ESP32 with 18650 battery
A single 18650 cell can power ESP32 projects for a long time if the firmware spends most of its time in deep sleep. Runtime depends on average current, regulator efficiency and real battery capacity.
Why capacity is not enough
A 2500 mAh cell does not automatically mean long runtime. WiFi transmit current, wake duration and the regulator quiescent current can dominate total power use.
Good design approach
Use deep sleep, wake briefly, transmit data in batches and choose a regulator that has low quiescent current. Measure the real board current instead of relying only on datasheets.
Calculate your real values
Use the related ESP32 calculator to test your battery, regulator or brownout numbers.
Open calculator