Amazon.com: HXJNLDC DC 3.7V 3000mAh 103665 Rechargeable Lithium Polymer Replacement Battery for DIY 3.7-5V Electronic Product, Mobile Energy Storage Power Supply : Electronics
3000mAh iPod 4th gen battery test results - I got 40 hours of playback on a single charge! More info in the comments. : r/ipod