Thorn Cycles Forum

Technical => General Technical => Topic started by: Andre Jute on January 27, 2019, 08:54:49 PM

Title: How far will a battery drive an ebike?
Post by: Andre Jute on January 27, 2019, 08:54:49 PM
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!
Title: Re: How far will a battery drive an ebike?
Post by: Andre Jute on January 27, 2019, 08:55:52 PM
Okay, here are some actual numbers to work with:

When I was limp from heart surgery, I had a 250W motor (nominal; it was the highest torque motor available) which I bought in a kit with an 8.8Ah battery fitted into a large water bottle. I live in the Rome of West Cork. There is no flat road here that sensible bicyclists ride on because only the main roads, which are dangerous and unpleasant, are flat. So any ride I take has hills, and any wind-free ride is across valleys, so ups and downs are pretty constant, but they're generally not very long and we know which direction to ride the circle or figure eight to avoid the steepest hills and ride their shallower sides. I'm a retired rugby player weighing 215 pounds, and a masher rather than butterfly. My bike with painting gear, water, motor and battery is probably about 35-40 pounds, and I think nothing of hauling six bottles of wine plus other groceries in the pannier baskets. I gave up the car in 1992 and haven't missed it even now that I'm old.

That 8.8Ah battery from the beginning, when I was weak after heart surgery, easily handles a 22km round round trip, with more than half of it left over. At one stage I was eyeing the Bosch Professional Tool Batteries, which are 36V and 3.2Ah (I think without looking it up again) but decided it would be too tiresome to switch them in and out. My first electric motor burned out (the controller couldn't take the heat) after about 3500km. But by then I had determined that at a push I could take it 40km with moderate use, and possibly (I never actually did this) 50km if I were resigned to abusing battery and motor so hard that they probably would clock out early.

I replaced the first front hub motor with an even higher-torque centre motor, and the 8.8Ah battery with a 14Ah battery. The shortest ride I take for exercise is 8.5km, and there are several of them, generally figures of eight; all are very hilly, and two take in the steepest hills in my part of the country. From none of them does the 4-lamp battery reserve show any lamp off. On the 22km round trip ride, one lamp or none may be off, depending on how I used the motor. On none of these rides do I run the motor 100% of the time: that's not why I fitted a motor. Instead I use the motor when my heart rate reaches the permissible limit, in short as an aid to keeping up with pretty slow (15kph average, what German housewives are assumed to "speed" by their legislators.) This is despite the fact that the Coulomb meter, the instantaneous discharge meter which I have directly in my eyeline next to the heart rate monitor) shows fractional deliveries at maximum rate.

However, on one occasion when I rode around and around the very large hilly block on which I live with the battery full on all the time, it was half-empty just under 20km. I didn't run it down past that, especially not at full discharge rate, but -- focus now -- the lower half of any battery will not hold the same power as the upper half, so to flat under the same circumstances wouldn't have been 40km altogether. My opinion is that with my normal careful use of the battery, it might in an emergency carry me 40-50km before recharching -- and it is also my opinion that a demand that large will decrease the battery life. Need I say say in this company that the battery is actually the most expensive part of an electric bike?

Notice that I'm using kilometres, not miles.
Title: Re: How far will a battery drive an bike?
Post by: Danneaux on January 27, 2019, 09:15:52 PM
Need I say say in this company that the battery is actually the most expensive part of an electric bike?
<nods> A small purveyor of e-Bikes has opened a couple of kilometers from my home. His prices for replacement Bosch e-Bike batteries are in line with the figures cited at the Google Shopping link below, with a Bosch Power Pack 500 running the better part of a thousand US dollars ($945). His Yamaha 400wH batteries are right up there too, selling for about $900. As you might imagine, shortly after his store opened and demand for e-Bikes ramped up, so did a thriving trade in stolen batteries, driven by replacements for thefts (sometimes the same batteries are sold back to the unwitting owner by the thieves who stole them) and for spares that can remain pre-charged and so quickly swapped at home for more riding without waiting for recharging -- I do the same with my rechargeable lawn mower batteries.


Title: Re: How far will a battery drive an bike?
Post by: bobs on January 27, 2019, 10:18:29 PM
I recently purchased a Giant Explore e +1 Ebike. Tech is  :-
Giant SyncDrive Sport, 80Nm powered by YAMAHA, 350% tuneablesupport 250 W motor.
Giant EnergyPak 500, 36V 11.3Ah Rechargeable Lithium-Ion.
When the battery is full charged the on-board computer shows it has a range of over 100km in the lowest assist mode , eco . The bike has 5 modes. The range is dependent on mode used, the terrain and the load carried.
I have done over 300 miles in the past 2 months and I have yet to determine the range . For example on one run I had travelled over 20 miles before I used any assist. Without assist the bike travels like a loaded touring bike and assist is not required on the flat or downhill . I think that over 80km would be possible  if assist was used sensibly.


PS   The battery is securly locked to the frame.
Title: Re: How far will a battery drive an bike?
Post by: mickeg on January 27, 2019, 11:09:48 PM
Or, think big.
Title: Re: How far will a battery drive an ebike?
Post by: Andre Jute on January 28, 2019, 12:00:43 AM
Bob, what range does your bike promise for the highest-assist mode?

On the whole I've found the assist modes pretty economical as compared to throttle inputs, but that's because the pedelec assist modes work arse-about-end, matching their assistance proportionately to your input, while it is exactly when your relative input falls, as on hills, that you want more assistance from the motor rather than less.
Title: Re: How far will a battery drive an ebike?
Post by: bobs on January 28, 2019, 12:54:17 AM
The computer show the range left in each mode. The range is highest in eco then decreases in each of the other mode and is at it's  lowest in sport+ mode.  I realise that this only a guide and depends on a great many factors. In eco mode the motor uses 1% every 1km  which I find fairly accurate.