External power supplies feature greater capacity as compared to regular batteries which increases operation time of digital NV units and thermal imaging scopes several times. 
The EPS3 (2.4Ah) has a rigid plastic case and can be installed on any devices outfitted with a Weaver rail or ¼ tripod mount. The EPS3 unit is supplied with an extension cable. That allows to place it under clothes while using NV or thermal device in winter conditions as well as to increase operating time.