Could be a bad cell. Or just low cold cranking amps. Or a short elsewhere draining the charge.
But before you buy a new battery, a decent battery shop should be willing to test your old battery.
Also, if you end up needing a new battery, take the old battery to the store when you get it tested (i.e. don't take it in another vehicle...).
That way you will be able to get a prompt "core charge" if a new battery is required.
This would avoid the hassle of going back home to get the old battery etc etc.
Also, getting rid of an old battery can be difficult.
Last edited by Simon.Starkie; 06-28-2019 at 09:35 PM.