Brain Teaser: 72V set-up and 48V set-up Throttling

[broloch]

In a hypothetical situation, say you had two exact same mountain bikes, each with the same exact battery chemistry, same motor, except one battery was 72V 20Ah, the other was a 48V 30Ah, and on one bike you had a 72V, 20A controller, and the other you had a 48V, 20A controller (48V controller matched with 48V battery, and visa versa), and same throttles. So if you took the first bike (72V) and somehow were able to throttle the bike so that it was the same exact speed as the 48V bike at maximum throttle, which bike would travel farther with no pedalling, wind resistance, rider weight, accessories weight, and all other factors being equal?

:?

 

[steveo]

48v 30AH

 

[Link]

48v 30AH

+1. You don't have switching and associated losses in the controller and motor when you run at full throttle; the FETs are totally on and at minimum resistance all the time. Why do you ask, anyway?

 

[broloch]

Well, I don't really know Hylian. I don't even think that I fully understand the question.
I guess this is more of the efficiency.

But if a controller was sorta variable...like listed as 24V-72V, then would both bikes travel more or less the same distance before
the battery gave in?

 

[Drunkskunk]

The 48V. Full throttle is the controller's most efficent point.


Here's another intresting twist for you, though. On your 48V setup, run a 408 motor at a given speed. On the 72 volt, run a 4012 motor at the same speed. They should both have the same top speed. Which Do you think would be more efficent?

 

[broloch]

I don't know...the 408 I guess since it is normally faster per V applied, more efficient, but the other is more efficient at torque?

But your avatar scares me.

I see the 408/4012 in your sig line.

 

[Toshi]

48v 30AH

+1. You don't have switching and associated losses in the controller and motor when you run at full throttle; the FETs are totally on and at minimum resistance all the time. Why do you ask, anyway?

How much energy is lost to "switching and associated losses" in reality? Would this difference even be perceptible? At what point does it make sense to _not_ pedal with partial assist (lower efficiency, sure, but much lower power requirements in the first place) and use full throttle instead?

 

[Link]

How much energy is lost to "switching and associated losses" in reality? Would this difference even be perceptible? At what point does it make sense to _not_ pedal with partial assist (lower efficiency, sure, but much lower power requirements in the first place) and use full throttle instead?

Depends on the controller and parts: what switching frequency the controller uses, FET switching speed, internal resistance and size of the caps, inductive properties of the motor, etc. All kinds of crap. It shouldn't amount to much, though. I'd take a WAG and say perhaps around 5% difference? :?

Unless you're running enough power to push a car to 80mph, pedalling and the slower speed will make up many times for the PWM losses.

 

[broloch]

So if I had a 36V battery at 4.2Ah, and a 72V battery at 4.2Ah, if I run the 36V battery at full throttle, and the 72V battery at half throttle (generate speeds that would be generated by the 36V throttle at full throttle), then the 72V battery would last pretty close to twice the distance as the 36V battery could, at 36V full throttle?

Maybe the 36V battery could take me 5 miles.
And the 72V set-up running at half throttle, running the bike at the exact speed as the 36V battery and bike, then I would go at about 9.97 miles, or something to that effect?

(All other factors controlled)

A matter of the Watt-hour of the battery doubled I am guessing?

 

[Link]

Generally controller efficiency is high enough compared to other factors as to be ignored (95%+). So, yeah. With a battery with double the energy you should get pretty close to double the range if kept at the same speed.

 

[diver]

Boy i just went threw this with Justen at ebikeCA. he told me that my bike was to energy wasteful at 72 volts. He said that if i did not need the speed and where i carry 6 big marine deep cycle that if i made up two packs at 36 volts that i would increase my range way over the 72 volt set up at 25 mph. Im not dropping to 36 volts but i am building one other 60 volt pack to run with this pack out of the spare 14 amp 6 volt gels i have. If i get this right of what he said. I one spun there motor up free wheel and say it spun to 50mph, and with a load on it at full throttle it only went 30 then your motor and controller combo ain't right and your wasting power. He said you need to be in the 10 mph ball park at be energy efficient for better range.

 

[cerewa]

If i get this right of what he said. I one spun there motor up free wheel and say it spun to 50mph, and with a load on it at full throttle it only went 30 then your motor and controller combo ain't right and your wasting power. He said you need to be in the 10 mph ball park at be energy efficient for better range.

For the most part, brushless motors should run at 80% of unloaded speed for max efficiency. (Assuming constant voltage...) If you go slower and keep the number of amps the same, the (battery) power consumed stays the same, the torque stays the same. But if you produce the same amount of torque at half the speed, you're getting half the amount of usable energy out!

My impression is that by reducing the number of amps (lower the throttle) when you go at a lower speed, you are able to maintain efficiency of a motor at different speeds.

 

[Toshi]

My impression is that by reducing the number of amps (lower the throttle) when you go at a lower speed, you are able to maintain efficiency of a motor at different speeds.

Play around with the simulator and lower throttle % settings... The efficiency curve is compressed to the left, basically. If your load is low enough that you're at 80+% of the unloaded speed _at this part-throttle position_ then you'll have good efficiency. Otherwise you won't.

http://ebikes.ca/simulator/