check the acid in the battery, there are some tools for it, and those are very cheap, if it isn't good enough, or the amount of the acid is too low, then you can fill it up with "distillated" water, like your radiator without antifreeze stuffs, then you can charge it up with a charger
but if this things are ok, then check your generator, maybe it's broken, my car(29 year old vw
) has the same problems too, it sucks the battery in one week if i don't use the car, so i pick off the cables from it if i don't use it