I had a little fight with my Canon Powershot A570is a while ago. It seems to like electricity more than average cameras do. It has some sort of typical problem, but what can you do, second hand device, no garantee.
So, same thing as you. I use CHDK and timelapse. Power is gone before you get even 10 pictures, if batteries or rechargeable NiMH's are in use. I made a little LM317 based circuit and I use 6V 4,5Ah sealed acid battery. LM317 creates suitable amount of temperature, and in cold weather it is even a good thing, when it is in the same enclosure with the battery, under it.
LM317 is simple, but it is not most efficient way. What camera, and how much current it takes if/when display is blank? My camera takes between 200-450mA, depending of the state of the display.
Cheers,
Kari