You might have heard that 'insert vintage pedal name here' sounds so much better when run off batteries than a power supply unit and that may be correct! 
But you don't really want to:
a) Have to keep buying batteries each time one runs so low the pedal just sounds rubbish/doesn't work.
b) Have to keep unplugging the pedal which is awkwardly placed in the middle of your neat and tidy pedalboard in order to save battery life.
c) Be worrying during a gig that your pedal is just gonna run out of juice and leave you with nothing.
 
The dying battery simulator is a voltage sag for your pedal(s).
Just plug in your power supply and adjust the amount of volts you would like to send to your pedals. This solves all the issues mentioned above whilst giving you freedom to find new tones you probably didn't know your pedal could achieve.
The on board LED also helps to show what stage you are at by fading as it is lowered.
 
Requires a power supply and a power patch lead to work. (not included)
 
Works with 18v and below, but only use a maximum power that your pedal can handle!
 
Size:
59mmx57mm