A Dragino sensor can run for years on one battery pack, but the real life is not found by reading a simple percentage on the device. The ER26500 lithium-thionyl chloride cell has a flat discharge curve, the radio draws short high-current bursts, and temperature can change what the battery can deliver. That is why battery life needs a calculation, not a guess. This guide explains how the ER26500 + SPC1520 pack works, how to estimate run time, when to replace the pack, and why