Firstly you need to remember that voltage sags while under load. So if you ride up a steep hill for a few miles, check your battery level immediately, ride back down, and check you battery again and find that you have around 20% more life left then that 20% is probably more so from the fact that you aren't looking at the battery voltage while under load.
Secondly for the best post apocalyptic EUC-hill-charging-method you should rig find a ski lift/gondola near a stream/river. You can then construct a waterwheel powered by the stream/river that then powers the ski lift/gondola. This way you can ride up the lift up with your EUC and use the downhill to recharge your wheel.