Aha! I can answer this!
OK, a few pieces of terminology you're going to need to understand:
VoC: This is the maximum voltage the panel will produce with no load under perfect test conditions. This is a really important number you'll find on the data sticker of any panel.
PV Input Voltage: This is the maximum DC voltage your charge controller can take in before it ends in magic smoke/fried equipment/angry wife.
Temperature Coefficient: Solar panels will produce a set voltage at perfect test conditions, which is IIRC 25C. This is great if you live in the tropics, but if you're anywhere that gets cold like Alaska/Vermont/Wife's Feet then the voltage will go
UP as the panel gets
Colder. This will affect the voltage going into your charge controller.
Now, for understanding the concepts and the sake of easy napkin math, I'm going to say that your Growatt has a
PV Input Voltage of 200V max. This is the number to
NEVER exceed when hot, cold, or any other temperature.
Now, lets say you're using 100w panels that are 20v VoC and 5a each. 20v * 5a = 100w. You think to yourself "Self, 200v of input and these are 20v panels, I can put 10 of these
in series and be good to go!" And then your self would have released the magic smoke because it was a little cool the other day and your panels' voltage went up a bit and exceeded that 200v input limit.
As a rule of thumb we say 20% overhead unless you're someplace REALLY cold like Alaska/Vermont/Wife's Feet at which point you have to do some math.
But, I hear you thinking to yourself, you've only got 6 of those 100w panels. You could put THOSE panels in series and only be at 120v, well under the 200v limit. And you'd be right. 6 panels in series facing south in a single series string would make your controller Very Happy. Yaay!
Now, it's been a couple years and it's just not producing enough juice to top up your batteries and you're getting sick of feeding the generator every day in winter. So you jump online and get some used 500w Big Boy Panels! Well, now it's time to do the math because those panels turn out to be listed at 50VoC at 10a (nice big panels!) so if you tried to string 6 of those together you'd end up with magic smoke/burned parts/angry wife. Uh oh!
Well, what you would do then is start making multiple strings in series to keep the voltage down below that magical 200v limit, and you'd put THOSE strings in parallel. Since you don't want to push that 200v limit you decide to set up a 3s2p setup where you throw 3 in series for 150v @ 10a (50v per panel, 3 panels in series, 150v) each and parallel 2 of those strings to get 150v @ 20a (each string is 150v @ 10a, in parallel you add the amps so 150v @ 20a). Now your controller is Very Happy AND you've gone from 600w of panels to 3000w of panels without releasing the magic smoke OR making the wife angry.
From there, now that you have 2 strings you get to choose orientation. Do you face all the new panes south and generate ALL THE WATTS at noon, or do you swing some SE and the others SW so you get Some Watts from dawn to dusk? That's more personal preference and environment and really isn't important at this stage though.
Does that all make sense?
Don't sweat that part, you're using 2 12v batteries. Throw them on a regular car charger that has an AGM setting and let them get full, that'll get you to 95+% charged. Once you get your system put together and the settings in the Growatt will take over from there and top everything up. The big key is to have both batteries as closely charged as possible, i.e. don't have 1 at 95% and the other at 50%. Connect them in parallel, connect the charger from the positive on 1 battery and the negative on the other. They will both charge just fine and evenly.
The only time balancing a battery becomes a real issue is when you're building your own and you want all 8 or 16 cells to be perfectly topped up, what we call a "Top Balance". For factory built batteries, not a deal.
