a single 9v battery can happily power 2 EMG pickups for a year, if not more.... depends if you are sharp enough to remember to unplug your guitar after playing.
there are two ways to use two 9v batteries wit EMGs: 1. wired in series, 2. wired in paralell. with in series wiring you get 18v and noticably more output. i don't suggest it for thosee with high gain amps simply because the power you get from EMGs with one battery (9v) is plenty in 99% of all situations. with paralell wiring of two batteries you get about double the battery life but still only 9v, so this is worthwhile if you are really really lazy and the thought of having to change batteries once a year is too much for you to bear
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you'll know when the batteries are dying because the pick-ups themselves will start to flash with a glowing red light and will emit a siren sound to warn you of impending core meltdown.
seriously though, you will know because they will start to sound weak.