Hi Florian,
Ah yes, an age old question! The Ugobe Pleo battery and the Rb battery have the polarity "key" (the little rectangular protrusion on the bottom of the battery) in different positions.
In the battery compartment of the RB there are two slots for the battery polarity key, one for RB batteries and one for Ugobe batteries. Unfortunately, as it was made before there were RBs, the Ugobe battery well only has one battery polarity key, for Ugobe batteries!
Apparently Innvo didn't want owners of Ugobe Pleos using RB batteries in Ugobe Pleos, but didn't mind letting their new customers reuse their old batteries in RBs.
You CAN remove the polarity key from an RB battery with an razor knife etc, and then it COULD be used in a UGOBE pleo with two drawbacks:
1. IMPORTANT!! You will not have reverse polarity protection from installing the battery 180 degree from normal position, You will have to ensure correct orientation of battery on each installation. Not recommended if small children will be removing and reinstalling the battery unattended.
2. The standard rb battery charger WILL NOT recognize the battery without the polarity key. What I used to do is save the sawed off key from the battery and place it into the charger before installing the battery. All this is a bit fiddly, but I did it for many years without a single failure.
Hope this helps!
Cheers
Peter