question about amps and voltage regarding speed and torque

[tomtom123]

As a noob, reading up on batteries on this forum and wiki. There's one overwhelming theme, which is that voltage gives your motor more power/speed and amps will give you more torque/hill climb ability. Why is that?
Would a 36v 20ah given you the same speed or power as a 48v 15ah battery?
And would a 48v 15ah battery give you the same torqueand hill climb ability as a 36v 20ah battery?

The reason I ask is because I want to know whether or not the difference in voltages and amps between the batteries affected their respective areas since both batteries are the same Watt Hours

 

[major]

As a noob, reading up on batteries on this forum and wiki. There's one overwhelming theme, which is that voltage gives your motor more power/speed and amps will give you more torque/hill climb ability. Why is that?
Would a 36v 20ah given you the same speed or power as a 48v 15ah battery?
And would a 48v 15ah battery give you the same torqueand hill climb ability as a 36v 20ah battery?

The reason I ask is because I want to know whether or not the difference in voltages and amps between the batteries affected their respective areas since both batteries are the same Watt Hours

You're right; same Watt hours (Wh). Let's say you have 3 Volt, 5 Ah cells. You configure them for a 36V, 20Ah battery. That would be 4P12S or 48 total cells, 4 in parallel and 12 in series. You have a total of 720 Wh of energy. Each cell is 15Wh. Times 48 cells is 720 Wh.

Those 48 cells can be configured a number of ways altering the battery voltage, but will always give you a total of 720Wh. Like 3P16S for a 48V, 15Ah battery, still 720 Wh in energy. Or 6P8S for a 24V, 30Ah battery, still 720Wh.

If the cells were rated at 1C continuous and 2C peak, then you would have 720 Watts (W) of continuous power and peak power of 1440W regardless of how you configured your battery; 48V, 36V or 24V.

How does this relate to speed in the bike? Though the motor and controller. Let's assume the chain ratio and wheel size are fixed in the same bike. Then for the 48V battery, you need a 48V motor and controller rated at 15 Amps (A) continuous and 30A peak, say at 1000RPM. And for the 36V battery, a 36V motor and controller rated at 20A cont and 40A peak. For the 24V battery, 24V motor and controller rated at 30A cont and 60 peak. All at 1000 RPM. All combinations would give you the same performance, speed and gradeability, and range. Battery size would be the same, afterall, it is the same number of the same cells. Motor would be essentially the same size, just wound differently. The controller may need to be a bit larger for the lower voltage due to higher current and the cables will need to be larger for the higher current at the lower voltage. The higher voltage systems might be a bit more efficient and a bit lighter weight. But nothing drastic.

Now the theme you refer to 'higher voltage--->more power/speed and amps--->more torque' relates to changes to the system while keeping the motor the same. If you raise the voltage to the motor it will rotate faster and higher RPM at the same load will be higher power (P = Torq * RPM). It is also likely that raising the voltage to the motor and making it go faster will increase the speed of the bike causing the load to increase thereby drawing more current. This then adds to the power. So increasing voltage to the motor increase speed and power. Often an easy way to burn out a motor.

The second part of your stated theme is increasing motor torque by increasing motor current and this will result in better hill climbing and faster accelerations. Typically this would be done by increasing the current limit in the controller, or getting a bigger (higher rated) controller. Here again, the motor power will increase due to increase in torque (P = Torq * RPM).

Keep in mind that C-rate of the battery when you start increasing power/speed/torque of the motor. The system has to work together or parts of it will suffer. But say for example purpose you had cells which could tolerate much higher C-rates. Then if you used the 48V battery on the 36V motor, you'd increase your speed and power considerably over using the 36V motor with the 36V battery.

On the other hand, if you were to use the 36V battery and controller on the 48V motor, the extra current would give you additional torque. But you would suffer lower speeds. And the motor may not tolerate the extra current well.

 

[dogman dan]

Part of your confusion is the typical noobs blurring of the meaning of amps, and amp hours.

It gets confusing at first because the EV has two places that limit amps ( the battery, or the controller), plus there is amp hours which refers only to the capacity of the battery.

Battery capacity though, affects the batteries limit of how many amps it can put out without damage or excess sag. Pretty soon you start thinking of amp hours as amps, and off you go into confusion.

Took me at least two months of reading the forum to even begin to understand the basics. I was pretty uneducated about electronics in 2008.

A 48v 15 ah battery will nearly always give more torque and speed than a 36v 20ah. If the controller is 25 amps or less. Why? Because in most cases a 15 ah battery is big enough to run the controller, so there is no excess voltage sag or battery problem with using the 15 ah size. So in this particular example, 15 ah or 20 ah doesn't make much difference. The controller still can get all 25 amps of current from each one.

Then you go to the equation volts x amps = watts. 36v x 20amps controller = 720 watts. 48v x 20 amps =960 watts. So the 48v battery will provide more power. More watts will mean more torque. In addition, the 48v voltage gives you more speed.

It's not entirely cut and dried though. There is a relationship between wind resistance and wattage. So it could be possible to add voltage, but get less more speed while riding than you expected because the increase did not overcome the huge increase in wind drag as you go faster.

I'm sure I could keep writing , and confuse you even more. It just gets worse and worse, If you assume A B and C, then D will happen and all is well, but change C and the bike melts or the battery dies. Eventually, all answers become, "It depends on".

 

[major]

A 48v 15 ah battery will nearly always give more torque and speed than a 36v 20ah.

The 2 batteries are the same in terms of power capability. If you get more speed and torque (which is more power) from one or the other, it is due to the motor/controller choice and you'd increase the C-rate from the battery.

 

[drebikes]

I just wanted to add a few things on the motor control. It's being said that more Ah means more amps which leads to more torque; as well as higher voltage gives higher speed; very briefly, here's why it is:

*higher road speed, means more rotational speed in the motor, which means more backEMF - when the backEMF, which is linear with speed, reaches a value equal to the battery, no more power transit is possible. Imagine you have two voltage sources that oppose each other, one is the bEMF and one the VDC - if equal electrons don't pass from one to the other. The bike works in motor-mode when battery VDC is superior to the bEMF - so higher voltage means more margin versus the bEMF so higher MAX speed. If you have enough power to overcome friction, hills and wind the absolute speed is higher with higher voltage.

*at low speed the motor is torque-limited. As major said P = Torq * RPM - at low speed RPM is low. To transfer a lot of power to start fast, you need lots of torque; and torque pretty much equals Flux (so bEMF) x Current. bEMF is low due to low RPM, so magnetic flux is low, so all you have to give you torque is the current. For the same chemistry (say Lifepo 2C continuous discharge) a 20Ah battery gives you max 40Amps and 15Ah gives 30Amps; 40 is bigger than 30, so more torque with the 20Ah battery regardless of voltage. As you accelerate though, the bEMF rises so to achieve max torque you no longer need max current.

To sum up: to start fast you need lots of torque/current, to achieve max speed lots of volts. Just be careful not to toast the motor at low speed with all those Amps flowing, losses and thermals go with the square of current. And Ah is battery capacity, A(mp) is current, don't mix them you'll get the hang of it.

 

[major]

bEMF is low due to low RPM, so magnetic flux is low,

Hi drebikes,

Pretty good post, but..... Most motors on these ebikes use PM (permanent magnets) for the field, so the flux is essentially constant and not dependent on speed or bEMF.

 

[drebikes]

bEMF is low due to low RPM, so magnetic flux is low,

Hi drebikes,

Pretty good post, but..... Most motors on these ebikes use PM (permanent magnets) for the field, so the flux is essentially constant and not dependent on speed or bEMF.

Still, the variation of the magnetic flux gives the bEMF(dPhi/dt), not the magnitude flux itself, which is magnetic potential multiplied by speed. Magnets create induction not flux, it's the passage of induction thru a winding that is considered a flux I may have oversimplified, but everything else is oke.

cheers

 

[major]

bEMF is low due to low RPM, so magnetic flux is low,

Hi drebikes,

Pretty good post, but..... Most motors on these ebikes use PM (permanent magnets) for the field, so the flux is essentially constant and not dependent on speed or bEMF.

Still, the variation of the magnetic flux gives the bEMF(dPhi/dt), not the magnitude flux itself, which is magnetic potential multiplied by speed. Magnets create induction not flux, it's the passage of induction thru a winding that is considered a flux I may have oversimplified, but everything else is oke.

cheers

I think I disagree. But this is beyond the scope of the OP so let's leave it

 

[dogman dan]

48v is more torque than 36v, if the motor and controller remain the same, and both batteries can handle the controller in use. This is because 720w is less than 960w, in the 20 amps controller example. It's about 25% more power.

Increase volts to increase speed, and increase amps to increase torque is the rule of thumb. But, only if you increased volts AND decreased amps would you end up with identical watts. So increasing volts almost always increases both speed and torque.

It's just increasing the watts another way than changing amps. But yes, if you increase volts and decrease capacity, then you do indeed increase the c rate. Whether or not that is critical depends on the exact combination of motor controller and type of battery.

 

[major]

48v is more torque than 36v, if the motor and controller remain the same, and both batteries can handle the controller in use. This is because 720w is less than 960w, in the 20 amps controller example. It's about 25% more power.

48V is not more torque than 36V. If you have the same motor and controller, the current limit would be the same and it is the current which determines torque capability, so between the 48V and 36V battery, the maximum torque would be the same. Now the 48V battery, with the same motor and controller, would give the capability for high motor RPM and higher vehicle speed which would load the motor more causing higher running torque. I mentioned this towards the end of my post.

And going from 36V, 20A to 48V, 20A is a 33% increase in power, not 25. And it is this increase in power responsible for increased performance, not the increase in voltage. A similar increase in power (and performance) could be achieved by keeping the same voltage and increasing the current along with a gear ratio change.

The original question related to the increase in voltage with the battery energy (Wh) remaining the same. Therefore it is reasonable to assume the power capability would remain the same, unless he is willing to upgrade to more powerful (higher C-rate) cells.

 

[tomtom123]

48v is more torque than 36v, if the motor and controller remain the same, and both batteries can handle the controller in use. This is because 720w is less than 960w, in the 20 amps controller example. It's about 25% more power.

48V is not more torque than 36V. If you have the same motor and controller, the current limit would be the same and it is the current which determines torque capability, so between the 48V and 36V battery, the maximum torque would be the same. Now the 48V battery, with the same motor and controller, would give the capability for high motor RPM and higher vehicle speed which would load the motor more causing higher running torque. I mentioned this towards the end of my post.

And going from 36V, 20A to 48V, 20A is a 33% increase in power, not 25. And it is this increase in power responsible for increased performance, not the increase in voltage. A similar increase in power (and performance) could be achieved by keeping the same voltage and increasing the current along with a gear ratio change.

The original question related to the increase in voltage with the battery energy (Wh) remaining the same. Therefore it is reasonable to assume the power capability would remain the same, unless he is willing to upgrade to more powerful (higher C-rate) cells.

Yes ebike.ca simulator proves this. 48v 15ah is the same as a 36v 20ah battery under same loads/variables using 20amp controller. Actually ebike.ca shows the 48v has better range at the same speeds

 

[dogman dan]

Well, something is more. Go ride 10,000 miles and then tell me you disagree.

 

[gogo]

Yes ebike.ca simulator proves this. 48v 15ah is the same as a 36v 20ah battery under same loads/variables using 20amp controller. Actually ebike.ca shows the 48v has better range at the same speeds

There isn't an equivalent 48V 15AH and 36V 20AH battery option in the drop-down menu on ebike.ca simulator, so what values did you use? In any case, I wouldn't expect the difference in range to be significant.

Well, something is more. Go ride 10,000 miles and then tell me you disagree.

Assuming the motors are each wound differently for the same top speed, performance differences shouldn't be significant in the OP's example.

 

[dogman dan]

I was assuming the same motor and controller amps, changing only the voltage. Two variables in any experiment leads to junk science.

Capacity of the battery would not affect performance much, unless the smaller one was too small for the particular controller. So that isn't a significant variable with typical 20 amps controllers. Ideally, I would have done my 8,000 miles or so of riding comparing 36v 20 ah pingbattery with a 48v 20 ah pingbattery. But I had the 48v 15 ah. The 15 ah did not sag more under load than the 20, so I felt that it was not a variable.

I don't have much in the way of a method for measuring torque out on the road. Riding that 10K miles, I had a CA for the miles that were trying to gather data.

But I did notice that running 36v on a front hub, I rarely had any traction issues with a 36v 20 ah battery. From a stop, on dry pavement, there was little to no wheel slip getting started. But with a 48v 15 ah battery of identical cells, I soon learned to feather the throttle on a start up so as to not wear out my tire excessively fast. Lots of wheel spin on a front hub with 48v.

Would this not be a crude method for demonstrating that with the same motor, same controller, but different voltages, 48v produced more torque? Once cruising at a specific speed, say 20 mph, both batteries would of course produce the same torque. They would both produce exactly enough to make the bike move at 20 mph. No more. Only at the moment of maximum load would the motor pull more torque. This is fairly obvious to anybody who has ridden up hills with various voltages on the same bike.

As for range, the range of the 48v 15 ah battery and the 36v 20 ah battery are virtually identical, if both rides are done on identical bikes,identical terrain, identical speed, identical weather, identical rider weight, and identical pedaling.

I did not find that out with a simulator. I did it on the road, over and over and over.

There was one situation where I could get better range from the 48v battery. That was climbing a very long, very steep hill. The lack of power, about 400w less, with 36v would lead to riding below 12mph with 36v, putting the motor into the more inefficient rpm, creating more heat climbing the mountain. The 48v battery did better, able to keep going at least 14 mph.

 

[jonescg]

More volts does result in more amps for the same circuit resistance; V=IR/ If R is constant, then doubling V results in a doubling of I. Since electric motors rely on current to produce a force, the only way to increase the amps in the system (short of rewinding the motor to have a lower Ohmic resistance) is to increase the voltage.

The controller will be the limiting factor in this system though, as it has a limit to how many amps it can manage before burning up.

Hope this helps the original poster.

 

[major]

The controller will be the limiting factor in this system though, as it has a limit to how many amps it can manage before burning up.

Exactly. The controller limits the current and this is independent of the battery voltage. The current limit in the controller sets the maximum motor torque. Now the experience which dogman describes about the wheel breaking traction with the higher voltage battery could be attributed to a faster ramp (increase in rate (volts/second) from zero voltage to maximum motor voltage). This would appear to be more wheel torque, but really is the same torque. I have noticed a similar effect when running in different gears with the same battery/motor/controller. A WOT pulls a wheelie in third and not in first. Obviously first gear has more wheel torque. But in third, the ramp up in the controller increases the wheel speed faster and pulls up the front before the vehicle starts to travel. I know, seems strange.

 

[dogman dan]

I agree, but it's perceived as more torque for sure. As John in CR knows well, I'm not the guy who knows how this stuff works. I constantly scramble up motor theory and have to be whacked back down for being full of crap. I'll never be smart enough to fully understand the simulator.

It's definitely more power, that why you take off faster, climb steeper hills without bogging down etc.

I guess I will never understand how more watts, more power, however you get it doesn't make more torque AT 0 MPH. Once rolling at any given speed on flat ground, then you would definitely see torque be equal, exactly as much as that given speed requires. It cannot be more, or less. You ride enough, you feel that, and see it on a CA.

The rules of thumb are still valid.

To get more torque without increasing motor rpm, increase amps.

To get more motor rpm, increase volts.

Both increase watts of course. It just seemed to me that increasing power either way increased the torque, AT 0 RPM.

 

[dogman dan]

Go look at the simulator.

It agrees with me, at 0 rpm, 48v is more torque.

9 c 2807 , 20 amps controller, 36v 14 ah ezee battery. The blue line, motor torque, is 50
change battery to 48v 10 ah ezee, and torque goes to 57.5

Nuff said. unless you want to argue that 50=57.

 

[major]

Go look at the simulator.

It agrees with me, at 0 rpm, 48v is more torque.

9 c 2807 , 20 amps controller, 36v 14 ah ezee battery. The blue line, motor torque, is 50
change battery to 48v 10 ah ezee, and torque goes to 57.5

Nuff said. unless you want to argue that 50=57.

O.K. I did that. I had not really paid much attention to that simulator but thought the curves looked strange compared to what I'm used to with larger motor/controllers using motor phase current limiting. Then it hit me. The simulator must be using controllers with battery current limit. This is only a secondary feature of some larger EV controllers to limit power and something I didn't consider with the small ebike controllers. I am surprised that the controller makers can get away with doing it that way. I did find this explanation of battery vs phase current limit. http://www.endless-sphere.com/w/index.php/EBike_Controllers

You are correct when battery current limit is used. I am correct when phase (motor) current limit is used.