In 100ma charge rate (when connected to a computer and low battery), it powers down the SDRAM and only uses the smaller RAM in the SoC. You can read about it in the u-boot source code. It functions mostly as a battery charger in that mode. My experience with monitoring the serial port during charging is that fastboot mode seems to charge fastest, and periodically outputs the battery voltage on the serial port.
I really LIKE a built in solar cell, if we can eliminate the extra "solar oven" feature that cooks the kindle while charging.
