This sounds like a battery issue. It gets swollen and doesn’t store power anymore. Mine was turning off every 15 min, unless I was doing max volume and switching off immediately. So I found a replacement battery and thanks to this tutorial JBL Xtreme Akku Replacement high power , I changed it in no time. Problem solved! My advice, after you get the new battery, is to disconnect the charger as soon is you get it fully charged, every time.