on that immediately telling the guy how *he should be doing things* instead.
I don't do that unless he's asking to do the impossible. I try to answer *the question the guy actually asked*.
So, the OP has a 100w Renogy briefcase, which he still wants to use, but the charge controller failed. He wants to add another 100w fixed mount to the roof.
Okay, fine. Let's walk it through by the numbers...
Renogy offers several 100w "Portable Solar Suitcase". I don't know which one the OP has. But it doesn't matter - they are all around 18v Vmp.
So what is 18v Vmp? That's 36 x .5v cells in series = 18v.
There are two ways to do that. 18 cells per 50w panel, and the two 50w panels wired in series, or 36 cells per 50w panel and the two panels wired in parallel.
The only one that shows a wiring diagram shows this:
18 cells per 50w panel, two 50w panels in series to equal 36 cells in series for 18v Vmp.
So he wants to add a 100w panel. No problem. Renogy offers a four different 100w non-folding panels.
One has a Vmp of 17.8v.
https://www.renogy.com/renogy-100-watt-12-volt-polycrystalline-solar-panel/
One has a Vmp of 16v.
https://www.renogy.com/renogy-100-watt-12-volt-monocrystalline-solar-panel-slim-design/
One has a Vmp of 18.9v.
https://www.renogy.com/renogy-100-watt-12-volt-monocrystalline-solar-panel/
Once has a Vmp of 17.7v.
https://www.renogy.com/renogy-eclipse-100-watt-12-volt-monocrystalline-solar-panel/
Except for that one at 16v, any of the other three would work just fine in parallel with the 18v suitcase panel(s).
A 100w panel is exactly the same electrically as two 50s in series. It's going to be 36 x .5v cells in series either way.
The only real difference is that with the suitcase, half the cells are in one frame and half in the other, and with the fixed panel, all the cells are in one frame.
But it's still just 36 cells in series. (Well...not that 16v panel, that's going to be 32 cells in series.)
Personally, I'd probably go with the 17.8v just because that's closest to the 18v of the suitcase. But it really doesn't matter, a slight mismatch in Vmp is no big deal. A 2v mismatch (18v/16v) would still work, but if you can get closer, you might as well.
Okay, so he's already stated in the OP that he's going with a Victron MPPT. Fine. Get a fixed 100w panel with a Vmp close to the suitcase Vmp. Install the fixed 100w panel, rig a parallel to be able to plug in the suitcase as needed and run the parallel into the Victron at 18v.
Easy. Done deal.
He'll scavenge more power w/separate controllers too, the diodes will basically cut off his fixed panels when hes using the portable ones because the'll have higher voltage.. If his fixed panels are getting 1A, and his portable are getting 4A his controller will be outputting 4A, but two controllers could handle the two different voltage discrepancies and would output 5A combined.
Sorry man, but that's BS. Exactly how will which diodes to do what to "cut off his fixed panels"? That's not how it works.
The diodes are internal to the panels, and they are bypass diodes. The diodes in one panel don't affect the diodes in the other panel at all. They sure as hell do not "cut out" the other panel. It the cells of one panel get shaded and a bypass diode kicks in, that is only going to reduce the voltage of that one panel - it won't do anything to the other panel.
If a panel has a Vmp of 18v and has say two bypass diodes, if one diode bypasses, then that panel will only be putting out 9v and it won't contribute anything to charging a 12v battery. With two of them rigged in parallel, the other will still be putting out 18v.
Two controllers won't change that. One panel on one controller will still be putting out only 9v and not contributing to battery charging, while the other is running at 18v.
You seem to be basing this erroneous BS on the assumption that he'll have panels with two wildly different Vmp ratings, like one 18v and one 36v.
