With wireless turned off, battery life remains at two weeks.
Kindle uses wireless to receive books and other content.
"Battery power management for portable wireless devices is a complex technical area, and the battery life improvement announced today is the result of a six month firmware improvement and testing program," company officials said.
The improvement applies to the current 6in Kindle, and existing owners will automatically receive the new firmware via wireless.
Another firmware change provides a PDF reader, allowing the reader to display files in the popular format. This means such files can be transferred directly to a Kindle via USB.
Previously, they had to be emailed to the users kindle.com address for automatic conversion to the Kindle's native format and then either transferred to the device via wireless (charged at $US0.15/MB in the US, and a whopping $US0.99/MB elsewhere), or relayed to another email address so they could be downloaded to a computer and then copied to the Kindle.
Some earlier Kindle versions will also automatically receive the PDF update.
File types already supported for direct USB transfer are Kindle, text, unprotected Mobipocket, Audible, and MP3.
"Kindle, already the number one bestselling, most wished for, and most gifted product on all of Amazon.com, is now even better - with 85% more battery life and a built-in PDF reader," said Ian Freed, vice president, Amazon Kindle. "These two significant enhancements are available now."
Kindle costs $US259, and is available in a number of countries including the US, Australia, Canada and the UK.