The important details for diagnosis are missing.
We need to know exactly what engine you have.
This means you need to look on the engine and locate the engine model number, and type number (if it is a Briggs & Stratton brand) or the engine model number (if it is a Kohler brand).
Your symptoms sound very much like a Briggs & Stratton engine in need of the annual valve adjustment (necessary to maintain the compression release effectiveness).
You might also include the brand and model number of the mower in case it is needed later.

It's an 18.5 hp B&S Intek

Well, that's an improvement but it's still not the engine model and type numbers from the engine.
The numbers above are merely "sales point declarations" intended to be easily seen from about 8 to 10 feet away.
The technical model and type numbers can only be found by doing a "close in" inspection of the engine itself.
Typically the numbers will be found on a cooling shroud or on a valve cover decal.
But............now that we know it is a Briggs Intek, it's a safe bet that the engine needs a proper valve adjustment.
If you think you might be capable of doing this as a DIY project, beware that it requires at least a basic understanding of 4-stroke engine operation and feeler gauges of the specific dimension for your engine.
I can't send you the instruction procedure via e-mail because my e-mail is not working today.
There is another member (Walt Conner) who will be glad to help out if you send him an e-mail.
You will need to put this address into the "proper protocol" for it to work. (at=@, dot=.)
wconner5 at frontier dot net
Be sure to explain what you need (Briggs OHV valve adjustment procedure) and he will send you the facts.
Then you decide if you can do it or if you need a shop to.
Hi Walt!

"...the battery tests OK..."

What test is it testing OK? Voltmeter? Load test?

There's an old diagnostic mantra... when you hear hooves think horses, not Zerbras.

When diagnosing a charging problem always start at square #1. You need to know that the battery is good... not think it's good.

What is the static voltage of the battery? Should be 12.7 volts DC. If the static voltage of the battery is OK then take the battery to an auto parts store and have it load tested. If the battery passes a load test then it IS a good battery and you move on to square #2.

here is the model/type# from my mowers engine 31P777-0293E1-031120ZD.

lurker; battery voltage and load are within spec and are "ok"

Well, it looks like I'm getting goofier every day.
I laid out Walt's e-mail address as ending with "dot net",
it should have been "dot com".
My own e-mail address ends with "dot net" and I have had some problems with our e-mail service lately.
I guess I have typed or said "dot net" so many times the past few days it just came out by habit.
Sorry Walt.