With the battery out of my Zephyr B is it possible to check the voltage from the Pos & Neg terminals on top of the battery using a multi meter?
I would have said that the half-way point in terms of how far you can go is somewhere around 37 volts because it's watt-hours that count. If you've already done, say 30 miles, there's no way in the world that you'll get another 3 when your battery is down to 31.5v, other than by pedalling hard.40.5V: 90% remaining
39.5V: 80% remaining
37.5V: 70% remaining
36.5V: 60% remaining
35.5V: 50% remaining
34.5V: 40% remaining
33.5V: 30% remaining
32.5V: 20% remaining
31.5V: 10% remaining