You can buy only a replacement mug (without coaster/charger). Also, get in contact with customer support mentioning the battery, they usually offer a discount code.
I had to deal twice with them: coaster stopped working 2 years ago and more recently paint inside the cup came out. Both time, they replaced the broken part at no cost. Probably the first time in my life I'm using and happily recommending customer support of any kind.
