I am always looking to charge my Epic 4G Phone.
I can't wait until they build phones that offer more power.
For now I picked up a second battery on amazon that didn't break the bank.
Here is the link: Amazon.com: High Capacity Lithium-Ion Battery (EB575152VA) for Samsung Galaxy S (Samsung Captivate i897, Vibrant T959, Epic 4G, and Fascinate): Electronics
The battery works as well as the original and recommend buying one.
Also pick up a clip charger they can charge the battery outside the phone.