Hacks, Mods, and Parts for your favorite Robot
Presented here is a small test jig that can be used to perform a true load test on your Roomba or Scooba battery.  The components used can be obtained at most electrical parts stores as well as some RV camper stores.

It is important to utilize a small digital volt meter to monitor the overall voltage level and prevent the battery from being discharged below 12 volts dc.

An 1800 mAH to 2100 mAH battery should be able to turn on the lamp and keep it lit for at least an hour.  The higher mAH batteries will run the lamp for longer periods depending on the rated capacity.  The 12 volt dc 25 watt light bulb used in this load test will simulate a 2 amp load which is very similar to what a Roomba or Scooba would utilize during a cleaning mission.