Controller performance and battery consumption on hills?

[Fordulike]

I understand that hub motors like to run at fairly high speed to gain optimum performance and efficiency.
In this scenario there are two similar bikes. Both using the same battery and hub motor, but one has a 20A controller and the other 40A.
Now if both were to tackle the same steep hill on full throttle only, which would use the least amount of battery power?
I'm kind of guessing that even though the 40A controller has the potential to draw more amps, it would be in the high efficiency range more of the time and it wouldn't draw its max all the time.
The 20A would bog down more and therefore be drawing it's maximum amps most of the time.
There is also the time it takes for each set-up to reach the top. The quicker it achieves this, the quicker the motor and battery has a rest.
I could be utterly wrong about this, but is more better in this case?

 

[flecc]

I think too much depends on the battery in this comparison, to give any accurate controller performance assessment we would have to assume a theoretical battery that could deliver any scale of current with equal freedom.

In practice with commonly used batteries, the stress the 40 Amp controller placed on the cells could result in a level of chemical exhaustion that left the battery with less residual charge at the end of the climb than if the 20 Amp controller had been used.

And of course it's that residual charge at the end of climb which defines the system's relative efficiency.

 

[Fordulike]

I think too much depends on the battery in this comparison, to give any accurate controller performance assessment we would have to assume a theoretical battery that could deliver any scale of current with equal freedom.

In practice with commonly used batteries, the stress the 40 Amp controller placed on the cells could result in a level of chemical exhaustion that left the battery with less residual charge at the end of the climb than if the 20 Amp controller had been used.

And of course it's that residual charge at the end of climb which defines the system's relative efficiency.

Yes sorry, the battery would be along the lines of an A123 pack, which should easily provide 40A up until its BMS cut-off.
I realise a lot of this is purely theoretical. But in the real world, hill climbing ability coupled with lower battery consumption, is probably at the top of most people's list.

 

[flecc]

In that case, I'm inclined to say that probably neither would have the advantage, the efficiency gain of the 40 Amp system balancing the higher consumption on any given gradient of substance.

 

[Fordulike]

In that case, I'm inclined to say that probably neither would have the advantage, the efficiency gain of the 40 Amp system balancing the higher consumption on any given gradient of substance.

Thanks flecc. It's always good to get your opinion.
I suppose the downside is that with the 40A controller, one would be less inclined to pedal assist, therefore negating any benefits.

 

[flecc]

That's certainly the effect this has on me. I only really get any workout on the Panasonic type of systems where they make me do my share. With powerful hub motors I just get lazy, partly because keeping a low throttle setting on bumpy roads is so difficult.

 

[Deleted member 4366]

That's certainly the effect this has on me. I only really get any workout on the Panasonic type of systems where they make me do my share. With powerful hub motors I just get lazy, partly because keeping a low throttle setting on bumpy roads is so difficult.

If you were operating in a vacuum and were using super-conductors, it would be approximately even-stevens, but you're not. The higher the current, the more heat you produce in the wires, and the higher the speed, the more are the losses due to rolling and air resistance. Therefore, the lower speed motor will use the least battery as long as the motor is not working right out of it's normal range.

20 amps is enough to get most people up most hills without much effort. 25 amps will get you up most hills without pedalling - even if you're a heavyweight. 40 amps is overkill. for which you'll need fatter tyres and you'll probably still fall off regularly. That's my opinion for what its worth! I've done it and got the T-shirt and the scabs to prove it.

 

[Fordulike]

Thanks for your replies
Slightly off the topic.
When a running hub motor is not allowed to turn, as in stalled on a hill, would the current draw from the battery be the maximum rating of the controller, or does it in effect short the battery?

 

[flecc]

If you were operating in a vacuum and were using super-conductors, it would be approximately even-stevens, but you're not. The higher the current, the more heat you produce in the wires, and the higher the speed, the more are the losses due to rolling and air resistance. Therefore, the lower speed motor will use the least battery as long as the motor is not working right out of it's normal range.

20 amps is enough to get most people up most hills without much effort. 25 amps will get you up most hills without pedalling - even if you're a heavyweight. 40 amps is overkill. for which you'll need fatter tyres and you'll probably still fall off regularly. That's my opinion for what its worth! I've done it and got the T-shirt and the scabs to prove it.

Fair comment, I'll leave the falling off to you though, at 75 I can't risk snapping bones!

 

[Scottyf]

As soon as the hub motor starts to slow down the controller feeds more current.
At stall motors make there greatest amount of torque using as much current as possible

 

[Fordulike]

As soon as the hub motor starts to slow down the controller feeds more current.
At stall motors make there greatest amount of torque using as much current as possible

Would the stalled current be the maximum rated output of the controller?

 

[Kudoscycles]

Pulling more power by uprating the amps of the controller seems to me a negative loop...the extra current can only be fed by higher power batteries,which are so heavy as to need more power to drag themselves up hills,also the current drain is such that these big batteries are needed to give an acceptable range.
The current drain introduces other problems such as hot wires,higher amp battery fuses and holders and higher loads on all the bike systems....we experimented with pushing higher current through the controller and it just made the motor noisier,reliabilility suffered and the range plummeted...all diminishing returns and I think other manufacturers are risking their reputations by pushing the extremes to achieve limited additional hill climbing power.
Dave
KudosCycles

 

[NRG]

Would the stalled current be the maximum rated output of the controller?

More or less, I've found in practice though peak current is reached with the motor in use on a steep hill....

 

[Scottyf]

Depends on the batteries but generally the peak amps are pulled at the lower speed rpm.
Torque is linear with amps.

If you have a controller that reaches peaks of 10amp and you increase it to 20amp with the same hub you double the torque. Unfortunately you don't just double the heat it increases 4 fold.

But on the legal bikes as kudos says there comes a point where you don't really need all that torque at the lower rpm anyway. It would help acceleration. At the cost of range and life of batteries