[Popeye]

You should get about 14.6v with engine running.
It is possible that you have a bad earth connection but.......

 

[Popeye]

You can measure it at the battery. No need for any contortions

 

[Popeye]

Dirk, I had a problem with my battery constantly dropping voltage (below 12v).
I discovered I had what's called " A parasitic battery drain ". (look it up)
With your drain of 500ma. I think you may have the same.
Mine was a fault in the dash-cam connection. Much cheaper and easier to fix than a new alternator.
I hope you are as lucky as I was.

Paul.

PS I should mention that 5 months after that I had bigger problems - NO electrics.
I had to have a new BCM --Body control module fitted. Wonder if they were connected ?

 

[Popeye]

" I was really upset that I couldn't start the engine . It feels like you didn't cross the Finnish line."

Just think how good you will feel when you win

Paul.

 

[Popeye]

I hate to state the obvious as I'm sure you are doing this ..BUT..
You are pressing the brake pedal when pushing the start button ?

Paul.