The battery life of the Vuzix M400/M4000 is 2 to 12 hours, depending on the external battery choice. The Vuzix M400/M4000 has a head-worn 750mAh USB-C external battery. This means that when you are wearing the glasses one half is the battery and the other half is the viewer. The Vuzix M400/M4000 also has an internal battery of 135mAh that provides 5 minutes to hot swap external batteries. You can also power the Vuzix M400/M4000 with 3rd-party USB battery packs that you can wear as a collar around your neck.