crapula

Hi I have a 2016 CCM GP 450 Adventure with about 10000 miles on it. It has recently had a factory service. Last week I was on the motorway at about 70mph. I started losing power and speed dropped to 55mph then the engine died completely (didn't notice a lock up). I pulled in the clutch and coasted to a stop. Tried to start it and nothing much happened maybe a weak click or clunk. I guessed it was battery not charging properly and got recovery home. I checked the battery and it was a bit low around 12v. I then tried to start with a fully charged car battery and jump leads and there was just a clunk and nothing else. All lights etc are still working. I'm sort of thinking it's something catastrophic, what are things to check and what order. TIA