No you don't need a battery as the GK kit runs from the powered 13 pin cable from a Roland synth unit (GR-55,20 or VB
The GK kit pickup is only an analog hexaphonic pickup where the 6 analog signals are fed to a synth unit for processing into midi.
The GK kit does not do midi itself.
The GK kit does not provide analog sounds either, but the synth unit such as the VB
-99 or GR-55 with it's HRM modeling can use the hexaphonic signal to restructure different model basses or guitars.