Powered on during screen change

I was replacing the screen on my iphone 6s, and accidentally turned it on while re-connecting some of the cables (front camera, screen and speaker). It smouldered a little which is what made me realize. Turned it off again, finished the install, and..

Turned it back on. The apple logo appeared before turning black. I tried plugging it in to charge, and turning it on a few times but it just turned on by itself later for a little while. I was able to surf the web and access my stuff, so thought good, and turned it back off to set it back to charging. The battery read 35%.

I later tried to turn it on again and once it didn't make it past the apple logo. Then later once more, and this time it fully booted, showed 24% battery. I noticed that after plugging in the charging cable, it would show it's charging and then stop. Tried it a few times, and the same thing happened. Tested the cable and it works. The charge port looks debris-free, and didn't have a problem with it before the "repair," so I don't think it's the issue.

Do you think I shorted the battery? I don't own a voltmeter, and don't know exactly how to tell. I've replaced the battery before so if that's the issue, it might be worth doing. Otherwise, not sure it's worth taking to a repair shop given the phone is only worth $100 new these days.

Thoughts? Thank you