I've got more questions than answers for you, but cannot give help until I can see what you are doing. First, as stated, we need engine numbers. They are located on the valve cover, please give us all numbers you see. Second, how are you testing the voltage regulator? Third, have you tested stator output, if so how? Fourth, have you measured battery voltage while the engine is running? If so what were your readings? From what you have told us 2 & 2 do not add up to 4, we are not getting an accurate picture. Been solving electrical problems for years and know of no design flaws or weak charging systems on Briggs engines. Over all they are very reliable. After you reply to these questions we should be able to point you in the right direction.
I love Briggs, I've torn em down and rebuilt em and they're great engines for sure. I was in a hurry so I gave a half-assed reply, that's my bad. Alright here we go:
Model: 407777
Type: 2217G5
Code: 120423YG
Every test I've done was to the repair manual specification and how they wanted me to test it. Have not load-tested the battery yet but I do get 12.5 volts across the terminals every time I've checked. But I don't know if it can carry a charge properly (yet).
These tests were done after each time I had a flat battery problem and with the engine running at full throttle (haven't tested everything today as I put it on a charger and had to be out the door, but tests were the same each previous (three) times this issue came up):
-Stator output was a solid 26+ volts DC. That part is fine.
-Regulator/rectifier showed no output (tested without a shunt, because I do not have one. However a new regulator/rectifier showed at least some output as opposed to getting nothing from the fried one when I tested it. Could mean nothing though?)
-Battery voltage with engine running was slowly dropping down to at least 11.24 volts. After replacing regulator/rectifier, I only got a slow climb of voltage across the terminals to 13.5 (is the slow build normal or should I see an instant 13.5 with the engine running?)
-I have continuity across the board where it needs to be. (Regulator red lead to battery, battery ground to frame etc.)
-Ground leads to the frame were checked and cleaned, everything was screwed tightly to battery etc. I'm not sure about the keyswitch/seat killswitch/PTO killswitch/Reverse killswitch/clutch killswitch because usually it's snug tight and I have no way of knowing if it wasn't connected properly or how I'd even check that.
Thank you for the replies, gentleman.