So far I've only been on hilly streets around Cheyenne Mountain (former home of NORAD). What kind of range estimate is your bike giving you in all 4 modes? After todays ride and then a full charge, for my next ride it's estimating 12/15/20/40 (from Turbo/Sport/Tour/Eco). Every time I ride in my neighborhood this estimate for range on the next ride gets smaller, because there's so many hills. But I still believe that I'll have enough juice for that 32 mile ride I mentioned above, for which I will only use OFF, ECO, and TOUR on the 16 mile ride out out, and then on the way back I'll only need some pedal assist on two small hills. On previous rides we'd just get off and walk the bikes for those two small hills, but I'd have my oxygen with me (while getting a shuttle ride for the 16 miles uphill to the mid-point, skipping the uphill bike ride). In any case, it would be easier for me to put a 5lb spare 500WH battery in my Camelback than a 10lb oxygen tank. So it looks like a spare battery may be in the cards for me.