The three constant heat power settings, and the fourth Power Boost setting, can be selected using the Power button on the battery packs or very conveniently with the HOTRONIC Heat app via a Bluetooth-enabled smartphone 6000 mAh capacity offers a category-best 24 hours of constant heating power High-performance Lithium-Ion battery with USB recharging cable Cambrelle covers for easy integration with all types of insoles, whether preshaped or customized The HOTRONIC Heat app provides an overview of the current charge status and heating levels. The app is available free of charge from the App Store and Google Play Store Hotronic XLP 2C BT Custom Foot Warmers | Christy Sports