NewerTech offers a replacement battery for 1st and 2nd generation iPods that can extend the battery-life to 21 hours.
Accelerate your Macintosh has a short review of the NewerTech iPod battery with battery life time test. They achieved a run-time of 21 hours and 3 min. Opening the iPod and replacing the battery takes 5 min. The NewerTech 2100mAh battery costs $39.99.
More details on xlr8yourmac. (thanks Russell)
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