Even with the 15-hour charge time, which I do not plan to repeat, my bike was fully charged by the time I was ready to ride in the morning.
So I am still not ready to lug around an extra charger, as finding 120V outlets that people will you use for an hour or two, is really tough and I would just prefer to manage my route and riding style.
One thing that I have noticed is that after having run the battery pack on my Zero down to zero, the bike now charges to 100% before turning off, whereas it used to turn off at 98% and then finish charging to 100% a day later. The 98% business started after the power tank was installed. So now I guess the BMS is now fully adapted to having the power tank on board.
I would like to emphasis that, unlike the 2012 models that gave you a few more miles at slow speed when all of the power bars were gone, when the 2014 bikes say 0%, they mean it. When the gauge reaches "0%", expect for the bike to stall out about 100 yards later.