The Gymboss Charge is our most durable timer and was designed to be used under the most demanding conditions. While it is the same size and offers all the functions as our Classic Gymboss, it is also weatherproof allowing for use in heavy continuous rain as well as extreme sweat and humid conditions. A backlit LCD makes it easy to see in any light exposure. It also offers the loudest beep alarm of any of our timers. Finally, the Charge has a built in rechargeable battery that will last for months on a single charge. You can time one or two intervals and repeat them 1-99 times, allowing you to time both your work and rest periods.