A 18650 lithium ion battery can appear in Bluetooth speakers, solar lights, smart locks, GPS trackers, POS machines, and IoT devices, but the same cell format behaves differently once it is placed inside each finished device.
The difference between 3500mAh, 3800mAh, and 4000mAh 18650 batteries is not just runtime; it also changes how you read current, energy density, and device fit.
A 3.7V 850mAh lithium polymer battery specification becomes clearer when voltage, capacity, cutoff limits, charging method, and temperature are read together.