Lightweight Retro Electric Motorcycle Design

Hello!

After weeks of scouring this amazing website soaking up as much information as possible, I have decided the take the plunge with my first post.

I am currently designing a lightweight electric motorcycle/moped for use in the UK. The aim is for it to be a commuter vehicle for use in cities, retro-styled, with a removable battery. As such the minimum performance stats are as follows:

Max speed: 40 mph
Range: 50 miles
Battery maximum weight: 10 kg
Target vehicle weight (including battery): 50 kg

It can’t be slower than that, can’t have a lower range, and the battery can’t be any heavier than that. If the vehicle ended up being heavier than 50 kg, so be it, but I would rather sacrifice any performance stats above those already listed in order to achieve a low weight. In designing it I am taking inspiration from:

Meijs Motorman - https://www.google.com/search?q=meijs+moto&rlz=1C1CHBF_en-GBGB779GB779&source=lnms&tbm=isch&sa=X&ved=0ahUKEwiBlsftzL7iAhWyXRUIHajhAWMQ_AUIDygC&biw=1920&bih=969#imgrc=YZhZQI25Qmb2BM:
Munro Motor 2.0 - https://www.google.com/search?q=munro+motor+2.0&rlz=1C1CHBF_en-GBGB779GB779&source=lnms&tbm=isch&sa=X&ved=0ahUKEwjyx4v4zL7iAhU9QxUIHdx3Cz8Q_AUIDygC&biw=1920&bih=969#imgrc=lpIsZF982AU8jM:

I started this project with very little design or electrical experience but a) this site is incredibly useful and b) isn’t that half the fun!?

However, I have reached a stage where I could use some specific help. Each of these issues could probably use its own thread, but because of how interconnected they all are I have decided to do it on one thread and hope some people have the time and inclination to read the whole thing.

Battery

As it is the truly fixed input, it makes sense to start with the battery. I am almost certainly using 18650 cells, though am looking at a range given the trade off of high capacity vs high discharge rate. By my calculations, depending on the cell and depending on the weight of the other battery components, I will be able to get 200 cells into the battery and keep it below 10 kg. This means I have 200 to split into either series or parallel, and thus prioritise voltage or current/capacity. So this is where my lack of electrical knowledge, and therefore the first question comes in…

Given my desired top speed of 40 mph, and the low weight of the bike, what battery voltage should I be looking at? I realise of course that controller and hub motor play a major part in this, but I can’t progress choosing those that much until I have decided what voltage battery I should be working from. If say I was looking at a QS V3 3000 watt motor, 17 inch rims and a Sabvoton controller, do I need to go with 72 volts or would 52 or even 48 volts suffice? Being able to stick to 48 volts would free up plenty of cells to have in parallel, upping current and capacity, which (as far as I understand) would improve torque and range respectively.

Linked to this, do volts need go up in multiples of 12? I understand that this is a hangover from lead acid batteries, but do I need to build the battery in this way from a compatibility perspective? For example if I built a 62 volt battery, would that work with a variety of controllers or only if there was a 62 volt controller? Phrased another way, what does a controller/motor’s voltage mean? Is that maximum voltage, minimum voltage, or the only voltage at which that it will work?

Finally, when choosing cells, if I choose cells with high max continuous output but lower capacity, am I right in thinking I will be able to output higher current to the controller and then motor, resulting in more torque? What should be the minimum “maximum continuous current” for the battery? By my calculations, if I use LG HB6 I can get 330A max continuous current with 72 volt but only have capacity of 16.5Ah capacity (1.19kWh), whereas if I use LG MJ1 cells I can only get 90A max continuous current with 72 volt, but capacity is huge at 31.5Ah (2.27kWh). Would 90A max continuous current feel sluggish given my parameters or is that adequate?

Motor

I am currently planning for a hub motor, predominantly for weight reasons, but also to avoid having the additional moving parts or having to think about chain servicing. I could be persuaded on this, but given the relatively low performance aspirations it’s where I am currently heading. Within this, I am currently expecting to need a 2000w – 3000w motor. Again, I realise that the bike’s ability to reach this speed will heavily depend on the controller and battery, but will 2000w suffice? If not, then will 3000w? According to this chart (http://www.cnqsmotor.com/en/article_read/QS%20Motor%20205%2050H%20V3%20Motor%203000W%20Electric%20High%20Power%20Bike%20Spoke%20Hub%20Motor/259.html), a QS V3 2000w motor with 48V and 70A of current will do 65 kmph, which is 40 mph, but is this reliable? Can anyone in general tell me about the trade-off of motor sizes in the context of the battery discussion? Finally, can anyone recommend any good hub motor brands? I would like to avoid Chinese suppliers such as QS if at all possible, but there don’t seem to be that many others to choose from (especially relative to mid drive system).

Controller

As above, my unsureness stems from a lack of true understanding of the fundamentals. So to phrase the question very simply – given the above, and let’s say I went for a 48v 150A, 53 Ah battery and a QS v3 motor, what voltage/amperage controller do I need to go for to achieve my target max speed of 40mph? My understanding is the more current the controller can draw the better the torque/acceleration, if I am trying to optimise for price within a given max speed constraint, what is the minimum amperage of the controller I need to be going for, and is there a maximum? Finally (back to my voltage confusion and the earlier question, does it need to be specifically rated for 48v, or if it’s rated for 72v but my battery is 48v is that fine? For example, I would love to get the Mobipus 72200, but would that only work with a 72V battery?

And I think that is it for now. So to summarise the questions for easier answering now context has been given:

1) Can a 48 volt battery make a lightweight moped go 40mph if it has the right current/controller/moped? Is this a bad idea
2) Do battery voltages need to correspond to controllers/motors, and therefor move up in fixed numbers of series (48V, 52V, 72V), or can you make up a voltage as long as it fits within a controller’s/motor’s range
3) Is there a minimum or maximum “max continuous current” for my bike’s performance purposes that I should be aware of when designing my battery, or can I just totally prioritise capacity?
4) Will a 2000 watt motor get me to 40 mph, and if not will a 3000 watt motor (assuming adequate battery/controller)?
5) Any hub motor brands other than QS that anyone can recommend?
6) What amperage controller do I need to go for at a minimum to achieve 40mph?
7) Do I need to think about any controller brands other than Kelly, Sabvoton, Mobipus, Adapto?

Thanks in advance for all of your help!

First, I strongly recommend first picking your motor so that it can perform the job you need it to do.

Then pick a controller that can make the motor do that job, *and* has the control input and display output features you want in it.

Then pick a battery that can supply more power than the controller will need to do it's job, and that has the capacity to give you the range you need.

If you do things in a different order, you may end up with stuff that doesn't really do the job you need it to do. Or spend more money than you needed to for stuff that is overkill for the job.

skinardlasvegas1 said:

1) Can a 48 volt battery make a lightweight moped go 40mph if it has the right current/controller/moped?

Click to expand...

You can use any voltage, as long as the gearing of the motor to the ground (sprocket ratios, etc., wheel size, etc) allows that motor to provide enough torque at the necessary RPM. See http://ebikes.ca/simulator for how this works. Read the whole page first, then play with different systems in the simulator, including the advanced options.

2) Do battery voltages need to correspond to controllers/motors, and therefor move up in fixed numbers of series (48V, 52V, 72V), or can you make up a voltage as long as it fits within a controller’s/motor’s range

Click to expand...

I'd recommend using a battery of the specific voltage the controller you pick requires. Otherwise the controller's HVC and LVC won't correspond to your battery's, and you won't be able to use the full range of the battery pack, or else you'll ahve to disable the controlelr's HVC and LVC and depend entirely on the battery's BMS to protect it (which means only one layer of protection, and increasing the possibility of damaging the battery.

3) Is there a minimum or maximum “max continuous current” for my bike’s performance purposes that I should be aware of when designing my battery, or can I just totally prioritise capacity?
4) Will a 2000 watt motor get me to 40 mph, and if not will a 3000 watt motor (assuming adequate battery/controller)?
6) What amperage controller do I need to go for at a minimum to achieve 40mph?

Click to expand...

It's not just about the battery capability, controller current, or motor wattage, etc. It is a combination of motor winding and power capability, gearing to the road, controller capability, battery voltage and capability, etc. vs the terrain, wind, vehicle rider and weight, aerodynamics, etc. It's a system, with interdependent factors. See the simulator to get good guesstimates of what you need to do what you want.

You have to also specify the terrain, winds (head or tail or side all affect aero), vehicle and rider weight, acceleration needed, range required at specific speed(s), and so on, to be able to create a system that will really do what you're after.

If you only need to ride on the flats, and there's never any wind of any kind, and you're not riding in traffic so acceleration doesnt' matter (you can take half a mile to get up to speed, for instance), then you can make a system with lower torque geared for higher speed, and use less power, needing only the power maintain the max speed you want.

But in the real world there's almost always some slopes, even hills, and there's always some wind, so you need mroe power than just enough to reach the speed. Usually there's also some requirement to accelerate at some rate to keep traffic behind you from running you over as it speeds up from a stop; this is also related to the power needed to climb hills and maintain speed.

7) Do I need to think about any controller brands other than Kelly, Sabvoton, Mobipus, Adapto?

Click to expand...

You need to think about the options you need, and then determine which controller does what you want. It's not about brand, except for how that company behaves for when you have a problem.

For example, while each of these issues may or may not be resolved these days: Kellys are known for bricking themselves if you spin the motor while it's hooked up to the computer for programming/etc. Adappto is known for failed units sent back to the company to never be seen or heard from again; I'm not sure they're even in business anymore. Sabvoton's are known for wierd sudden failures, and the company for failing to respond to communications. Mobipus as a company is known for just disappearing for months (maybe years) and not responding to communications, even from people trying to buy stuff from them. Other companies have their own issues you'd have to look into.

Regarding battery size (weight):

It's probably going to take around 100wh/mile or more to maintain 40MPH on the flats with no wind, assuming continuous speed and not reaccelerating to that speed from a stop. (acceleration takes more power than continuous speed)

Assuming the above, then to go 50 miles you'd need 50 x 100wh = 5000wh, minimum.

I have a just-bigger-than 2kwh battery (14s2p EIG NMC, 52v 40Ah) that weighs almost 40lbs. That's around 18kg.

So a battery not even half the size you need it to be already weighs twice as much as you've allowed for, and doesn't account for extra capacity needed for headwinds or hills. To do what you want, it's probably going to have to be more like 40kg (or more).

A 10kg battery, assuming average capacity-and-performance-per-kg, will only get you about a quarter of the range you're after.


If you build the vehicle to be much more aerodynamic, like the Vetter fairings, etc., you can improve the wh/mile which will decrease the size of the battery needed to get the range you're after, but that's going to drastically alter it's appearance, and it wont' look anything like what you're after.

My long term around town average energy consumption is about 52wh/mile over the past 20,000km. That's rapid acceleration, 35-40 mph cruise, bike weight of 42kg, QA v3 in 17" rim, 18650 pack, 20s 36ah from Samsung 30q cells. Top speed is >70mph. Power peak is around 12kw in daily ridden form. I weigh about 72kg with all my gear and backpack on, probably less than the average brit.

At 52wh/mile my pack can theoretically do 50 miles, however in practice you do not want to use the entire pack. Cell longevity increases exponentially with reduction in charge voltage, reducing from 4.2v to 4.1v charge (~90% SOC) doubles battery lifespan. Not using <3.5v also makes a substantial difference. In practice I have found you want to size 30% over your 'spec'. This allows for a lower charge cutoff, reduced likelihood of substantial imbalance by fully discharging/damaging the cells, plus it will ensure that you're still able to meet the spec after a year or two of normal cell degradation.

In your case, using my average consumption figures, you then require a ~3.4kwh pack, or put another way 320 cells using 3ah 18650's. Rule of thumb I use for pack sizing is 50g per cell with termination, putting this in the ballpark of 16kg. Revise your cell choice for a 3.5ah offering and you can reduce your pack weight to ~14kg at the expense of peak power potential.

Your speed, bike weight <50kg, range are all doable - however 10kg battery weight gives you some hard constraints. In a heavily optimised package (aerodynamic, rolling resistance, low power output, hard limited top speed, aggressive discharge profile (4.2->3.0v), smooth riding behaviour) then it's *possible* to achieve the specification with a 10kg battery, however the final product would suck in the real world. If your battery weight limit is due to being able to comfortably remove and carry the battery, I would suggest splitting it into two identical 10s packs that are connected in series on the bike. This is how mine is wired, though I don't care about making it removable - I ride straight in my front door.

Regarding voltage/ah - this is a complex topic, you will want to think carefully about your selection here. The important thing is that your battery voltage, controller and motor are all selected to provide the appropriate speed range for your desired vehicle specification. Specifically for the purposes of power output and range, the configuration of series/parallel does not matter - only the quantity of cells. Each cell contains a fixed quantity of energy and is capable of outputting that energy at a specific rate while maintaining a reasonable temperature. The energy and power output of a pack configured as 4s20p and 20s4p are identical, given appropriate controller limits. Continuous power output is usually constrained by temperature, this will vary significantly depending upon packaging of the cells and cooling available. Peak power output is significantly higher than this, bursts of power for acceleration can safely be a multiple of continuous power output.

Many thanks for the super helpful replies - it may have taken me months to respond, but your input has very much shaped where this project has progressed to and has been hugely appreciated.

I am currently leaning towards a QS v3 3000w motor run at 48V, a Votol EM100SP controller, and building my own 13S11P 18650 battery from LG MJ1 cells, which would mean 48V 38.5Ah (1.85 kW).

A couple of things I am unsure of:

1) The maximum continuous discharge of an MJ1 cell is 10A. As I will have 11 in parallel on the battery, does this mean the max continues current discharge is 110A? Which would mean that max continuous power of the battery is 48 x 110 = 5280W?

2) What does the battery mean for the motor? That because the battery has the ability to put out 5280W continuously, but the motor will only draw 3000W continuously, the battery will be absolutely fine? Or does it mean that the battery will be putting out 5280W and might blow the motor?

3) What is the controller's role in the above? Please see below for the controller's specs.

Will I be able to configure the controller to not go above 40 mph? Is this controller appropriate for the bike? Is it under/over powered?

I would like to be able to configure the bike to have great torque/acceleration (which I believe the motor and high current should provide?), but then max the top speed out at 40mph so that range is preserved. Am I going about this the right way?

Any thoughts would be greatly appreciated!

skinardlasvegas1 said:

48V 38.5Ah (1.85 kW).

Click to expand...

1.85 kWh
Very important distinction.

kW is a measure of power.

kWh is a measure of capacity.

1) The maximum continuous discharge of an MJ1 cell is 10A. As I will have 11 in parallel on the battery, does this mean the max continues current discharge is 110A? Which would mean that max continuous power of the battery is 48 x 110 = 5280W?

Click to expand...

Theoretically, yes.

However, that's the rating for a single cell, probably in open air or with ventilation cooling, in a lab, at 72F.

Inside a pack, where the cells will heat each other up, the temperature will get much hotter, and you may want to derate that current for that, based on whatever the spec sheet shows for thermal derating (if it has any such spec; if not, you'd have to guesstimate, or look at other cells spec sheets that do have that spec). Or build the pack with forced-air cooling.

If you need 110A continuous, I'd recommend building a bigger pack, more parallel cells, to reduce the load on any single cell (cuz you'll need it for the range anyway, at the speed you want).

The closer you run cells to their max specs, the harder it is on them, and the shorter lifespan they will have.

2) What does the battery mean for the motor? That because the battery has the ability to put out 5280W continuously, but the motor will only draw 3000W continuously, the battery will be absolutely fine? Or does it mean that the battery will be putting out 5280W and might blow the motor?

Click to expand...

THe controller limits what the motor pulls. So it is up to the controller to limit the power to what the battery can handle, but still output the power the motor needs to do what you ask of it.

Will I be able to configure the controller to not go above 40 mph?

Click to expand...

That you'll have to ask the manufacturer or seller, if they have software to program it's settings with, and what settings you can change, and what their ranges are.

Is this controller appropriate for the bike? Is it under/over powered?

Click to expand...

If it will provide the power you need to go the speed you want, and accelerate at teh rate you want, then it's not under powered.

Overpowered only matters if you have a size/weight restriction for the controller on the bike (becuase generally higher powered controllers are larger and heavier), and/or it's not programmable for current limits to prevent battery damage.

I would like to be able to configure the bike to have great torque/acceleration (which I believe the motor and high current should provide?), but then max the top speed out at 40mph so that range is preserved.

Click to expand...

For the motor, you should verify with the manufacturer that the version you want has a winding that will give you 40mph at 48v in the wheel size you're going to use.

And that at 48v it will draw the current and provide the torque you want in the wheel size you're going to use to give you the acceleration you want.

To figure out what it will take to accelerate at the rate you want, you need to know the total weight, and the Nm of torque the motor has, and the wheel diameter. Then there's some math I don't know that will give you the accleration rate from that. (It's also dependent on the slope of teh ground and the windspeed, and the aerodynamics of the whole setup, and how long the systme can force the motor to provide the torque, but most of those matter more as you speed up, and less at the startup from a stop.)

Am I going about this the right way?

Click to expand...

Have you looked at the http://ebikes.ca/simulator yet, as suggested? You can play with differnet systems to see what kind of power it will need to do the job you're after under the conditions you have. It doesn't have the specific motor or controller you list, but you can use "custom motor" and "custom controller" (and custom battery) with guesstimates of those things (if you don't ahve specs for them from the manufacturers), to get a good estimate.

It will also *show* you the relationships between battery, motor, controller, and other things in teh system and environment, as you change one thing you'll see changes in the results.