T100 2017 Battery

I've had my T100 new since august 2017, and its always lived outside and I've always done short journeys- now over the last few weeks battery keeps on going flat. Recharged battery on weds with optimate charger and measured voltage across terminals with voltmeter ( 13.7), bike started fine, drove one journey of 3 about miles and another of around 2. Measured voltage today ( sunday) and its down to 12.6 and bike wont start ( almost did ! )…
Should it lose its charge after two starts and 3 days resting ?
Battery obviously isn't totally knackered as it does take charge from optimate.
Any ideas folks-how do I check if the bike is charging the battery




P.S no extra electrical gubbins fitted like heated grips- just bog standard factory spec

 

my optimate is supposed to indicate if its sulphated and go into recovery mode. It just shows " charging " and eventually after 24 hours goes to steady green light for "charged". Thanks for advice about what voltage- will give that a go tomorrow. I know I had a similar problem on a classic mini a few years back and turned out to be a bad earth ,,,,!

 

ended up getting new battery, 5 days of optimate charging and the thing still wouldn't start.
Anyway new battery in today and it showing around 14+ v at 3000 rpm, so hopefully just battery buggered..