why does my phone have a burning smell and doenst turn on

so i was on my samsung galaxy alpha doing random stuff and all of a sudden the screen goes black and a burning smell came out. the phone going black was normal to me but not turning on later was new. like 1 month ago this all started, my phones battery would say its at 70% and it would die immediately and would after say 0%, if id remove the battery and put it back it would say 50% and then it would charge to 80% and stop. someone help since i dont want to buy a new phone because im saving up.

i left the phone without the battery for 12 hours and still nothing i seriously need help with this. anything is helpfull

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