NOTE: The product previously used batteries, but has since been revamped to have a built-in lithium battery. There is no need to use additional batteries. The back cover of the battery has been sealed, please buyers do not pry it open to avoid affecting the experience, if you mind, please do not order. Thanks. Features: Multi-functional & Professional: Tens Electric Massager with 6 Modes, 19 Levels Intensity for Pain and Muscle Relief. TENS unit help relieve pain associated with sore and aching muscles in the shoulder, waist, back, neck, upper / lower extremities(arm/leg) due to strain from exercise, overwork or normal household work activities. Activate specific natural pain gate mechanism, relax your body anytime you feel the need with our professional grade TENS electrotherapy pulse massager and accessories. Updated hydrogel: Our TENS Unit Replacement Patches has been upgraded to the industry leading adhesive, which can provide much better self-stick performance and easy clean up for longer lifespan. It quite easy to find compatible electrodes (2mm pigtail tens unit pads) on the market for you to choose from. Powerful Battery: Package includes a USB cable (No Adapter) that can be connected to a wall charger, power bank or laptop USB port to charge. One charge provides 40 hours of useage, which is more environmentally friendly than AA batteries and lasts longer than other machines.