1996 Other Mitsubishi Models Battery not holding charge

Hi, the car in question is a 2800 turbo diesel Shogun (Pajero). I've been having serious problems with the car not starting. I charged the battery overnight and the it started straight away afterwards. When I tried it again 5 mins later the battery was flat again. The battery is at least 6 years old and doesn't look too happy. White deposits etc. I was wondering if the problem was simply to change the battery or would filling it up with distilled water do the trick. I'm worried that it may be the alternator (ie more expensive problem) but that was changed a few years ago. With a booster (wireless charger pack) I can get enough power into the battery to start the car while rolling down hill but this is obviously not ideal. And dangerous. Whatever advice you can give me would be much appreciated. Thanks.