Power vs range?

[manneokoko]

Concerning the maximum range: if I have a cute motor and run it with 36V 12A and full throttle, compared to using the same motor on. 48V 16A using partial throttle so I get the same speed - Is there any difference in the maximum range?
My question is: do I benefit from using less amps and volts concerning the range per energy when driving at the same speed?

 

[Jeremy Harris]

Range is really mainly a function of speed and battery capacity, motor choice and battery voltage only has a small effect overall.

For example, let's say you have a bike and rider that weighs 100kg, the bike has MTB tyres and you ride upright. With no wind and no gradient this bike and rider combination would need the following power at each speed:

10 mph = 80 W
20 mph = 306 W
30 mph = 826 W
40 mph = 1786 W

Range will be given roughly by the watt/hour capacity of the battery pack (amp hours x volts) divided by (power needed / efficiency).

Using a higher voltage system with the same motor won't change the efficiency much, if at all, as the majority of the losses are in the motor, and these will remain the same for both cases. Using a higher voltage with a motor wound for a lower Kv may give a slight efficiency improvement, but this is unlikely to be more than a few percent.

Generally overall efficiency tends to run between 70% and 80%, maybe a bit higher if you have a really good motor and run it at it the peak efficiency point. Efficiency wiull vary a lot with torque demand though, more torque = more current = less efficiency as a general rule.

For your example it's motor current that matters, and that stays the same unless you change the motor. Battery current will reduce with the increased voltage, but battery and wiring losses are only a tiny part of the overall losses.

 

[wesnewell]

Typically, as long as you run at the same speed, you will use the same amount of battery energy regardless of anything else. The 48V battery will just have the potential for more speed ver the 36V battery. More speed= less range.

 

[dogman dan]

Exactly, given the same battery capacity in watthours, the same speed is going to take the same watthours per mile regardless of the motor type or controller amps. Sure, there are some differences that can be measured. But you need a really sensitive measuring device to do the measuring. As I like to say, an differences that there may be would be made up for by pedaling two strokes. Basicly, the energy needed to go the same speed is the same.

However, there are a few situations that would make the difference bigger.

You will kill your range, and your battery for that matter, if your larger wattage controller is too big for the battery. So you could have a 10 ah battery capable of about 15 amps that would get good range with a 15 amp controller. But put a 30 amp controller on that battery, and the battery starts to sag like grannies tits and runs out a lot faster, has the bms shut it down on hills, and eventually kills the battery.

The other situation is if you have extremely steep hills. In this case changing the motor instead of the controller, you might find one motor making more heat climbing hills than the other. So the "wrong" motor would be less efficient. This is really different than the original question, because the speed is likely to change from one motor to the other.

 

[d8veh]

The way I see it as that if you were doing an endurance race at the same steady speed with each battery/controller, you would have approximately the same range, but in the real word on normal journeys with stop/start and frequent acceleration, the 36v will have the better range because it will be working with higher efficiency provided that you don't go too fast. 48v will make it try and go faster than what it has enough torque for, so, unless you're very light, you'll get more speed down-hill and with the wind behind you, otherwise you'll be converting a lot of battery power to heat rather than motion.

12amps sounds a bit gutless. If you have a cheapo 10aH or less battery, you might not want to go for too much more, but if it were mine, I'd solder 1/3 to 1/2 the shunt in the controller to get it up to about 16 to 17amps for better hill-climbing.

 

[TylerDurden]

Not exactly the OP criteria, but an example of differences between 36V and 48V systems:

http://ebikes.ca/simulator/

 

[John in CR]

Put the 48V in a smaller wheel for an identical top speed as the 36, and the 48 will eat its lunch in every category.... range, overall efficiency, hill climbing, fun quotient, etc.

 

[wesnewell]

Something is very wrong with the simulator. Note that the 48V battery uses less wh per km, yet the sim says the 36v system has a greater range. That's impossible since the 48V systems battery has 33% more wh than the 36v system.

 

[Jeremy Harris]

Something is very wrong with the simulator. Note that the 48V battery uses less wh per km, yet the sim says the 36v system has a greater range. That's impossible since the 48V systems battery has 33% more wh than the 36v system.

Better look again...................

(clue: the 36V system uses a 14 Ah battery................)

 

[wesnewell]

Didn't notice that. So the whole test is flawed to start with since the 36v battery has 504Wh while the 48v battery only has 480Wh.

 

[Jeremy Harris]

Didn't notice that. So the whole test is flawed to start with since the 36v battery has 504Wh while the 48v battery only has 480Wh.

To be fair, TD did make it clear that this was only a comparison of the two voltages, and was not representative of the OP's whole question.

 

[dogman dan]

So do the math and convert each one to wh/mi or wh/km .

Pretty much good enough to say speed kills range. Johns comment about a smaller wheel at higher voltage is valid. In my opinion, the typical 26" ebike motor wheel they sell us is not the correct winding to use for efficiency. I'm a slow winding fanboy since I don't want small wheels. Slower windings at 48v work great if you don't want to go faster than is legal to in most places. Both result in about the same thing, better hill performance and less heating of the motor.

But assuming you have the same incorrect winding in two 26" wheels, using 400w to go about 20 mph doesn't change regardless of the motors overall size. You will tend to use more power getting to 20 mph if you have it though.

 

[TylerDurden]

Let's look at the small wheel scenario.

This time the 48V throttle is 100% and the slope is increased slightly to keep the simulator honest.


It appears the main effect is to shift the curves to lower speeds; so much so, that no useful output is generated over 32kph.

http://ebikes.ca/simulator/[/quote]

 

[dogman dan]

Yep, much like a slower winding would do. It's a more efficient way to ride when you have a stop sign every 100 feet you actually have to stop for, or you are climbing grades above 10%. You don't waste as much into heat on each start, or when climbing steep enough hills to stall a 26" wheel with a fast winding.

For me, going to a slower winding got me efficeincy two ways. One, I made less heat, but in addition slowing down really improved my wh/mi.

But yeah, at the price of a 20 mph max speed instead of a 25 mph max.