left airpod running out of battery quickly

hello! im usually not one to do this type of thing, but i have an issue. a close friend gifted me their gen 1 airpods since they didnt use it anymore, which means i dont know how old they are. so, whenever i use them, the left airpod keeps running out of battery after two or one and a half song, even if its "fully charged". the right one works perfectly fine, and so does the case. why does this happen? and is there any way to fix it?