A post on Apple's Discussion boards by Benjamin Weaver indicates that the new 12" G4 iBook battery (part #M9337) has delivered dramatically increased uptime for his older 600 MHz iBook.
"I only installed the new battery after a clean install of Panther 10.3.1 (and resetting the power manager). I was amazed by the results: After calibrating and using the iBook for a few days, I regularly get 5 hours of time on my usual settings (Energy saver set to Automatic, screen dimmed down halfway and Airport on). And 5 hours are also what I get out of it, even when using the screen at full brightness and doing some Photoshop work during that time. [...] (The old battery lasted about 2.5 hours)."
According to Apple's documentation, the 50 watt-hour battery in the 12.1-inch display G4 iBook model uses six lithium ion cells and provides up to 6 hours of operation, depending on configuration and usage.
The newer-model battery is compatible only with 12-inch iBook having two USB ports, is priced at US$129, and available from the Apple Store.
Older iBook batteries will have a part number M8956 or lower.