In the thread
"Complexities in touring" ebikes are offered as a solution to emergencies in touring for those of us on whom age is creeping up. I've
a contrary opinion; I don't think ebikes are ready yet except in places where huge infrastructure investment has already been made.
We'd better determine how much distance you can expect from every amp-hour of battery capacity on you ebike.
First of all, battery capability isn't straightforward, for normal marketing reasons, because battery power delivery isn't linear, for ambient situational reasons, and for a whole variety of lesser but still important reasons. However, since ebike batteries sold in Europe are generally speaking of a high quality, it will be adequate if we bypass some of these technical difficulties and simply measure distance per amp-hour on various topologies.
All the same, it is well worth keeping in mind that two riders will get different mileages from the same battery, sometimes radically different mileages. What is important about a battery in the first instance is the Amp-hour (Ah) rating. This tells you roughly how big the battery is. In theory you can calculate how much weight of cyclist and cycle and touring gear so many Ah will pull at what speed, and adjust for the cyclist's input. However, once again, it isn't that simple. There's something called the coulomb rate, which tells you how fast the power is being delivered. Twice the amperege-rate of discharge will drain the battery's remaining amp-hours disproportionately faster than a slower half-ampere discharge rate. In particular, if you're mashing down a throttle or running an electronic pedelec programme at max output all the time, you will run the battery down to flat in much, much less than half the distance you were expecting.
There's an upside to all this. If you use the battery infrequently and never at more than half the theoretically possible discharge rate, it will last nearly as long as the dealer told you. This is why the better controller/reporters have a meter of some kind on the dashboard for the rate at which you're consuming the battery, while the remainder of the charge may be reported on the battery, or on a second level of performance reporting.
It is because in practice the coulomb or instant discharge rate is so important that it becomes important to buy a much bigger battery than you calculate from consideration of your distances, weights, and types of terrain. As an initial minimum, I want to impress on you (earnestly!) that you should be aiming at no less than twice the apparent battery requirement. An interesting experiment is to ride up a steep hill with maximum battery engagement (high delivery rate over time) and then to watch both the meters at the top, the coulomb or delivery-rate meter settling fast, the "reserve power" or "available power" meter rising from some frightening low reserve to a higher one. If you know how to wire in a digital volt meter, you can watch a nominally 36V system fall from 42V (full charge delivery) at the bottom of a long, steep hill, through the nominal delivery of 36V as soon as the draw begins to bite really hard (high coulombs), to nearer 30V (pretty near self-damaging for a 36V LiPo battery).
One more thing: Electrical motor size is a tricky thing. You can forget the ratings engraved on the electric motors sold in Europe; they're basically meaningless because the EU permits the manufacturers to decide the rating of each motor. However, a tourer has two imperatives, and one of them is counterintuitive. If a motor is too small for the purpose, it will lose a good deal of its Amp-hours in wasted heat. It therefore pays to buy a motor that is bigger (more Watts) than strictly necessary, and preferably selected by, if you can find a reliable comparison, the largest possible benison of torque (Nm, Newton meter, foot pounds) because that is what breaks away a heavily loaded touring bike from a standstill, and hauls it up hills. The biggest motor you can legally fit also requires a bigger battery.
A bigger battery is HEAVIER. But with the present stage of battery development even a commuter cannot afford to buy too small a battery, and a tourer must perforce, considering the additional unknowns he faces, buy the biggest battery he can carry. That is what we want to discover: How far any available battery will carry you.
That's the outline of an electric motor/battery decision for a tourer, or a commuter who does more than 12-15 miles one way every day.
In the next post I'll relate some personal experience, and then everyone else can kick in, and I hope that between us we'll build up a useful base of knowledge.
Oh, one last thing. Coulomb's Law has to do with electrostatic forces, which I know about because my hobby is DIY high-voltage audiophilia (tube amps and electrostatic speakers). He's just the famous electrical law-giver with a constant nearest to what is required in these calculations. I can't even remember if it was me who appropriated him first, or an American engineer deserving of a Darwin Prize. Safer to blame an American!
