Motor Wattage VS Battery Drain

Burnt75

100 µW
Joined
Nov 2, 2017
Messages
7
Location
Monroe, Louisiana USA
Had a question I haven't found the answer to. Lets say you have 2 identical bikes and batteries, only 1 bike has a 3000 watt hub motor and the other has a 5000 watt hub motor. If driven identically, which will use the battery faster? Does being a bigger motor mean it will drain faster? Is it like comparing a V6 to a straight 4cyl in a vehicle? :?:
 
If driven identically, the battery drain will be very close. A bigger motor is presumably heavier and may have slightly more drag, so might take a bit more than a smaller one, but it will be very close.
 
Burnt75 said:
Had a question I haven't found the answer to. Lets say you have 2 identical bikes and batteries, only 1 bike has a 3000 watt hub motor and the other has a 5000 watt hub motor. If driven identically, which will use the battery faster? Does being a bigger motor mean it will drain faster? Is it like comparing a V6 to a straight 4cyl in a vehicle? :?:
Depends on a lot of things.

Drag - the larger motor will usually have more mechanical (and magnetic) drag.
Match to voltage - a motor running closer to its base speed will be more efficient.
Regen - if one motor supports regen and one doesn't (i.e. one is geared) then the one that supports regen will use less battery over the course of a trip.

So the larger motor will generally take more power. But it's not as bad as a V6 vs a smaller 4 cylinder engine - it's generally closer than that.
 
If the bikes, including the controller, are identical, then power drain will effectively be the same, because it is the controller, not the motor, that really determines the amount of power used.


You can use a 5kw controller on a 3kw motor, and vice-versa, and the battery will still have to supply the 5kw to the controller if it's demanded by the conditiosn at the time. The 3kw motor on the 5kw controller might not make the same amount of torque or power to the ground that the 5kw motor on the same contorller would make, but the battery would still have to supply the 5kw power either way.




If the controllers are different (even if the motors are the same), then the higher wattage system will use more power faster, because the higher power controller will pull more from the battery to feed to the motor.

How much more depends on the riding style, terrain, weather (winds), stops/starts, speed, etc.

The more acceleration that has to happen, the more high-current situations that will occur, and the bigger the difference between the two would be.
 
billvon said:
Regen - if one motor supports regen and one doesn't (i.e. one is geared) then the one that supports regen will use less battery over the course of a trip.
This part is more complicated than that.

A geared (freewheeling) motor vs an equivalent DD motor might use less battery over a trip, if the trip has a lot of coasting, and not many stops or decelerations (because the regen of the DD would not be able to recover much power, while the geared motor freewheeling when not in use would create less drag than the DD motor would when not in use).

There will be a balance point somewhere where the DD with regen would recover enough power via regen to make up for it's drag losses, making overall less battery usage than the geared hub.


If two identical DD motor setups are used on the same trip, except regen is used for braking on stops/decelerations on one but not the other, then the one with regen would regain some of the power used on the trip while the one without regen would not, resulting in less battery usage for the one with regen.
 
fechter said:
If driven identically, the battery drain will be very close. A bigger motor is presumably heavier and may have slightly more drag, so might take a bit more than a smaller one, but it will be very close.

On the other hand, if the trip in question involves a decent amount of hill climbing or hard acceleration, then the larger motor will almost always be doing that with greater efficiency than the small one which will be a more significant effect than this, and will have LESS battery drain than the bike with the smaller motor.

This is counter-intuitive to most people but it holds true both in theory and in practice. Ride up a hill at a given speed with a small hub motor, then do the same thing at the same speed with a large hub motor, and you'll find that you use less amp-hours and have better wh/km on the climb with the larger motor. A bigger motor doesn't automatically mean that you'll have less range and higher drain on the battery pack, it often means the very opposite.
 
justin_le said:
fechter said:
If driven identically, the battery drain will be very close. A bigger motor is presumably heavier and may have slightly more drag, so might take a bit more than a smaller one, but it will be very close.

On the other hand, if the trip in question involves a decent amount of hill climbing or hard acceleration, then the larger motor will almost always be doing that with greater efficiency than the small one which will be a more significant effect than this, and will have LESS battery drain than the bike with the smaller motor.

This is counter-intuitive to most people but it holds true both in theory and in practice. Ride up a hill at a given speed with a small hub motor, then do the same thing at the same speed with a large hub motor, and you'll find that you use less amp-hours and have better wh/km on the climb with the larger motor. A bigger motor doesn't automatically mean that you'll have less range and higher drain on the battery pack, it often means the very opposite.

How about the same motor, ridden at the same speed, but at different voltages?

In particular, I'm thinking about a smallish geared hub (Q128H) at 36V and 48V but without utilising any extra speed that might come from the higher voltage?
 
The two main factors are: 1. The kV of the motors, which affects the efficiency. 2. The controllers, i.e. how much current they allow. The power of the motor 3kw vs 2kw means very little. Some motors are inherently more efficient than others by design. That could have some effect, but would most likely be insignificant compared with the other two factors. The kV and controller current determine the maximum speed. Obviously, the faster you go, the higher and more wasteful your battery consumption becomes, except when going slow up hills.
 
Same thing.. bottom line is a certain cruise speed, say 20 mph, takes about 400-500watts. This is set in stone by the weight, and the aero drag of the bike and rider.

At cruise, efficiency of differing setups is very similar. But big differences exist at other times, starting up, climbing hills, or pulling big loads going slow. But by 20 mph or so, all that levels out and systems tend to all be fairly efficient. There are differences still, but they are very small at cruise speeds, and can be erased by pedaling with the motor off, or nearly off, for a few blocks.

So a big motor running at 500w pulls the same 500w as the small motor.

At 48v, 500w is 10.4 amps. at 100v its 5 amps. but either way, its 500w to go 20 mph. Because of the load, the combined weight and drag to go 20 mph does not change.

Different bike can have a huge effect, by changing the drag. Upright commuter much worse drag than a lay down recumbent bike. And how you ride can make a huge difference.

If you ride faster in general with the big motor, you will eat up more battery because drag increases with speed. Most of us, with a bigger motor, tend to ride a bit faster, or at least use more power taking off, and climbing hills. But on the flip side, undersize the system enough, and not only do you ride inefficient on big hills, you can also melt down the motor if its really too small for the hill, and the weight involved.

What you need to do is ask this question, how big a system do I need to do this? Give us the total weight, the grade and length of hills, and the speed you want to travel. Then we can give advice what type system you need. If you are already thinking in terms of 2000w and above, then no problem, you can tow a trailer up the rocky mountains with 2000w, and you live in Louisiana, so your biggest hill is a bridge.

Both the 3000w and 5000w setups will devour a battery capacity in 15 min, however you ride them at full power. 40 mph takes about 3000w, and your ride is over in about 17 min for a 72v 10 ah.
 
d8veh said:
The two main factors are the kV of the motors that affects the efficiency, and the controllers, i.e. how much current they allow. The power of the motor 3kw vs 2kw means very little. Some motors are inherently more efficient than others by design. That could have some effect, but would most likely be insignificant compared with the other two factors. The kV and controller current determine the maximum speed. Obviously, the faster you go, the higher and more wasteful your battery consumption becomes, except when going slow up hills.

Understood, but I was wondering about when not using aby extra speed that might come from the higher voltage. Ie. Same controller, motor, journey, hills and speed, but different battery pack?
 
500w cruise speed drain on the battery equals 500w drain. Sure, one setup might pull 490w, and the other 510, but this is a tiny difference.

Same thing if you are cruising pulling 3000w, but in this case a similar percentage difference is more watts total. But yer still looking at one pack lasts 15 min, and the other 15 min 45 seconds.

At cruise, all the setups are fairly efficient. You can set up particular circumstances where one motor is 5% better, but in the real world you cannot ride lab conditions. Things tend to even out, and all will average about the same efficiency, or in any case close enough to not extend your run time or range significantly.

HOW YOU RIDE MAKES THE DIFFERENCE. You can get 20% to 50% better range easy, by riding different, slower, adding pedaling, tucking, etc.
 
Buk___ said:
Understood, but I was wondering about when not using aby extra speed that might come from the higher voltage. Ie. Same controller, motor, journey, hills and speed, but different battery pack?

In this case, the answer is mostly that the consumption and efficiency would be identical, but there is an exception. If you are riding a bike with a sinewave of field oriented controller where the mosfets are always doing PWM switching, or are riding a trapezoidal controller but at less than full throttle (as would be the case if you are speed or power limited with a Cycle Analyst for instance), then it's going to be exactly the same. If you switch from a 36V to a 48V pack but keep your motor power and speed the same then your efficiency and watt-hours per km will remain unchanged.

The one exception is that if while riding with the 36V pack you are going full throttle with a trapezoidal controller where the mosfets are at 100% PWM duty cycle. Here, you have almost no switching losses in the controller, only the I^2R conduction losses in the mosfets, so the controller will be operating with very good efficiency. When you then switch to the 48V battery pack, the controller will have to be running at 75% PWM duty cycle to match the speed and power of your 36V setup, and that means that the controller will get warmer since in addition to the conduction losses (which will be roughly the same) you also have switching losses each time the fets turn on and off. So while your motor efficiency will be the same, your controller efficiency will be worse, and that would slightly increase your wh/km energy usage from the battery pack. In addition, there could be slightly higher losses in the motor due to the current ripple at the PWM switching frequency, but this is usually really insignificant.

Hopefully that's the specific answer you were looking for to your very specific inquiry!
 
The worst efficiency loss is the result of a bad connection.
Sometimes my throttle wrist is connected to my balls instead of my brain, battery gets a beating. :twisted:
 
justin_le said:
Hopefully that's the specific answer you were looking for to your very specific inquiry!

Indeed it does. Thankyou.

My thinking -- tentative as it is -- is this. I have a 36V battery pack and a Q128H on order; which will be perfect for most of my around town riding; but for my occasional forays out to nearby towns and villages, it would be nice to have the extra speed that comes from using a 48V pack.

I think the above confirms my suspicion that running a 48V battery for everything would likely be less efficient for the low speed stuff that will be the majority of my riding; and is extra weight when riding without assistance.

(Which is my intention for much of the time, reserving the motor for steep hills, particularly the unavoidable last kilometer up to my hilltop home when I'm knackered out. That's the intent. Whether I'll be able to resist the throttle only time will tell :) )

I see 3 possible approaches to avoiding the weight:

  1. Buy an additional 48V pack and use that for the out of town runs.
Extra cost for the pack and charger.
  • Buy a DC-DC booster and turn it up/switch it in for OOT runs.
Using a booster for 36V-36V would actually cost more than a direct connection.
  • Buy a 12V pack and add it in series for OOT runs.
Cheaper than a full 48V, and perhaps easier to manage, but would need good connectors.
 
OK at this point Buk your logic is making little sense. A 48V battery is no heavier than a 36V battery for the same energy capacity and range, so you don't have any extra weight when riding it without assistance. If you want the extra speed that 48V provides you, then should get a 48V battery. If you want this extra speed some times but not when riding around in the city, then simply setup your controller or CA device to a limited speed mode so that you aren't tempted to use it except on the longer out of town trips. DC-DC boost converters or extra 12V packs make little sense here unless you've already invested in a 36V pack, and are veering towards a path of unnecessarily over-complicating things.

If your range requirements are such that you need a lot more battery capacity for the out of town trips than the inner city trips and don't want to carry that battery weight all the time, then you can get two smaller 48V packs. Use one battery for less weight on the daily around town use and double them in parallel for the longer trips.

MadRhino said:
Sometimes my throttle wrist is connected to my balls instead of my brain, battery gets a beating.

Ha! I don't have a fix for that kind of mis-wiring.
 
justin_le said:
OK at this point Buk your logic is making little sense. ... unless you've already invested in a 36V pack ...

That's the nail on the head.

I already have a 36V pack. Delivered DOA, and written off with full refund by the supplier. Recovered through the kind auspices of this place and some patience. Effectively, £230 worth of battery for free!

So, how to make best use of that £230 worth; whilst benefiting from the extra few mph afforded by running the motor I've chosen at 48V?

(I'm on a very limited budget; so throwing money at the problem isn't a viable option.)

The DC-DC booster (ala wturber)is the cheapest option.

The 12V add-on pack has the merit of adding capacity and voltage. Which helps on OOT runs with both speed and duration.

The new 48V pack is a 'when finances allow' option; dependent upon stock performance etc.
 
Burnt75 said:
Thanks Dogman. Most helpful. Trying to decide between 3000 or 5000 motor. Would like mostly to ride about 25 to 35 mph with some fun run of about 45mph.
40kph-55kph-72kph.

http://www.ebikes.ca/tools/simulator.html?bopen=false&motor=MX4503&batt=B7223_AC&cont=cust_50_115_0.03_V&hp=0&wheel=18i&mass=110

Buy the MXUS 3000W V2 or V3 they are both excellent choices. Get the 3T and lace it into a 17" motorcycle rim, with Shinko 244 tire. Install suitable battery which will be 20S 72V(nominal) 84V(max) for your 45mph requirement. The smaller wheel diameter allows you to keep everything cooler. Take a look at the motor simulator, fits your needs perfectly.

This is with a 5% grade added, and you are still flying fast, over heat in 39 minutes, at 62kph which is like 38mph.
http://www.ebikes.ca/tools/simulator.html?bopen=false&motor=MX4503&batt=cust_82_0.2_8&cont=cust_50_115_0.03_V&hp=0&wheel=20i&mass=110&grade=5

Controller - If you want to save money, buy the ebay seller hksunwin he sells 72V 1000W on up are all the same, there is no difference between them.
https://www.ebay.com/sch/m.html?_odkw=48V&_ssn=hksunwin&_armrs=1&_osacat=0&_from=R40&_trksid=p2046732.m570.l1313.TR12.TRC2.A0.H0.X72V.TRS0&_nkw=72V&_sacat=0

Where to buy your battery and torque arms - www.ebikes.ca
 
markz said:
Burnt75 said:
Thanks Dogman. Most helpful. Trying to decide between 3000 or 5000 motor. Would like mostly to ride about 25 to 35 mph with some fun run of about 45mph.
40kph-55kph-72kph.

http://www.ebikes.ca/tools/simulator.html?bopen=false&motor=MX4503&batt=B7223_AC&cont=cust_50_115_0.03_V&hp=0&wheel=18i&mass=110

Buy the MXUS 3000W V2 or V3 they are both excellent choices. Get the 3T and lace it into a 17" motorcycle rim, with Shinko 244 tire. Install suitable battery which will be 20S 72V(nominal) 84V(max) for your 45mph requirement. The smaller wheel diameter allows you to keep everything cooler. Take a look at the motor simulator, fits your needs perfectly.

This is with a 5% grade added, and you are still flying fast, over heat in 39 minutes, at 62kph which is like 38mph.
http://www.ebikes.ca/tools/simulator.html?bopen=false&motor=MX4503&batt=cust_82_0.2_8&cont=cust_50_115_0.03_V&hp=0&wheel=20i&mass=110&grade=5

Controller - If you want to save money, buy the ebay seller hksunwin he sells 72V 1000W on up are all the same, there is no difference between them.
https://www.ebay.com/sch/m.html?_odkw=48V&_ssn=hksunwin&_armrs=1&_osacat=0&_from=R40&_trksid=p2046732.m570.l1313.TR12.TRC2.A0.H0.X72V.TRS0&_nkw=72V&_sacat=0

Where to buy your battery and torque arms - http://www.ebikes.ca


Was looking at that motor. Qulbix 140 recommended a 19 inch wheel. Thanks for info.
 
5000w motor would only be needed to go 50 mph +.

It will be quite possible to run either large motor on less, using controller amps limiting, and still be quite efficient at 15 mph, 20 mph. The big motor will only tend to suck more on start up.

I use the amps limiting a lot on my 2000w bike, because I also have that bad right hand. Mine was so bad I had to give up motorcycles entirely for 35 years, or die. So when I want to go farthest, I'll set my controller as low as 5 amps. (60 volts, so 300w to get max range) Typically I run about 1000w, to keep my top speed legal.

FWIW, I rode a 10000 rear hub bike once. It kind of sucked how hard it was to keep the front wheel on the ground. You kind of had to hang yourself way out over the bars to keep it down and steer at all.

A less touchy throttle would have helped, this bike was kind of like off, or 3000w, in the first crack of the throttle. ( on 10,000w controller unlimited) Fun on a straight road though, where you could just wheelie a while.
 
High power does require proper geometry, or some mean of throttle taming, or both for some.
I believe in balance: You need to be able to lift or drift at will, with minimal body displacement.

Yet, bikes can be built for a specialty, thus very difficult to lift, or impossible to drift without front braking. Being in balance you are not optimal for either, but you double your options and cut your reaction delay in emergency situations. The more power you have, the more details need to be improved. You can ride 30 a frame that has the tail 2mm off to one side, but at 60 it will lay you down anytime when you brake hard or get out of a turn aggressively. You can start 2 kw on a frame that is too short, but at 10 kw it will double back flip unless you are kissing your front tire. :wink:
 
Yeah I bet your bikes ride better than that one I took a spin on.

It just needed to be turned down to about 3000w for any kind of dirt riding, or have a better throttle for less than 60 mph. Just needed a three speed switch at least.

On the street, the wheel came down fine when you got to about 60 mph. You just needed a quarter mile of straight road to grab the gas.
 
I begin to be satisfied with a bike I am building, when I feel safe leaving the handlebar and look behind at 50 Mph. 8)

I am very happy with it, when I can throttle in acceleration keeping my front wheel 2 inch off the ground for 100 ft, and brake with my rear wheel 2 inch off the ground for 30 ft. :twisted:
 
Agree, that sounds heavenly. I just meant that more is not always better, if the result is a 3 foot wheelie, barely touching the throttle.

3000w will rarely do that, but I did build a spectacularly poorly balanced ride once that did wheelies on 1000w. 3000w will be plenty of power for a fast bike, without getting too much into sensitive throttles on full power.

He can still choose the 5000w motor, I don't object. I just say his stated needs will be met by the smaller one.
 
Back
Top