Galaxy S5 :: Only Charging To 50% And Randomly Showing That Connected Via USB When Actually Not

Recently spent a long time outdoors in the rain, but phone was never wet. Got home and when I tried to charge it, it wouldn't work. Then when I finally got it to charge (left it overnight) it had only charged to 50% and would stop immediately after reaching 50%. Now the "USB connector connected" notification randomly shows up, even when the phone is not connected to anything. Several seconds later it says it's disconnected. Is the phone damaged, or is it my charger? Just bought another battery on amazon and an external charger in case it's the phone. I read somewhere to hard reset it but I have too much stuff to back-up. If nothing else works, I'll have to do it, but I'm saving that as a last resort.