2 Mode Controller

[salsa]

"Each cell has to carry the same load either way."

This is where we differ. When you get 10 amps at low speed from 36 volts, (If you parallel the batteries) each cell only delivers 5 amps. If you have them in series at 72 volts, each cell delivers 10 amps.

When you exceed the 36 volt speed, switch to 72 volts. You save half the Amp-Hours when you are in the parallel mode.

Don

[xyster]

"Each cell has to carry the same load either way."

This is where we differ.

Ok. At least we're clear on where we disagree.

I'm going to use my system for this example. Let me know if you think it fundamentally different for other systems. I'm going to define each parallel bank of my batteries as putting out 4 volts, though the range is 3.6-4.2.

1) I have 20 banks of 15 cells. Each discrete bank of 15-cells is wired in parallel. I like to call these banks "subpacks". When these banks of paralleled-cells are wired together in series, I call that my "pack".

2) Each cell is 2.2 amp-hours and 4 volts. Therefore each subpack has 33 amp-hours of capacity (15 x 2.2 amp-hours), at 4 volts.

3)I could wire two subpacks together in parallel to create a 66ah 4 volt super-subpack, and I since I have 20 subpacks, I could do this 10 times, wiring 10 "super-subpacks" in series to create a 40-volt 66ah "pack".

4)Or I could wire all 20 subpacks together in series to create an 80 volt, 33ah pack.

5)Either way, my pack is the same watt-hours of capacity. Let's see how many:

a) 66 amp-hours X 40 volts = 2,640 watt hours

b) 33 amp-hours X 80 volts = 2,640 watt hours

this makes sense because I've got 300, 2.2ah 4 volts batteries either way and 300 X 2.2 amp-hours X 4 volts = 2,640 watt hours of energy.

With me so far? Are we in agreement so far?

5)So let's say I'm cruising down the road at 20mph with my 66 amp-hour 40 volt pack. That bike calculator, and my on-board ammeter, both tell me I need to suck from my pack a steady 10 amps to maintain this 20mph.

Agreed?

6)This means that each 30-cell "super sub-pack" is contributing 10 amps at 4 volts and each cell is contributing 1/3rd of an amp (10 amps / 30 cells). The volts then add together for 40 volts at 10 amps which equals 400 watts.

Agreed?

7)So each cell in this 40 volt 66ah pack is contributing 1/3rd of an amp.

8 ) If instead I'm cruising down the same road at the same 20mph with my 33 amp-hour 80 volt pack, that bike calculator, and my on-board ammeter, both tell me I need to suck from my pack a steady 5 amps to maintain this 20mph.

9) This means that each 15-cell sub-pack is contributing 5 amps at 4 volts, and each cell is contributing 1/3rd of an amp (5 amps / 15 cells). The volts then add together for 80 volts at 5 amps which equals 400 watts.

10)So each cell in this 80 volt 33ah pack is contributing 1/3rd of an amp.

Line 7 = Line 10, and would for any battery chemistry and arrangement where the voltage is halved and the paralleled-cell capacity is doubled (meaning total energy available for the entire pack is constant).

[Ypedal]

The equalizer in your " Specific " situation is that your motor runs better at higher voltage.... 5 amps at 80v.. or 10 amps at 40v... equals things out.. so the amp load on each cell is the same..

If you max out the controllers max amps, then it makes a difference. As the pack has to provide 20 or 40 amps depending on the controller.. at what ever voltage.

[xyster]

The equalizer in your " Specific " situation is that your motor runs better at higher voltage.... 5 amps at 80v.. or 10 amps at 40v... equals things out.. so the amp load on each cell is the same..

All motors are the same in this respect. Doubling the volts and halving the amps gets the same amount of power to the motor. When there's a load (as there always is to varying degrees) it's the power that's proportionate to speed, not just the amps and not just the volts. The hubmotor simulator also clearly demonstrates this.
http://www.ebikes.ca/simulator/
Find the peak power for your system, and then plug that number into the "bicycle speed and power calculator" to find the top speed you can expect to achieve on a given incline:
http://www.kreuzotter.de/english/espeed.htm

Notice the calculator does not care the amps/volts, only their product, the power.

[fechter]

There is a tiny advantage to running a lower voltage if you're going slow. The greater the difference between the pack voltage and what the motor's getting, the more losses there are in the inductor (controller).

I don't think the difference is very big, and usually not worth the bother of a dual voltage switch.

[safe]

Current Limits are often associated with motor limitations and battery limitations. If you want to boost low end torque and your motor can handle the extra amps then the lower voltage / higher amps scenerio might give an advantage.

It comes down to "powerband shape".

For the same total power output in "Watts" you can either have a powerband that is powerful at lower rpms or one that peaks late in the rpms. Efficiency goes up with rpms so from the efficiency standpoint the high revving, high voltage, low amps scenario is better. But for sheer low end torque (that's shifted because of the fact you are further up the parabola of power) you are better off with less voltage, but more amps.

Gears allow you to use the "peaky" powerbands better. You get higher efficiency and you "find" torque with the gears and not the motors powerband.

Do you want an Indy Car or a Tractor?

[salsa]

"8 ) If instead I'm cruising down the same road at the same 20mph with my 33 amp-hour 80 volt pack, that bike calculator, and my on-board ammeter, both tell me I need to suck from my pack a steady 5 amps to maintain this 20mph. "

You either have a magic motor or a magic controller.

If your motor is magic, it is a dual winding motor that magically switches from parallel mode when you have 40 volts on it to series mode when you have 80 volts connected to it. That way you would have 5 amps in each winding at your 20 mph condition and you would efficiently use your batteries would be used efficiently.

If you have a magic controller, It has a current multiplier (RC circuit) that takes the 5 amps from the 80 volt batteries and changes it to 10 amps before it sends it to the motor. This has been discussed before, but I have not seen a controller that claims to do it.

You can get a hint of what happens in the motor by looking at the performance numbers at the Crystallite web site. The data is not what you need to compare the performance because they do not have part throttle performance. The best you can get from the data is that torque is proportional to current (friction losses and winding losses not included).

The calculator only gives you what actual (delivered) power you need at 20 mph, so it does not care how you get it. You can't use it in your comparison.

Your volt times amp calculation only tells how much power is taken from the battery. It does NOT tell how much power the motor delivers to the road.

Now tell me about your magic motor and magic controller.

Don

[xyster]

You either have a magic motor or a magic controller.

Now tell me about your magic motor and magic controller.

Don

I've done all the explaining I can do. It's simple, practical math at work, not magic. Fechter noted there's a slight advantage at low speeds to going with low voltage/high amperage versus high voltage/low amps.

There is a tiny advantage to running a lower voltage if you're going slow. The greater the difference between the pack voltage and what the motor's getting, the more losses there are in the inductor (controller).

I don't think the difference is very big, and usually not worth the bother of a dual voltage switch.

[salsa]

The "Hub Motor Simulator" is an idealized 100% efficient "Magic controller".

Who sells them????

Don

[safe]

It's math.... that's all.

Go back and look at the charts I posted. For the same power output (and the same motor) you get different "powerband shapes". That's all there is to it! You lose a little more in efficiency with the lower voltage, but you get more power at lower rpms.

[fechter]

"8 )
If you have a magic controller, It has a current multiplier (RC circuit) that takes the 5 amps from the 80 volt batteries and changes it to 10 amps before it sends it to the motor. This has been discussed before, but I have not seen a controller that claims to do it.

Don

All PWM controllers do this when they are at partial throttle.

The power needed to go a certain speed is a fixed number of watts.
The current needed by the motor will be the same regardless of pack voltage.

The current drawn from the pack will be roughly half if the voltage is doulbled. 5 amps from the battery will be 10 amps at the motor. At the higher voltage, the throttle setting will be much less to get same speed.

At reduced duty cycles (less than full throttle) the controller / motor work like a buck switching power supply. The current can be multiplied several times when the output voltage is much less than the input. At full throttle, the current input = current output.

Another way to think about it is by looking at the power. To go 20mph, let's say it takes 400W.
80v @ 5a = 400W
40v @ 10a = 400W

Since the controller isn't getting hot and dissipating a bunch of watts, power input = power output. Of course there is some loss, but it's a small percentage.

[barts]

Indeed, ignoring I*i*r losses it doesn't make any difference if you run half voltage and double the current, the result is the same. There are significant advantages going with higher voltages, however; the loses are significantly less. There's a reason the ev1 ran at 300V.

The trick is, of course, that the motor needs to deliver the right speed at the desired voltage. It cannot help efficiency to run at 10% duty cycle all the time. As to the current mode throttles, this seems like a really good idea for these over-voltaged motors. When doing precision motor control systems in grad school, we would use a motor with a current amplifier, fed w/ an analog velocity error signal. The current amp simply had a shunt in the output across which the current was measured; this was fed into the - side of the op amp and the input velocity signal into the + side.

I haven't looked at the circuit on these controllers, but unless these devices are pure digital (including throttle) one should be able to do something similar very easily.

You may find the best drivability to be a mix of voltage and current; capping the maximum speed of the motor at low throttle openings will make it feel more like an IC engine than a pure torque source.

- Bart