The battery is replaceable yourself but is a right bugger to do & usually results in the plastic clips breaking if you don't do it in the right order

The retaining screws are located under the rubber feet on the bottom & then you have to flip the unit over & try to unclip the top plate from the back cover.

9 times out of 10 though it is usually the metal contacts on the remote or cradle that cause charging issues, if you clean these with a bit of alcohol & also make sure the pins on the cradle are sitting proud you might find it will solve the issue of it not fully charging