You may need to act quickly. You say in your feedback that the battery failed after three weeks. To use eBay's money back guarantee you must open a case thrpugh the resolution centre within 30 days after the actual (or latest estimated) delivery date.
By the way, PLEASE remember that feedback isn't just for complaints. You're fully entitled to leave negative and neutral feedback, but do think to support and thank good sellers by leaving them positive feedback when deserved.
