Information is from O2-Cool.com:

With Smart Power™ technology, you’ll have *up to 75 hours of combined alkaline and rechargeable battery life. With four power sources and extra built-in features, you’ll never want to leave home without it.

• Smart Power™ Technology: Built-in rechargeable batteries (included), AC Adapter (included), Car Charger (not included) or uses 4 D batteries (not included)
• Variable-speed 5” (blade size) fan
• Pivoting fan cage
• Compact Design
• Low battery indicator lights
• Rechargeable batteries can charge up to 500 times, equivalent to using over 150 alkaline batteries
• Rechargeable batteries are energy-efficient and can be recycled
• * Up to 72 hour alkaline battery life (typical use on low) + up to 3 hour rechargeable battery life (typical use on low)
• Dimensions (L x W x H): 8.5 x 6.7 x 8.26
• Patents Pending

* Final battery life depends on the age and quality of batteries used. Battery life is determined using low speed under typical periodic on/off use.

 O2 COOL brand new Smart Power™ technology automatically chooses the least expensive and most efficient power source possible to operate your fan. The built-in rechargeable batteries can charge up to 500 times, which is equivalent to using over 150 alkaline batteries. With Smart Power™, you'll have *up to 75 hours of cooling no matter where you choose to go.
* Final battery life depends on product and the age and quality of batteries used. Battery life is determined using low speed under typical periodic on/off use.