This paper presents a new multi-mode multi-input–multi-output (MIMO) converter with a battery for operating an energy harvesting (EH) system for gas sensors. In this EH system, the MIMO converter has a single DC–DC structure with multiple inputs and outputs. The converter is designed and tuned to harvest ambient light energy from a photovoltaic module, and it then provides regulated power at biases of 3.3 and 5 V to charge the battery and/or to drive a gas sensor. The battery serves as a load to be charged by the MIMO or as a power source when the ambient light energy is insufficient to drive the gas sensor. For regulating the DC–DC converter, maximum power point tracking is implemented for each power path from inputs and outputs. The proposed MIMO converter has the least number of switches among existing converters. Experimental results reveal that the MIMO converter exhibits a high efficiency of 89.73% for a large power throughput of 0.9 W for a gas sensor.