Do not leave your robot uncharged, ever.
Lately, whenever I left town, I'd unplug the Roomba's charging unit. When you do this the battery eventually drains, and then it sits uncharged until you plug it back in again, right?
When you do this you destroy the battery and it won't ever hold a charge again.
iRobot tells you this, (here and here) and woe unto you who ignore (or forget) this advice. I can show you the receipt for my $50 replacement battery if you want proof. (I tried resetting the Roomba's power manager to no avail, if you find yourself in this situation, give it a try before replacing the battery.)
If you won't be using your Robot for a while, the correct thing to do is to completely remove the battery, per one of the articles linked to above.
Ugh. But I still loves our Roombas. And the new Scooba is working out well so far. (More about the Scooba after I've used it more.)