The AG3 battery is a 1.5 volt alkaline button cell battery commonly used in small consumer electronics.  AG3 is a common name and is identical to a LR41 battery.

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About the AG3 Battery
The AG prefix stands for the alkaline chemistry of the battery.  It has a 7.9mm diameter and  a 3.6mm height.  The nominal voltage is 1.5 volts.  Typical capacity is 35 mAh.

AG3 Specifications

Nominal Voltage1.5V
Dimensions7.6mm x 3.6mm
Weight0.7 grams
Capacity35 mAh

Equivalent Batteries
Several companies use other names that may be interchanged with AG3 batteries.
General common names: SG3, LR41, LR736, and SR736 are common compatible batteries.
Brand names:  Energizer and Duracell will use names of 192, 384, or 392.

How to Get Longer Life
For electronics where you want longer life, then substitute with a silver oxide battery instead of using the alkaline version.  SG3, and SR736 are such batteries with silver oxide chemistry and have a typical capacity of 45 mAh. The higher capacity equals to about 29% longer life.

Saving Money
Since button cell batteries will have the same capacity, a cheaper generic battery will perform just as well as a more expensive brand name battery.  Also, you can easily find AG3 batteries sold in packs of 10 cheaper than a 2 to 3 pack of the brand names.  They normally have a shelf life of 2 to 4 years.