Bose QuietComfort headphones use a computer chip inside the headphones to identify and cancel out background noise. In order for this chip and the noise-canceling hardware to work, a small amount of electricity is required, and Bose provides this in a removable battery located inside one of the headphones. The QuietComfort 3 has a custom-built battery that you must recharge, while the QuietComfort 15 uses an AAA battery that lasts about 35 hours before it needs to be replaced.
Bose QuietComfort 3
Press the power switch to "off" on the right headphone. The right side will have a small "R" near the headphone on the headband.
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Rotate the right headphone so that the headband connector is away from the top of the headphone. Grip the very top of the headphone with your fingers and pull straight up. The battery will slide out.
Insert a new battery, or replace the same one after it has charged, by sliding it straight down into the battery cavity.
Bose QuietComfort 15
Hold the Quiet Comfort 15 in front of you, so that the right earcup is to your right and the left earcup is to your left.
Rotate the right earcup in a little, so that the top of the earcup is away from the headband support structure.
Pry open the small plastic flap located on the front edge of the ear cup, just above where the headband connects to the earcup. Tilt the headphone over and the battery will fall out of the compartment that was covered by the plastic flap.
Insert a new AAA battery into the compartment with the positive terminal facing down. Close the flap to secure the battery.