The Ember Mug was designed to bring a hot beverage to a resting set temperature and maintain that set temperature for a period of time. This is best achieved by pouring in freshly brewed coffee or hot tea. For the best Ember experience, this is how we always recommend using your mug.
When you pour in a beverage hotter than your desired temperature, Ember’s battery won’t be needed until your beverage reaches that set temperature. Once reached, the heaters in the Ember will turn on to maintain that temperature. This also allows you to get the best battery life possible while using your mug off the coaster.
Pouring in a colder beverage may result in the mug not heating or reading as empty. More information on that here.