# Electric moped advice



## akumabito (Jun 23, 2008)

Hello everyone!

I am new here and looking into the options of building an electric moped from scratch. What I am hoping to end up with is a recumbent e-bike inspired by the Riley Ground Hugger XR2. The final design will be adapted for e-power only though. 

I am wondering about what motor power I should aim for. Speed is hardly a priority; in the Netherlands mopeds are limited to a 45km/h top speed (28mph). Recumbents are also relatively aerodynamic when compared to normal bikes. On the other hand, the area where I live has a lot of hills, some if which quite steep. I think the motors commonly found in light scooters may not be up to the job. I have seen some larger e-scooters with 1,500 or 2,000W motors. I think these might be more suitable, although bulkier batteries would be called for. 

I suppose the motor could be less powerful if I were to opt for gears instead of direct drive. The rear wheel could easily be fitted with a 5 or 6 speed Shimano transmission as commonly found on MTB's. Has anyone experimented with CVT transmissions? I recently discovered there are models suitable for bicycle use (NuVinci), and I suppose these could be used in conjunction with an electric motor. They are quite a bit more expensive than traditional gears, so would they be worth the extra investment? 

Any advice, tips, suggestions, etc., etc.,?

I am aiming for a 45km/h (28mph) top speed, if possible an 80km range (50 miles, half of my ordinary moped) and a moped weight of 25 to 30 kgs (55 to 65 lbs) + batteries.


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## Gravitic Anomaly (Jun 19, 2008)

FWIW,
In the USA the same 45km/h rule for Moped's applies. The Sach's, Puch's, and Honda 2 cycle motors are about 2.25 HP or 1680 watts. The 30 km/h bikes are 1.1 HP. The powerband on the engines is quite narrow so gearing or a CVT is required.
A brushless motor run off of a good PWM controller will have a flatter torque speed curve and need at most two speeds. I would think about using a Sturmy-Archer 3 speed hub just to keep it simple. 
From a power stand point you will not need 2.25 HP continuously. Unless of course you run a gas Moped wide open throttle all the time. I at least throttle back for turns 
An 800W output brushless motor should equal the performance of the two cycle engine if the batteries can give it the current it needs.
A brushed motor would have to be in the 1100W output range.
I have a Sachs Westlake (similar to a Heracles) and a Motron Moped I have been thinking of converting to electric or hybrid.
Let us know how the project goes.


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## Gravitic Anomaly (Jun 19, 2008)

Oh, I forgot.
Be cautious when looking at the scooter motors that say they are 1.8 or 2 KW. Unlike gas engines there is no standard rating system. Some manufactures state the peak power capability of the motor and not the continuous rating. The Chinese motors in particular are often rated this way. Model airplane motors are usually rated by the power INPUT watts giving huge numbers for the most ineffcient motors.
Reputable motor manufactures rate the motor by the tempurature rise and the insulation system thermal capacity. They are designed to run the rated power constantly. Peak capacity is usually 2x to 4x of rated as long as the average power is within the rated power.


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## akumabito (Jun 23, 2008)

Thanks for the advice, it's much appreciated!

I am looking into the formulas now to decide on the battery pack needed to get my range, and motor power to get my top speed. I think I may be doing something wrong though.. would you mind looking into the numbers a bit?

First for top speed:


```
POWER NEEDED FOR TOP SPEED OF 50 km/h

((Mass in kg) (9.8m/s²) (Velocity in m/s) (Rolling Resistance)) + ((0.6465) (Coefficient of Drag) (Area in m²) (Velocity^3))
(160 x 9.8 x 13.9 x 0.001) + (0.6465 x 0.5 x 0.3 x 2686) = 21.8 + 260.5 = 282.3 Watt
```
These figures seem to make sense. I based them on data on drag coefficient and frontal area of a similar recumbent bike. I guesstimated the final weight including rider and the rolling resistance of the tires. The figure for this speed seems consistent with what I read of other recumbents at similar speeds. No problems so far, I think.

Moving on to the battery sizing:

I had to guesstimate a lot more here. I could really use some pointers to improve my estimate. I estimated the figure based on an example of another electric motorcycle, which yielded an efficiency of 75W/mile. 

My project will be significantly lighter, more aerodynamic, have less rolling resistance and it will be slower. I am not sure how much influence this would have on the efficiency though. 

I have a gut feeling that says the efficiency could probably be improved with 1/3, giving 50W/mile. But I am reluctant to go with overly optimistic estimates as I do not want to be disappointed in the final product. 

*Are there any formulas to improve the estimate of efficiency?* 

Anyway, going with the original 75W/mile (converted to 50W/km - sorry, I'm a metric guy)


```
Estimated efficiency: 50 Watt/hr per km.
Power consumption: 50 x 80 = 4,000 Watt
4,000 Watt / 48 Volts = 84 Amps

Multiplied by 1.25 to compensate for battery inefficiency =  105 amps
```
That just seems like an awful lot. Or maybe not. Maybe it's spot on. I don't know for sure yet. It _is_ a figure that has me slightly worried though. The batteries would be pretty big and heavy.

But perhaps I am just confusing some numbers here.. it doesn't seem to make much sense. Going with 800W and a range of 80km, that would roughly equal to two hours full throttle. Wouldn't it follow then that I'd need 1600W battery storage, or divided by 48V, only 33.34Amp/hr? (plus reserve to account for battery inefficiency) What am I missing here? Obviously some of my numbers are FUBAR. 




Ahhh, this makes me wish I paid more attention back in high school.



Thanks a ton for any input!


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## 3dplane (Feb 27, 2008)

Akumabito!
That is what I get. Based on 800W continuous power usage to reach your goal of 50 miles at roughly 25 mph you need a usable energy of 1600 Wh pack. At 48 volts that's 33.4 Ah. I would go with some easily obtainable 55 Ah batteries.(roughly 17 Kg each if lead) However that puts your Wh/mile to 32.( I think..) Barna.


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## akumabito (Jun 23, 2008)

Thanks for that!

Where is that much larger number coming from though? Am I just misunderstanding the formula, or its application?


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## 3dplane (Feb 27, 2008)

If you go with a 34Ah battery that will be 100% discharged and that will kill it(unless some exotic battery chemistry is used).So to have some "overhang" I would use the higher capacity.Barna.


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## akumabito (Jun 23, 2008)

How much reserve is usually factored in? 20 to 25%?


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## tidykiwi (Jun 28, 2008)

akumabito said:


> The rear wheel could easily be fitted with a 5 or 6 speed Shimano transmission as commonly found on MTB's. Has anyone experimented with CVT transmissions? I recently discovered there are models suitable for bicycle use (NuVinci), and I suppose these could be used in conjunction with an electric motor. They are quite a bit more expensive than traditional gears, so would they be worth the extra investment?
> 
> Any advice, tips, suggestions, etc., etc.,?


I have yet to get a nuvinci hub myself but when the funds become available i will be getting that and an elation gear drive kit or similar. i have read that gear drive kits can be hard on drive trains and also read that nuvinci are very strong and can also be bought as a belt drive unit. i figure i should be able to climb and pull most everything with that setup.


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## akumabito (Jun 23, 2008)

tidykiwi said:


> I have yet to get a nuvinci hub myself but when the funds become available i will be getting that and an elation gear drive kit or similar. i have read that gear drive kits can be hard on drive trains and also read that nuvinci are very strong and can also be bought as a belt drive unit. i figure i should be able to climb and pull most everything with that setup.


My thoughts were similar. It may be overkill, and a three-speed may do the job just fine.. but it's nice to know you won't get bogged down on some hill. 

I'm think a nuvince hub may be beneficial because you can leave the motor at an optimal speed as well when cruising and let the hub do all the speed-changing work. Maybe this would increase efficiency just a tiny bit?


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## akumabito (Jun 23, 2008)

Sorry for the late reply!



3dplane said:


> Akumabito!
> That is what I get. Based on 800W continuous power usage to reach your goal of 50 miles at roughly 25 mph you need a usable energy of 1600 Wh pack. At 48 volts that's 33.4 Ah. I would go with some easily obtainable 55 Ah batteries.(roughly 17 Kg each if lead) However that puts your Wh/mile to 32.( I think..) Barna.


yeah, that's what I'm getting, too.

1,600WH / 50 miles = 32 Wh/mile.

That would be pretty darn good I think! I believe most motorcycle conversions are somewhere in the 70's, aren't they? Seems like a recumbent on skinny tires is the way to go. 

I just hope the numbers all work out..

I've found a company near me that specializes in carbon fiber manufacturing. If someone could confirm the calculations so far, I can start thinking about frame design so I can request a quote from them.


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## Qer (May 7, 2008)

akumabito said:


> How much reserve is usually factored in? 20 to 25%?


How long is a string? (Swedish saying to illustrate an impossible question) 

The less you discharge a lead-acid battery the more cycles they will live. From my memory (known to be flawed for details occationally) I believe I've seen numbers of about 1000 cycles for 50% DoD (Depth of Discharge) and 500 cycles for 80%. Of course that probably varies between different brands and makes and I'd guess that the discharge current (very low in your case, which is good) also is a key factor, but the general advice seems to be to aim for 50% DoD for normal trips and keep that extra 30% down to 80% DoD for special occations.

Don't go below 80% if you can avoid it. The life length of the batteries gets severely shortened by every percent DoD below that level and the closer you go towards 100% the bigger the risk that one of the individual cells will get completely drained and possibly killed forever. The individual cells varies a bit (of course) so even if you still can drain current from the battery as a whole there's no guarantee that it means that all the cells still holds charge, so even if the battery is providing power it might still be dying in the process.


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## akumabito (Jun 23, 2008)

Thanks for that. A 50% reserve it is then. Now I just have to find a way to cram it all in a small and narrow design and keep the weight down as much as possible


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