MacBook Pro 2012 - Burning Smell

I have a MacBook Pro 2012 13 inch. I only use this to DJ with and I've had it for just over 5 years. It did go though a long period of time where I did not turn it on and as such it sat for long periods of time.

The issue I have with it is that there is a "burning" smell coming from it. My first thought of course was that it was caused by the battery. A couple of days ago I replaced it with a new one. I am still getting this burning smell. I can smell it from the left side of the keyboard.

I figured changing the battery would fix it. The old battery had no leaks or any damage on it so I am confused as to what the cause could be. Is it possible that there is damage to the mainboard that was caused by the MacBook sitting for long periods? And when I say long periods I would say up to 6 months at a time.

I really don't want to spent $3000 on a new MacBook right now. The MacBook runs perfectly fine. it just has this smell. And it seems to only occur when it's running on battery power. Not sure if that is significant or not.

Any thoughts or suggestions on what I should do?