When evaluating rechargeable fans, the battery capacity (mAh) and battery type (Lithium-ion vs. Lead-acid) are paramount. Lithium-ion batteries are preferred for their longer lifespan and lighter weight. You should also verify the continuous working time at different speed settings (e.g., 5-10 hours on low) and the charging time (ideally under 6 hours). Additionally, check the motor type; Brushless DC (BLDC) motors are highly recommended as they are more energy-efficient, quieter, and have a longer service life compared to traditional motors.