Edge Explore Battery Life

Hi can anyone help. Hubby and I both have an Edge Explore and both are about 18 months old.  At first battery life wasn't bad but now both units only hold for a 2 hour ride

before going into "Save battery mode".

I sent mine away for a new battery but they couldn't source a new battery and returned the unit. Surely we don't have to buy new one's already? Can anyone shed some

light on why the battery life is so poor.

I'd be grateful for some help. Thanks