My AA batteries had been in my guitar for a while so before work I checked them with a Radio Shack battery checker I keep in my stuff. It has a green arc, small yellow arc, and a red area. This particular type battery tester will check various different types of batteries. Button, AA/AAA, 9V, etc. Last night my batteries checked at the low end of the green so I looked for the flashing orange light during and after each set. I found it. I replaced the batteries on break and ran them through the checker again. Just below the green. I have found my rule of thumb. Hope this helps others.