I have three Canon Powershot A540 cameras. All three behave the same, so this has to be a characteristic of the camera and not a defect in one camera. Please post your experiences with the A540 battery life to see if we all are getting the same results.
If I put in a fresh set of Duracell or Energizer Alkaline batteries, I will get close to the 90 pictures claimed by Canon for flash shots with the LCD on. But, if I use Energizer Ni-MH 2500 Rechargables (which have a lower initial voltage than alkalines) the life is shorter. Sometimes the low battery indicator will come on after just a few shots.
It acts as if the low battery indicator is set for too high a voltage. If I take a set of Ni-MH batteries that show low in the A540 and put them in my old Canon A40, the A40 will run for days, taking over 100 shots. In fact, I could put a charged set of Ni-MH batteries in my A40 and come back a month later and they would still work fine, but in the A540 the normal shelf discharge would drop the battery voltage too low to use.
I would expect that the A540, with its larger LCD would take a lot more current than the A40, but even so the shutoff voltage is set much higher. Some devices have a setup that allows you to set the cutoff voltage based on the battery type to allow for rechargables, but I have not found anything in the manual or by going through the menus.
I haven't tried Lithium batteries yet. Just got a set. With their higher voltage they may work better. However, Canon recommends against lithium batteries and they are not rechargable and cost a lot more than alkalines.
So, what has been your experience with the A540 and batteries? List the brand and type of battery (include the milliamp rating if it is a rechargable) and how many shots you typically get on a set of batteries. List whether these shots are mostly with the LCD on or off, and with or without a flash.
I would like have the details, but if you just want to make a quick response, use the poll which is set for Rechargable Batteries, LCD on and Mostly Flash pictures.