I've read somewhere that mirror is held up by some electromagnetic mechanism, and hence will drain the battery quickly when using long exposure. Is that true?
It was also suggested to press shutter and take the battery out as I recall.
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I don't have personal experience with the EOS 5 - but Bob does and his report is that your max is 6 hours before the battery runs out and the shutter closes.
You'll want to watch the temperature as well. If you're shooting skies at night in really cold temps - expect less battery performance.