Thanks for the replies.
I can't figure out why the FLOW needs a receiver battery ... the SSX version 2 didn't need it, and woorked perfectly with 1S batteries. I can't see a receiver battery on several pictures from the worlds - including Ronalds car, and furthermore the presentation on the LRP site says, that it work with 3,7-7,4 volt.
I am aware of, that a receiverbattery can boost the performance a tiny, tiny bit, and ad around 8-10 sec more drivetime, if you are on the limit of enough power in the battery.
Jens