Canon Powershot A540, batteries need replaced constantly. I am using 2 AA alkaline as instructed. As soon as I put them in it says batteries are low and shuts off. This is happening with all brands of new batteries. What's the problem? Occasionally I may get to take 1 or 2 pictures before this happens, usually none.

According to Canon, your camera needs servicing.

I have a Canon PowerShot A75 camera that had the same problem with the "Replace Battery" message appearing immediately even after I had put in brand new out of the package recommended batteries. After trying all the remedies I had read about, I bought and put the Eneloop hybrid NiMH Rechargeable batteries in my camera. There have been no problems since then even though I have taken many, many photos since putting in that type of battery. The AA alkaline batteries just don't seem to work even though they are recommended in the instructions that came with the camera. I see the date of your question is quite some time ago. I hope you have found a solution by now to your battery problem. If not, give this a try.