Battery Or Alternator Or Reg/rec?

If its showing 13.8v at idle (is that across the battery terminals?) and presumably over 14v when you rev it up, then it sounds like the alternator is working OK. Could it be your battery has seen better days?.

 

I always use a trickle charger for both bikes - the Honda has its HISS so pulls the battery down if not, the triumph has no reason (alarm etc) to need it technically, but the charger keeps it at its optimum. Left to its own devices it’ll turn but not catch after a week or so.

EDIT - meant to say after two weeks it’s dead as a doornail

 

I have my Street triple on a trickle charger most of the time. If I don't put it on the charger, especially in winter time, it goes flat in a few days and the starter motor won't spin the engine. Its on my to do list to change the battery, but at the moment whilst ever it fires up when I want to ride it I've got other stuff requiring my wallet's attention.