Can I leave the batteries in my camera?
I recently bought a Canon 1000d DSLR. I'm just an amateur and don't use my camera daily. I use it once or twice a week. Is it okay if I leave the batteries in the camera during this idle time?
Leaving the batteries in your camera for a week or two between uses will be no problem. If you plan to leave your camera unused for six months to a year, you might want to take them out.
And, the lithium battery pack used in your camera (like most current dSLRs) should hold its charge on the shelf fairly well, so you won't have to worry too much about it being drained just sitting there.
(If you do need to shelve it for a long time, the battery will be best if stored at a little less than half full.)
A few references for further reading: Tektronix's Lithium Ion Battery Maintenance Guidelines and Ars Technica on What is the best way to use a Li-ion battery?.
Some rare cameras may have problems where the battery is drained by the camera even when the camera is off. You'll know if you have one of these by whether your batteries still work after two weeks. In that case, there's still unlikely to be any actual harm; battery life might be shortened somewhat, but not more than it would be by actual heavy use.
If it's just sitting in the house, I even leave the camera 'on' and let it go into standby on a timer. One less step since it wakes up as soon as I hit the shutter button. Been doing this for almost 3 years rotating between 2 batteries and haven't noticed any battery life issues.
+1 I agree, my K20D is showing full charge 3 months after I stopped using it in favour of my K-5.
One thing I don't like about answers to this question is that you give advice for 1-2 weeks (leave battery in) and for 26+ weeks (take battery out). What's the advice for the intervening period? At what point does the advice change? What if I leave the battery in an unused camera for 4 weeks? What about 10 weeks? Is that OK? What's the actual rationale for removing the battery in any case? Is the advice any different for a DSLR and for a Mirrorless camera? Without having analysed it, it seems to me that my Sony mirrorless camera somehow gradually drains the battery even when switched off.