Testing the charging system is extremely simple and can be done with the most basic of knowledge and equipment. This is what I would be doing first, if all's well then look toward the battery, it may be it's incapable of holding a charge.

Also check the electrolyte if you haven't done so, you would be amazed at the amount of people who neglect battery's and think they are maintenance free, when it's one of the most important components!

Martin