Which is more efficient, 36V or 48V?

[Rusty123]

I know that if power remains constant, the higher voltage will be more efficient, since there would be lower amps, and therefore lower I^2R loss. However, in the real world, it seems like high voltage = higher speed = higher power = less efficiency.

I'm planning to put a Bafang BBS0X crank drive on a recumbent, with the primary goal of long range. I have the BBS01 (350W/36V) on my commuter bike, and it provides me with adequate power, so I'm inclined to install the same unit on the recumbent. But I can't help but wonder how the BBS02 (750W/48V) would compare in terms of efficiency. (Although to be honest, I'm not totally confident that I'd have the discipline to stay out of the throttle).

Since the BBS01 and BBS02 are different motors, with different windings and such, the answer in this case might be different than if I were simply comparing different voltages for the same motor (like a hub motor, for example).

Comments?

 

[zener]

I can just speak for gearless DD hub.
More voltage + slower motor winding = more efficient

 

[spinningmagnets]

It's difficult for some riders to exercise restraint when they have a lot of power on tap, but I agree with the previous poster. If you cap the amps to the lowest possible level that works, you will get more miles from the same battery.

So...to get the same speed, use the highest volts that accomplish your goal, and then use a motor with the appropriate Kv.

 

[amberwolf]

I know that if power remains constant, the higher voltage will be more efficient, since there would be lower amps, and therefore lower I^2R loss. However, in the real world, it seems like high voltage = higher speed = higher power = less efficiency.

That depends on the motor winding, gearing ratio, wheel size, etc. Given all identical things other than voltage, then yes, higher voltage does equal higher speed for the same throttle setting. But if the system is wound/geared/whatever for the same speed at the same throttle setting at the higher voltage, then the "equation" stated is not necessarily true.


There are other factors that also affect whether it's more efficient at a higher or lower voltage, including which FETs are used in the controller. Sometimes higher voltage FETs have a higher RDSon than lower voltage FETs, and thus more losses to heat, or else require a higher gate charge to turn on, or other factors that may decrease their efficiency vs the lower voltage types.


So the answer to your thread title is...it depends, and on a lot of little factors, and some big ones.

 

[d8veh]

The voltage that's most efficient depends on your motor and the way you use it. The ideal voltage is the one that causes your motor to spin at a maximum speed of about 25% faster than you actual use it. With a crank drive, you want the max crank speed to be 25 - 30% more than your normal cadence. If you have two 90 rpm crank motors, one with 48v and the other 36v, the 48v one might be slightly more mechanically efficient, but I think you'll end up using it in a less efficient way.