A cycler is a device that discharges batteries at a rate simulating an operational load on the pack. When the battery voltage drops to a (generally user selectable) minimum voltage, it stops the discharge and starts charging the batteries. The discharge function is coupled with a timer so you know how long the batteries would last under operational conditions.

Here is a link to one, there are many on the market.
http://www3.towerhobbies.com/cgi-bin...?&I=LXMAH9&P=0