I am sure that you need to install a rechargable 9V. Ipower USA sells a lithium 9V which has 500ma of power (about 2X that of a stock 9V) The Chumby is sending power down to the 9V so it is designed for a rechargable cell.

I am amazed that a chumby will stay on if the power is removed with a 9 v inplace.
I have installed a lithium 9 volt battery. I have no idea if this is a good thing or not.
I should time the chumby to see how long it stays on if unplugged. The Ipower 9V I am using holds 500 ma and is ths longest lasting 9 Volt battery I know of.
The voltage present on the 9 Volt clip is the same as on the 9Volt charger. 11.5 volts. This voltage may change when a battery is installed (resistance).
I suspect the Chumby would run MUCH longer in "night" mode. I also suspect that the chumby will stay on much longer if the brightness is turned down.
IMHO the system should be smart enough to detect the drop in voltage and automatically switch to "night mode or lower the back light brightness in order to survive a longer power outage.
Guys remember  the Chumby is designed to have a RECHARGABLE battery installed NOT an ALKAINE Cell. that way the battery recovers to save the day when there is another power outage. Some times they happen in the middle of the night and you would never know to switch the battery.
I suspect the results will be just fine if one of these is used. They are.
Yaesumofo