Well, there's no place for new batteries, specially another D (R20) battery. But perhaps I can glue a battery holder somewhere else on the robot, since its large. And rewire the whole thing.
I do not quite get that it has 3xD batteries in each leg + 2xAA batteries. Perhaps they do different functions (D ones for movement and the AA ones for talk and leds..?) I'll trace the wiring...
I have to check which ones are causing problems, the big ones or the AA ones.
Another idea: could I just stick two 5V power banks in each leg (I think they should fit or I can make them fit) and wire them to the 3xD battery holder? Then each leg would have 5V instead of 4.5V and it would be easy to charge via two USB's. Any downsides here?