I have an S2. It is the lens cap that acts as the o/off switch. Batteries seem to last a long time, and I don't think it's sensitive to voltage variations too much. I'm using an alkaline battery in mine and it's woking well now for more than a year. I don't use it much and the lens cap stays on the lens when I'm not using it, so I can't say definitively that when the battery voltage starts to drop that it will continue to be accurate. No matter really though. I'd rather take an a few incident readings around the area where I'm shooting, make a mental note of the readings, and set the camera accordingly. As long as the light doesn't change, I'm set.