Oh, and this is what the manual says about the topic:
Does the Field Kit work on a 9V battery?
The Field Kit is designed to work with a 9V power source, so a 9V
battery could be used if the user fnds a way to connect it to the
power input. The Field Kit draws approximately 80mA of current
in normal working conditions without driving any external loads,
such as an external speaker or a motor. An 8Ω speaker driven
with full volume draws up to 220mA of current, while the DC Interface
can draw up to 500mA of current. This adds up to 800mA
of current consumption in total which would give the user with
a “standard” 9V alkaline battery with 550mAh capacity a usage
time of ~ 40 min in full work mode, although the voltage of the
battery would drop as it discharges. We are investigating different
ways of powering the Field Kit with a portable solution, keep
an eye on our media channels to check the progress!
I'll be getting a pre-soldered centre-negative barrel plug adapter this or next week, and I'll try it out then.