I've found http://data.robbe-online.net/robbe_pdf/P1122_1-8101.pdf attached to the link you gave above.

"The second set of instructions uses the output sockets of the decoder as mere switches. The centre terminal of each socket is connected to the negative terminal of a seperate battery and the load (bulb/motor/pump etc) to the positive terminal. The ground side of the load is then connected back to the outer terminal of the decoder output. Two loads being connected to each decoder output."

Can you provide the link where you saw the above?
I see no "separate" battery in the instruction link above. I DO see circuit boards labeled for polarity. Is an "Akku" a battery?