Dies and will not restart

I'm assuming the battery or voltage regulator, but just to check with people who know a lot more than I do...

1 year old xt1. Heard a very loud POP while mowing, and a couple minutes later it started acting as if it was going to die. I shut it down and tried to restart it, nothing. Lights on, battery at 11.8. Shop told me to jump it. Took a minute, but it started. Ran it for a few minutes, shut it down and tried to restart it and nothing. Jumped it again and decided to try mowing again. 30 seconds, started to die. Turned off the PTO (lever style) and it ran fine for several minutes until I parked it. Next day, battery showed only 8.9. The loud POP throws me off. Obviously not holding a charge, so still battery first? Thank you.