Battery life

I've had my 735XT for about 4 years now and have loved it, however in the last few weeks the battery life has drained significantly. to the point I can't do a 1 hr run with the watch on full charge.

I understand that batteries don't last forever, however this is such a dramatic drop in a short space. I'm just getting going in my marathon training, i dont need to be worrying about the battery on training runs. 

I'd love to get this fixed as I feel the prices have gone up so much since i bought this.