'Sane' driving of the i3 generally means the REx can maintain battery charge. But, depending on what you are doing and when it is engaged, you can get into a situation where you could run out of enough battery capacity to keep doing what you're doing! High speeds, climbing a grade, and the cabin heat on high (especially with the REx which doesn't have a heat pump) are probably your biggest power consumption sources. There is a power flow diagram that you can call up for the display to show you what's going on. Driving at night with the high beams on, the wipers running, the rear defogger on along with seat heater(s) just compound the energy use. A 34hp engine is about what the first VW bug had when introduced, or a 2CV...try to run up a grade and you'll not be able to fly up there once the battery reaches a certain low SOC. Once the battery charge gets critical, that's all you have to keep going. As you get close to that point, the car will warn you then start to shut down non-critical functions, but cross that magic threshold, and you'll be like you hit a wall, and slow down radically, not stop, but you wont' be able to maintain full speed while the load is still there. Pull over, let the REx rebuild some battery charge, and you could go again, or limp along until you reach the crest and can build some charge with regen on the way back down. Stay within the limits and capabilities of the car, and you may never notice. It is, after all, designed as a city car with a commute into the suburbs...not cross-country use even though people have and can do that with proper planning and understanding of how things work. The i8 and the next I-car can keep going, essentially without compromises under all conditions (but even the i8 has problems if you are using it on a track...even with over 200Hp, it can't keep up full performance while trying to keep the battery fully charged, but until the battery runs down, it goes really well!).
2011 535i x-drive GT, 2014 i3 BEV
Soon (hopefully!) A 2021 X5 45e will replace the above