what if the xda's battery gets empty??

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Regular mobile devices keeps all your information (contacts etc.) if the battery gets empty. I noticed that XDA doesn't that this way. It will loose a lot of info.

Anyone knows which info will be disappear?

everything ;)) it will look like if you hard-resetted the device.
I don't have experience with this on XDA/MDA, but on iPAQ if the main battery goes dry, you have a backup battery in place. Alas, it is not meant to keep your data for long, 2 days at most I guess.
don't know whether XDA has one though.
If you have important information you alway want to keep, buy a MMC card. they are cheap enough now. I also use the flash backup function so at least my contacts always stay in place :)