I'm "jumping on something new" because you continue to ask for information I've already provided. I provided not one, but two examples of variables and values. Then you quote the example, and ask me for an example? In your lingo, that's an infinite loop.
I used the "multiserialmega" example to find out what the solar controller is spitting out. I pasted this in the opening post, as it appeared in the serial window. There is no information being withheld. That's it. I fail to see how I as a novice have some insight an experienced person doesn't have, when we both have the same data.
"What separates each line - a linefeed character?"
The post after that contains code that shows \n new line, so it would appear this question has been answered. The code is for another product from the same manufacturer but a different model which has different variables, hence the need to modify the code.
"What marks the end of the message?"
Well as you can see, it ends with "Checksum" but is there some invisible character after that? How can I know this? Is there possible modifications to the multiserialmega example that can tell me?
"Am I correct to assume that each complete message can have different numbers of characters depending on the data it contains?"
The variables I suspect are fixed. The values would range between single digit 0 and 5 digits, so yes.