Range and Wh....

[brucehawsker]

Reposted from another thread where it got lost and I was justly accused of hijack for which sorry.....


We have been working with Barcelona Tech and using our iPhone wireless network to monitor a large number of riders and routes around Barcelona City and out into the hills. Thus we have an extensive and unique we believe route database. This is using our own designed 36V10Ah 360Wh battery, our own controller and our own motor. All of them optimised for efficiency.

"I somewhat agree Flecc however, battery capacity does vary a bit between similar bike types and it's fairly safe to say battery consumption on average falls between 8 and 16Wh per mile. Now, yes, there are variations outside of those figures that throws a curve ball as our US friends would say but we know the larger battery capacities already available from the likes of Wisper don't make the claimed range in the article."

I agree too. The less efficient raw (i.e. not modified for UK use) seem to be in the 14-16kWh/mile range. I worked for two firms who imported such bikes during 2010 and this analysis was based (unscientifically ) on anecdotally listening to clients talk about their real world expereinces. Both were rear hub bike ranges.

I suspect, but I am not sure, that the efficiency of the cheaper Chinese motors fall off rapidly as the cogs wear, and that the batteries are not that efficiently managed, and that the controller is optimised for flat Chinese road running and for less than 60kg people!

For Chinese imports which have been worked on and developed (usually over a number of years of importing) by a responsive manufacturer, the number falls to the 12-14kWh/mile range.

I guess (David, can you confirm this? This is wild speculation as I have never ridden your bikes in anger...) the Wisper might fall into that category. Thus a 36V10Ah Wisper might typically do 25-30 miles?

We have exhaustively tested and researched ours with our team of DPhil students are BarTech. We are pretty consistently falling into the 7.2-8.7kWh/mile range. But to achieve that, we have been running an expensive three + year Uni project and started from the bottom up with efficiency in mind.

Thus our claim to be able to deliver 40 miles - or better perhaps, 3.5 hours - endurance.

Sorry for a long post, but this is such a complex area and a lot of urban myths are out there.

Bruce
Big Cat Electric

 

[NRG]

Hi Bruce, that was my quote for the record I do hope you meant Wh and not KWh

 

[Deleted member 4366]

As far as I'm concerned, any statements about range are meaningless as long as someone is pedalling at the same time. For most riders, a true assessment will come from comparing different bikes over their usual routes, when they pedal with the same effort, have the same weight, go at the same speed and have the same weather conditions.

Most motor/ drive train systems seem to have a known effeciency, with, relatively speaking, not a lot between them. I'm sure that some here will tell me that under some conditions, some systems would be more efficient. That's true, but does it really make that much difference overall?

Battery amp-hours can be measured. This is the most important factor for range. Yes, I know that the weight of the rider and bike and terrain also have a substantial impact, but for most of us normal people (overweight and not so fit), we can't change these things, and when you weigh 100kg, a few kg on the weight of the battery or bike makes only a small percentage difference, which is probably unnoticeable. It would be different if one was athletic with a good body mass index as a few Kg would make a bigger percentage difference.

We all know that you if you put a device on your e-bike that cuts down the assistance, then it will go further - but you have to pedal harder! If you want to pedal hard, then you can have unlimited range by cutting the assistance to zero. That's logical!

So, coming back to the point, concerning range for the same pedalling effort. The real battery watt-hours is the biggest factor, and there will always be a slight trade off between weight and effort as the battery size increases - more noticeable when you're a light-weight.

By way of example, I have a front hub 250w motor and 9ah battery. I weigh 110 kg. I'm unfit (58yrs) and commute 14.8 miles each way with a total rise of about 900ft. I have the unadjustable pedal assist on all the time. I can normally do this journey without re-charging, except occasionally, if it's windy or I'm tired, the battery goes flat while I'm climbing the final steep hill due to voltage sag. My meter normally shows between 5 and 6 ah at cut-off so I'd say my normal range is about 30 miles. Last journey, I decided to pedal a bit harder, and when I got home the meter showed I had only used 3.4ah, so, on that basis my range would be about 47 miles - nearly 50% difference - just by pedalling a bit harder. So, a fit light person should be able to get a substantially higher range - probably well over 60 miles. Now you can see why it's so irrelevant to make any claim about range for a bike.

 

[brucehawsker]

Forgive me NRG for lack of attribution and I did mean Wh.

 

[brucehawsker]

d8veh. Thank you for your comprehensive post. I am going to cut and paste below and add my comments in blue:

"Most motor/ drive train systems seem to have a known effeciency, with, relatively speaking, not a lot between them. I'm sure that some here will tell me that under some conditions, some systems would be more efficient. That's true, but does it really make that much difference overall?"

I would comment that even with the same system (eg rear hub or crank or front hub) there are extremes of efficiency - sub 7 to 16+ Wh/mile have been tested for rear hubs for example.

"So, coming back to the point, concerning range for the same pedalling effort. The real battery watt-hours is the biggest factor, and there will always be a slight trade off between weight and effort as the battery size increases - more noticeable when you're a light-weight."

I don't agree it is the only big factor. Efficiency, something we have focussed on, has been largely ignored in the posts so far...

"By way of example, I have a front hub 250w motor and 9ah battery. I weigh 110 kg. I'm unfit (58yrs) and commute 14.8 miles each way with a total rise of about 900ft. I have the unadjustable pedal assist on all the time. I can normally do this journey without re-charging, except occasionally, if it's windy or I'm tired, the battery goes flat while I'm climbing the final steep hill due to voltage sag. My meter normally shows between 5 and 6 ah at cut-off so I'd say my normal range is about 30 miles. Last journey, I decided to pedal a bit harder, and when I got home the meter showed I had only used 3.4ah, so, on that basis my range would be about 47 miles - nearly 50% difference - just by pedalling a bit harder. So, a fit light person should be able to get a substantially higher range - probably well over 60 miles. Now you can see why it's so irrelevant to make any claim about range for a bike."

Thank you for this - very helpful. What voltage are you running off? I assume 36V. This gives 6 - 7 Wh / mile, like ours.

When you pedal a lot, that turns into 4.2 Wh / mile. Again, similar to our experience.

I would love to see some logged journeys contributed to a central web site showing bike model, Wh used, distance, net elevation.... BEBA? Lets get some objectivity here.... A good efficient electric bike will show a sensible range with a sensible battery.

 

[NRG]

Yes, of course range statments are pretty meaningless as there are so many variables involved, however, I suspect if you took a poll the broad majority of ranges achieved in everyday use would lay within or somewhere close to 8~16Wh / mile. I base this on my own use and logging of data on my too hub powered bikes and more recently on the Panasonic powered Proconnect....which I'm finding can achieve 7Wh used in low assist mode.

Battery ah is not the most important to measure for range as it is not a measurment of energy and does not take into account volts, Wh is the most important and given as imperfect as our calculations are its the best we have to go on. Another way to extend distance is to travel slower which will happen naturally if the rider input stays the same and motor assist drops...

 

[jbond]

IMHO. E-Bikes are only just good enough at the moment. The set of compromises mean they're just too heavy, just too little range, just enough power, just too expensive. This means there's still plenty of room here for R&D. eg:-
- User interface improvements that discourage use of assist and eek out the power so you use the assist when you need it and only then
- UI that discourages the most inefficient parts of the usage cycle
- Motor efficiency. Both overall and in the bad parts of the duty cycle
- Finding ways of getting hill-climbing torque as well as assist at higher speeds
- Motor weight
- Motor drag when unpowered
- Battery capacity and weight
- Battery life
- Efficiency and effectiveness of the underlying bike, in terms of speed and hill-climbing ability with just pedal power

Actually doing this involves having some scientific approach to measurement. It's not enough just to say 5% improvement in X km range or Y WHr/mile in average use. That's marketing not science.

Even if we keep everything else the same as the current state of the market, I think there's opportunities around the user interface side. If we want to avoid powering up hills at high speed and so wasting battery capacity, this is something the electronics could limit or they could encourage us to go slower.

 

[Deleted member 4366]

NRG: Sorry, you're quite correct about wh being the important factory. I was assuming a comparison of like for like voltages, but I know not all bikes have 36v like mine. I did mention it further down the post, but perhaps I didn't make it clear enough. Thanks for pointing it out.
Bruce: You can't use wh/mile as a measure of efficiency as long as a rider is pedalling. You can only measure the efficiency of a motor by comparing the energy it consumes with the work it does. How can you measure how much work it's doing when someone is pedalling at the same time? How much power is he/she providing?

 

[brucehawsker]

d8veh - of course you are right for a single journey in isolation. But combine a set of widely differeing journeys with different rides on the same machine and you start to get an answer. Our iPhone enabled bike uses an iPhone app to record the journey via GPS and hence give us distance and elevation vs time. We also get Amps drawn vs time. Thus we can derives W vs time, or cumulative Wh vs time and vs distance or a mix of distance and elevation. We have refined our bikes to produce the lowest average Wh consumption for a series of typical routes ridden by people with typical riding skills.

Once we get to the point of lowest or close to lowest cumulative Wh averaged across people and routes, given the operating parameters we can tweak in the BMS and in the controller and in the engine itself, we conclude we have optimised things ofr our bike - and we get to the 7.2 to 8.7 Wh/mile range described below.

Hope this makes sense - it is the best we can do. I would love to hear about other firm's R & D....

Bruce

 

[Deleted member 4366]

Sorry, I can't agree at all.

If your motor is using less wh/mile then you're pedalling harder. It's the first law of thermodynamics: You can't get something for nothing.

Most good electric motors are claimed to be 85-90% efficient and drive trains about the same. With some x-fantastic breakthrough technology it might be possible to gain about 5% on both, which would make a differnce of about 1wh/mile, so any claims beyond that are probably unrealistic.

 

[jbond]

If your motor is using less wh/mile then you're pedalling harder. It's the first law of thermodynamics: You can't get something for nothing.

Yes, but, for the same basic bike you can still tweak controller current and speed limits and the motor wind. If you add some additional circuitry, you could make the current limits speed dependent. I don't think it's unreasonable that optimising all this for a specific rider over a specific route could get you 25% less energy use from an un-optimised solution. The difference is partly from just pedalling harder and partly from using the motor in a more efficient way. It may mean having less assist and going slower up the hills for instance. That might be a perfectly acceptable compromise in that you don't have to pedal that hard, you still get up the hill, but you're not wasting power going up the hill at 15mph purely on the throttle.

Some of the motors (like the small Bafangs) come in a bewilderingly large range of winds for different voltage and target wheel sizes. Rather than just pick the 36v26" version it might be more efficient to use a 24v20" laced in a 26" rim, run it at 36v and then limit the speed in the controller. Or not. Or some other combination.

If Bruce has discovered some tricks I'd love to hear them. But I guess it's probably commercially sensitive information.

 

[NRG]

For a given rider / bike / route / speed / conditions / distance, 'x' amount of work needs to be done. This doesn't change, the energy required either comes from the rider the motor & battery or a combination of both. Changing power profiles, current limits etc in the controller just shifts where the energy contribution come from either more from the rider or more from the motor and battery. Yes, there's benefit for optimising the controller for the given motor to minimise electrical losses but when done you are back to fudging the figures.

The only way to lower the required energy input is to change the dynamics of the environment, either through lower aerodynamic drag, rolling resistance, physical route (less hills), less average speed or praying for a 20mph tail wind! etc.

 

[Deleted member 4366]

Thanks, NRG. It's nice to know that at least one other person gets it.

 

[jbond]

Yes, indeed. And yet Panasonic based systems typically get a larger range by encouraging more use of pedal power and by more efficient use of the available motor power. Probably.

So even though the total energy used is the same and more of it comes from the human and less from the battery, it still results in a bike design that gets more range from it's battery and a lower battery energy use per mile for the same rider, trip and speed. Probably.

That suggest to me that work in the area of efficient use of the available energy and new controller stategies is worthwhile. At least while we're still in scarcity thinking mode where there's always too much weight, too little energy available, too little efficiency and too little power.

 

[brucehawsker]

Thank you for all your contributions. There are larger gains than you might think because inefficiencies lie in many areas in electric bike - not just in the electric (or electronic) bits. Will share more tomorrow but it is time to go to the pub

 

[Deleted member 4366]

Bruce, I look forward to any news on how it can be done, I heard on the TV last night that our world (and universe) may be a holographic projection of information that is circulating on the boundary of a black hole, where our reality lies, so I guess anything's possible. I try and keep an open mind, I'm trying to get my head round quantum mechanics, where all our normal understanding of principles of physics goes out the window, I wish I could come to the pub with you and discuss it. I'd buy you a pint.
Dave

 

[flecc]

inefficiencies lie in many areas in electric bike - not just in the electric (or electronic) bits.

Very true, poor design is more often present than not in e-bikes, and few designs maximise their overall efficiency on both the bicycle side and the power integration with that.
.

 

[Alex728]

I suspect, but I am not sure, that the efficiency of the cheaper Chinese motors fall off rapidly as the cogs wear, and that the batteries are not that efficiently managed, and that the controller is optimised for flat Chinese road running and for less than 60kg people!

it is true that frames from hotter parts of China are made for shorter people (people get shorter in hot countries, tigers are smaller but rats grow in size, its some sort of natural balance) but many Chinese weigh a bit more than that these days, they are a lot more prosperous and like eating I am half Chinese and wish I could still be that slim

In China as opposed to the EU, motors and ebikes do not need to be so efficient as their otherwise stringent authorities do not enforce traffic laws in some areas and tolerate what are essentially small mopeds (like Synthman's machine but more powerful and faster) to be ridden on the public road without license, test and protective headgear. most of these machines are 48V with up to 1kW motors and lead acid batteries, but no EU nation will permit these things on their roads.

in fact most Chinese load up their ebikes with anything from shopping to passengers, their work stuff (including large amounts of building materials!) and are carrying much more than 60kg..

 

[flecc]

many Chinese weigh a bit more than that these days, they are a lot more prosperous and like eating

True, but they have also been dropping e-bikes and adopting cars at the same rate as they are getting fatter! It's an odd parallel with our western development as we gave up cycling for cars, getting taller and fatter at the same time.

Seems we are doomed to a future of 20 stone 6' 6" people driving large people carriers and 4 x 4s.

That's large-people carriers, not large people-carriers.
.

 

[CeeGee]

Seems we are doomed to a future of 20 stone 6' 6" people driving large people carriers and 4 x 4s.

That's large-people carriers, not large people-carriers.
.

Current people carriers which advertise seven seats etc are not designed for every passenger to weigh 20 stone and be 6' 6" tall.
Therefore large people will need larger people-carriers than present, making them large-people-people-carriers.
Of course, if 20 stone and 6' 6" is the norm then they won't be large people any more, but ordinary sized - but needing large people-carriers .... if you see what I mean.

Colin