AA, AAA and other, less standard, batteries put out a nominal voltage of 1.5 volts. The voltage in these batteries will slowly decrease throughout the batteries' lifespans. The batteries will continue working until the voltage reaches 1.1v, at which point the batteries are no longer useful and must be replaced. You can use a standard multimeter to test the voltage and determine how much life your batteries have in them.
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Set the multimeter to DC voltage mode. This mode may be represented by a portion of the center dial with VDC written on it, or by a single line with three dashed lines underneath it.
Set the multimeter to the voltage closest to the anticipated voltage of 1.5v while remaining in the DC setting. Typically, multimeter dials will have values increasing by powers of 10. For example, a multimeter might have 2v, 20v and 200v AC settings. In this case, we would choose 2v because it is closest to 1.5v.
Touch the multimeter's red lead to the battery's positive terminal. On AA and AAA batteries, this is the elevated bump on one end of the cylinder. Touch the multimeter's black lead to the battery's negative terminal. The negative terminal is on the opposite side of the cylinder from the positive terminal.
Hold both leads in place and look at the multimeter's display. The multimeter should read somewhere between 1.1v and 1.5v. A battery at 1.5v is almost fully charged. A battery at 1.1v is completely dead.