With just the internal battery, our tester Pete Cottell managed to get over two hours of heat, which should keep a 16-ounce coffee at a toasty 135 degrees Fahrenheit for at least your morning commute. If you routinely drive even longer distances, there's a separately sold car charger that will use the built-in 12V port to keep your liquids heated for as long as you're stuck in the driver's seat.