The use of microbial electrochemical sensors, with electroactive biofilms (EABs) as sensing elements, is a promising strategy to timely measure the biochemical oxygen demand (BOD) of wastewater. However, accumulation of Coulombic yield over a complete degradation cycle is time-consuming. Therefore, understanding the correlation between current output and EAB metabolism is urgently needed. Here, we recognized a tail stage (TS) on a current-time curve according to current increase rate─a period with the least electron harvesting efficiency. EAB adopted a series of metabolic compensation strategies, including slow metabolism of residual BOD, suspended growth, reduced cell activity, and consumption of carbon storage polymers, to cope with substrate deficiency in TS. The supplementary electrons provided by the decomposition of glycogen and fatty acid polymers increased the Coulombic efficiencies of TS to >100%. The tail current produced by spontaneous metabolic compensation showed a trend of convergent exponential decay, independent of BOD concentration. Therefore, we proposed the TS prediction model (TSPM) to predict Coulombic yield, which shortened BOD measurement time by 96% (to ∼0.5 h) with deviation <4 mg/L when using real domestic wastewater. Our findings on current output in TS give insights into bacterial substrate storage and consumption, as well as regulation in substrate-deficient environment, and provide a basis for developing BOD sensors.