iRobot officially recommends as a charging technique for your Scooba :
"Fully charge Scooba’s battery before each cleaning cycle. When fully charged, Scooba’s battery will last longer than a full Clean cycle. If you would like to run Scooba more frequently, you can purchase new batteries at www.irobotstore.com”
What I used to do was to continue cleaning successfully without charging for a complete 2.5 full tanks sessions! I pushed Scooba's battery to it's limits (similar to Roomba's MAX mode), for me it did not make sense to recharge Scooba after a single full tank cleaning session as iRobot recommends. Flash forward aprox. 5 months and I witnessed the results for not following iRobot's recommendation. The battery charge barely held for 2 full tanks, in other words the charge is lessened! The Scooba battery suffered, somehow by my charging techique, premature aging/damage.
This was confirmed by iRobot's customer support on this Email:
"The Scooba’s battery is designed to last for hundreds of charging cycles with proper care and maintenance. Although you may have been able to run your Scooba through more than one cleaning cycle before charging the battery, this is not recommended. You need to charge your battery in between every 35-45 minute cleaning cycle. If you do not, your battery will not last as long as it is designed to and it will damage your battery. "
So please follow iRobot's recommendation on charging your Scooba's battery in order to prolong it's life and efficiency.