A 42 nJ/Conversion On-Demand State-of-Charge Indicator for Miniature IoT Li-Ion Batteries

An energy-efficient state-of-charge (SOC) indication algorithm and integrated system for low-power wireless sensor nodes with the miniature Internet of Things (IoT) batteries are introduced in this paper. Based on the key findings that the miniature Li-ion batteries exhibit a fast response to the battery current transient, we propose an instantaneous linear extrapolation (ILE) algorithm and circuit system based on the ILE algorithm allowing accurate on-demand estimation of SOC. Due to the on-demand operation, an always-ON current integration is avoided, reducing power and energy consumption by several orders of magnitude. Furthermore, the proposed SOC indicator does not require a battery disconnection from the load, ensuring continuous operation of the applications. The system is implemented in a 180-nm CMOS technology. The power consumption is 42 nW, and maximum SOC indication error is 1.7%. The minimum applicable battery capacity is as low as $2~\mu $ Ah.