It's stated on iRobot USA support FAQ that Scooba should not be left charging without use for 2 wks.
http://homesupport.irobot.com/app/answers/detail/a_id/154
Pre-OSMO firmware (around 2006) of Scooba will overcharge battery within 5-6 hrs, ie. 4 hrs of eating & 1-2 hrs of force-feeding.
Google "Foie gras" if u can't imagine what force-feeding does.

Current Scooba firmware won't overcharge so easily, but >2 wks of small bites without exercise is still considered over-eating.
So... disconnect battery after full charge, then charge a bit just before use to top-up any loss of stored chemical energy.
All rechargeable batteries exhibit self-discharge, except those Ultra-low self-discharge NiMH like Eneloop, Recyko & Hybrio. They got specially engineered microscopic membranes to prevent energy loss thru crystalization.
Analogy: Milo powder in an air-tight container vs a lousy leaky container.