When evaluating rechargeable fans, the battery capacity (mAh) and battery type (Lithium-ion vs. Lead-acid) are paramount. Prioritize Lithium-ion (Li-ion) batteries for longer lifecycles and faster charging. Ensure the motor is a Brushless DC (BLDC) motor, which offers higher energy efficiency, lower noise levels (typically <45dB), and a longer lifespan compared to brushed motors. Additionally, check the charging interface, where USB Type-C is now the industry standard for faster power delivery and universal compatibility.