The fuel gauge has different operating modes: POR, INITIALIZATION, NORMAL, CONFIG UPDATE, and SLEEP. Upon powering up from OFF or SHUTDOWN, a power-on reset (POR) occurs and the fuel gauge begins INITIALIZATION. In NORMAL mode, the fuel gauge is fully powered and can execute any allowable task. Configuration data in RAM can be updated by the host using the CONFIG UPDATE mode. In SLEEP mode, the fuel gauge turns off the high-frequency oscillator clock to enter a reduced-power state, periodically taking measurements and performing calculations.

The fuel gauge passes automatically between these modes, depending upon the occurrence of specific events, though a system processor can initiate some of these modes directly. For more details, see the BQ27426 Technical Reference Manual.