Hi everyone. I'm wondering if anyone here has experience shooting bulb exposures with the Canon Digital Rebel XT (350D), or other DSLR's. The question is: Assuming full charge, how long does a single Canon battery (i.e. no battery grip) typically last through an exposure? I'm planning to try a little astrophotography (star trails to be precise), so the exposures will probably be longer than half an hour.

Thanks!