Volts or Amps. Wheres the fun at!

[Kiwi]

Here is the scenario.

You have 2 36v packs with a continuous discharge rate of 30 amps. (we can ignore ah and max discharge for this exercise).

Now, we could run them in parallel, 36v capable of 60amps continuous.
or, we could run them in series 72v at 30amp continuous.
Either configuration outputs (36x60)or(72x30) = 2160 Watts.

Lets assume the controller can deliver 36v at 60amps, or 72v at 30amps.

Obviously, there is going to be a greater speed with the 72v set up.
My question is: How does the performance compare in the speed range of the 36v and for torque for climbing.
Presuming the output is the same 2160watts, will the climbing performance be the same in either configuration?

I am asking from personal experience, or principle.

Here are graphs from the simulator on ebikes.ca, and you can see that there is quite some difference. In fact it makes no sense to me. What did I do wrong here that 36v60a only shows 800w??
But what happens in the real world? Will a 72v30amp and a 36v60amp setup climb at the same rate?

72v30amps

36v60amps

graphs over layed.

Unfortunately I don't have a 60amp controller, though I see Methods might be making some available soon.

You thoughts welcomed.
Kiwi.

 

[dogman dan]

In the real world, climbing long hills, both scenarios would give you enough power to overheat the motor. But that's another issue.

I'd go with the 36v setup if the speed of 36v was fast enough. But the 72v would be fun, and at the same speed, say 20 mph, range would be similar on both. I just think I'd tend to overheat my motor if it was a small one running on 72v or 60 amps. Presumably you have an x5? Definitely go to 72v if you have one. Since I live in the desert, I tend to be happy enough climbing a 6% hill at 15 mph, or less at half throttle if it is more than a mile long, even when riding my 5304.

 

[Drunkskunk]

Since heating is a factor of Resistance X Amps Squared, there is more waste heat from the 36 setup, But since the 72 volt setup would run twice as fast, to run at the same speed as the 36 V setup, it would have to run at part throttle, and in the least efficent range of the controller.

I have a multi voltage setup, and found both work well, but the higher voltage is more fun.

 

[Lock]

"Performance"... includes brushes (maybe.) Just that overvolting (?) brushes will burn them up fast... YMMV
tks
Lock

 

[dogman dan]

Yes, the 72v would be faster up the hill, and stay in the efficient sweet spot. 36v at full throttle on steep hills will start to stall and make a lot of heat. Since the graphs are showing an x5 motor, I wouldn't be running 36 volt if I had access to 72 . The x5 can take a bit more abuse heat wise so I wouldn't be worrying much about it. Even high volt doc doesn't melt his x5.

The least heat is likely to be running the motor at 36v, AND slowing down so the pedaling can assist the motor so it stays in the sweet spot, preventing stalling. That way there is less heat per minuite, even if more heat is actually created over the whole climb. The extra time lets the temperature at the top of the hill be less in the end. This would only apply to a long long hill. Motor heating on short hills is not such a big deal.

But yeah, a clyte x5 needs at least 48v and 40 amps, and that's barely getting warmed up to what it can do. I'd love 72v of headways for my x5. 72v would be were the fun is at.

It just depends on the end goal. Justin rode across canada on 36v. Slower and steady was what he needed for that one.

 

[Kurt]

I don't think running at 72v with the intention of restricting your self to 1/2 throttle is an easy thing to do. Its so tempting to use the extra power. Especially when taking of from slow speeds. At 36v I like the fact I can run my 5304 at full throttle and have the voltage limit me to 40kmh . The wattage tapers off to around 600w once your moving at max speed on the flats. You can peddle along with it in top gear if your keen to.

I find my whr km at 36v to be around 15 to 20. At 72v I struggle to keep it under 30 to 35. Its easy to say that 72v it just as efficient as 36v. If you ride at the same power levels but the thing is you don't. Well I don't anyhow I allways seem to want to dip into that extra wattage.

Mind you this comparison is done at the same controller amp rating of 48amps .

I might have a different objective with my ebike than others. Range is important to me. I want at least 35km range no matter how I ride and 20ah of battery is needed to give me this. If I was t run my pack at 72v. I have tyred it before for a short time .In this con fig its only a 10ah pack. To give the battery's a easy life its only 7 or 8 usable Ah.

Guys like Doc that run at high voltage also have the capacity to back it up. I think he has 23ah .

Kurt

 

[dnmun]

most controllers will not run at 72V. you will have to upgrade your 36V controller to run at 72V.

http://endless-sphere.com/forums/viewtopic.php?f=2&t=10222

 

[317537]

I'd imagine how thick your coil core and magnet width and robustness, gearing and wheel ratio makes the difference.

Higher voltage has a longer throw and if you system is up to holding the current and can through put enough wattage, longer throw can equate to more power up hills or more heat loss. It depends on your motor and controller. A planetary geared hub or cyclone system linked to your rerailer would obviously give you much more usefull torque at higher voltage than than anything with good supply current and no gearing IMO.

 

[Kiwi]

Well, to answer my own question I did the test today.
I have a 72v48a crystalyte controller and 2 36v15ah v2.5 packs 30amp cont feeding an x5305

first 36 volts at 40amps, went up the hill at 14.6 km/h
then 72 volts at 20 amps, went up the hill at, you guessed it, 14.6 km/h.

Only differences i noticed, is it seemed smoother with 72v. Like it was just cruising, but at 36v sounded deeper like it was working. And more snappy throttle from stand still in 72v.

Now, I dont have a CA yet, but do have an amp meter, so my thumb is the current limiter, so i up it to 30amps, cool..then 35..now i know the batteries can give a little...pink..the ping cuts out.
I expected that.
I do have 4 36v15ah batts, and have been running it 72v 48a, and its fantastic fun, just a bit heavy and over capacity for the trips we do. So I was looking to cut it back to two batteries.
I plant to run it 72volts 35amps using the CA to limit.
I have been talking to Ping batteries and they have there new v3 batteries that put out 3c. Currently he is building with 10ah cells, but he says he is changing to 5ah cells to be able to offer different sizes.
He will soon have a 36v15ah that can deliver 45a cont, 100amp peak. I would be happy to have 2 of those and run them at 72v 48amps. 10kg.

Oh, and thanks to all who commented.

 

[317537]

Very cool testing.

You’ll find the current draw and the voltage was equal by the time its converted to watts in your motor coils.

But what if you were to use adjustable gearing on the output to the torque vs speed ratio. Maybe thats why formula one engines use higher rpm and much smaller motors than prime mover trucks with diesel motors, silly comparison yet two totally different needs. That RPM can be converted to torque and vice versa but there is a point where high RPM and voltage will be a better application to ebike use than the lower voltage at higher current. Both would require gearing to compete with each other yet the higher current motor would no doubt have to be heavier to deliver the torque needed to match the speed. Even the mounting needs to be larger, Small high rpm RC motors do have their advantages and their disadvantage could be heat and bearing wear and tear.

The prime mover truck is designed for performance over the long haul and the F1 is designed for performance over a shorter time. This maybe what you will encounter running the motor at higher voltage attempting to gain similar torque than the lower voltage settings. I think some pedal power would make some amends to what one would encounter at 72v.

It's interesting you found the 36v better on take offs as this would indicate to me that 36v on your motor has more torque than at 72v within this throw/speed ratio. Providing there is some speed involved, say over 10kph the energy and power is being used efficiently as the magnetic throw of the coils at both voltages is within your motors dimensions and tolerances.

The thing that confuses me is the smoothness experienced on the hill at 72v. My guess is that the motor is not as well equipped to the lower voltages and the sweet spot is in between 48v and 72v somewhere, and 14kph is about as good as you gonna get under this load at 36v.

It would be interesting to see a similar test done with 48v @ 45a @ 2160watts to see if your test created equal inefficiencies on both sides of the scale. I would guess you would yield a different result and possible around 48v and 60v just might be the target for you motors best efficiency at hill climbing..

 

[jkirkeboe]

I find my whr km at 36v to be around 15 to 20. At 72v I struggle to keep it under 30 to 35. Its easy to say that 72v it just as efficient as 36v. If you ride at the same power levels but the thing is you don't. Well I don't anyhow I allways seem to want to dip into that extra wattage.

I might have a different objective with my ebike than others. Range is important to me. I want at least 35km range no matter how I ride and 20ah of battery is needed to give me this. If I was t run my pack at 72v. I have tyred it before for a short time .In this con fig its only a 10ah pack. To give the battery's a easy life its only 7 or 8 usable Ah.

Pedaling is surely the thing for range then. On my morning commute, doing 19mhp on average (23mph top speed), my Ezee 38.4v (2.4v NiMh booster pack) setup draws 10-11 Wh/km (16-17 Wh/mile). This is with a low to moderate effort pedaling so I don't sweat.

 

[dogman dan]

Signifigant pedaling does increase range.

Slow speed increases range the most. Partly this is because you may simply be able to help with pedals at a slower speed if you have a fast motor, and or, the contribution of the total output from pedaling may be a bigger percentage as the motor's wattage output decreases. Example, you can pedal 100 watts for a long distance. If the motor puts out 500 watts, your contribution is 1/6. If at slower speed the motor puts out 300 watts, your 100 watts is 1/4 of the total.

The real biggie is wind resistance, that is simply much much much less at 15 mph compared to 30. I get my max range at about 12 mph, can't stand to ride slower, and is about the speed I could pedal a motorless bike for a long distance. With little effort, I can use a motored bike to go the same speed, and I'll get nearly double the range of full throttle riding at 25 mph. I also get double the distance before my motor gets hot riding slower.

The trick to increasing range at any speed, is to go a certain speed, and then pedal enough to go faster by 2-3 mph. This will lower the amp draw on the motor quite a bit. It works the same as riding down a hill.