Why would a laptop battery for an ASUS Vivobook seem to randomly lose charge at a rate of a percent every few seconds, then go days without a problem?

It usually occurs if the charge is already below about thirty percent. I've had this problem several times in the last month and occasionally at other times in the last year, where for some bizarre reason it loses power at this rate, but it will charge up just fine, and once it is charged, it usually seems to be just fine for several days without having this problem. It does not appear to be correlated with anything with what I actually do with the laptop. The laptop was made about two and a half years ago or so.