Sounds like you need a new battery....but first I would remove the battery and check the connections for corrosion or rust. If that fails, head for the local battery shop.
Newer style Lithium-ion batteries don't tend to die slowly like the old style batteries, losing the amount of time they would hold a charge over a period of weeks or months. They do in many instances tend to work right up to the end of their life with maybe only a slightly noticeable loss of charging capacity and then suddenly, they quit completely refusing to take a charge at all.