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Long exposures pose no threat to the longevity of Canon's CMOS sensors in EOS Digital SLRs, but they will run the camera's battery down. If the battery voltage falls below a prescribed limit, the camera will shut off and any image data in the buffer memory will be lost. I've commented on this topic several times in previous editions of Tech Tips, most recently in November 2008. Here is a relevant excerpt:
"Realistically, maximum bulb exposure time for an EOS Digital SLR is going to depend on the type of power supply and to some extent the ambient temperature conditions. By far the longest exposures are enabled when using the optional DC Couplers and AC Adapter Kits, since these accessories eliminate concerns about battery life. If you're doing astrophotography from your backyard or any other location where AC power is accessible, this is probably the most feasible solution. When AC power is not accessible, the next best solution is to use an optional battery grip and load it with two lithium-ion rechargeable battery packs. The battery grips and battery packs will vary according to the camera model involved, but in moderate temperatures it would be reasonable to expect no more than approximately 4 to 6 hours of battery life for time exposures with a two-battery power supply. In situations where only one battery pack is available, the total bulb exposure time in moderate temperatures will be somewhere in the range of 2 to 3 hours. Cooler temperatures down to the cameras' minimum rated operational range of 32 degrees Fahrenheit will reduce battery life somewhat. In answer to your second question, any image data in the camera's buffer memory will be lost if the camera loses power before the file is written to the memory card. Since long exposure noise reduction in the camera can last as long as the actual exposure, you'll need to factor that into your decision on setting the length of time exposures."