It's not your battery !!!!
I and a lot of others have had this same problem. I can start mine up, ride for 15 or 20 miles, let it sit for an hour and it won't start. Won't start as in won't turn over. Just a click and nothing.
This problem has been discussed on several internet sites, and to my knowledge, no solution has been found. Several years ago some Australians were thinking about a class action against Triumph because this problem was so widespread. Don't know what became of this.
Triumph knows about the problem, even if your dealer doesn't. Several people have achieved a fix (maybe not permanent) by replacing the cdi box. Some claim it's the crank position pick up. ( Inside right engine cover) Replacing the regulator/rectifier is another solution thats been tried.
I have quit riding my Scrambler because I'm never sure if it will get me home. I've been stranded four times. Tell me how a battery has less voltage after being ridden than it did before???
I hope you can get some response out of your dealer because, as much as I love my Scrambler, I can't own a bike I can't trust to ride.