ER26500 is the Lithium thionyl Chloride C Size  It is Primary Not Rechargeable Lithium battery. It has 26.5mm  length, 50mm diameter, cylindrical battery.

Lithium thionyl Chloride (Li/SOCl2) cells have the highest energy density of any lithium battery. They are not rechargeable, but have extremely long shelf-life, making them ideal for low power consumption devices that run for years. The ER series comes in three varieties, high capacity (high energy storage, suffix H or none), high current (high power, suffix M), and high temperature(suffix S) .
ER26500  is widely used as a source of power for wide variety purposes and  temperature environment such as AMR utility meters (Electricity meter, Gas meter, Water meter and Heat meter), Wireless alarm sensors, Remote monitor system, Memory backup, Military radio-communication, Night vision device, Tracking and positioning system, TPMS, Communication, Intelligent controlling IC etc.