We’ve only had ours for a couple weeks. I noticed a stretch yesterday at 55mph that the engine was off for about 1/2 a mile. Probably not gonna get much longer at those speeds. It is surprising how often it switches off at 50-70 in econ mode. As far as running without a battery no. The charging of the 12v system is done from the high voltage. There is times when the engine is directly powering the drive motor. It’s more of a serial hybrid with the engine engaging at 45mph, and it only has an overdrive gear with a hydraulic clutch to activate. Weber university does a good video of the gearbox that would probably help explain a lot of the power train.
