The NiCad batteries fit in plastic holders soldered to a printed circuit board, which fits under the chassis and has the connections brought out to screw terminals. The batteries have to be taken out for charging. Six NiCads produce about 7.2v, which is too high for the motors. It was the intention from the outset to use pwm motor control, however, and some is lost in the motor drivers. More would be lost in the drivers if H-bridges were to be used in the future.
At the back there is an external power connector. When used, this isolates the batteries so that the robot can be run from a bench power supply. There are also two switches: one switches off all the power, the other switches off power to the motors only - again, for convenience while working on the bench. Note the labels, here and on the controller board, below: when I started this I knew it was likely to be put aside for long periods, that I would forget what was what and that diagrams on scraps of paper would get lost. Laser printers are great for this sort of thing.
Connector blocks are glued to the underside to aid interconnection.