Most acoustic/electric guitars have an onboard preamp that allows you to control Volume and EQ directly from the guitar. Most of these pre-amps are powered by a 9V battery.

If your guitar pickup has a preamp that is powered by a 9V battery, be sure to unplug the patch cable when not in use. 

When you plug in the patch cable it turns on your pre-amp, if the cable is left in the guitar overnight, it WILL drain your battery.