Question - more volts or more amps ?

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Mike Allred   1 mW

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Question - more volts or more amps ?

Post by Mike Allred » Feb 09 2015 3:49am

Like everyone, I'm trying to get the most RANGE out of my battery pack which consists of (so far) 91 Samsung 29E batteries in a 13S, 7P configuration (at 45 kph - 28mph). Yesterday I was running a smaller 13S Panasonic test pack at the low end of its charge capacity and producing only 42V. It gave me a speed of 40kph (25mph) which is acceptable given my target of 45kph. Fully charged it gives about 52 kph but the speed limit on the controller should be set to 45kph in Switzerland. As I was flat out at 40kph (all numbers are without pedalling) I started to think that I may be wasting amps because 42V won't get me above 40kph anyway. I also thought that when fully charged, the 52V may also be wasted because I'm limited to 45kph anyway.

So I wondered if someone could help me with answers on the following questions. By the way, I'm running a MAC 10T, the test controller was an EB312 and I also have an Infineon 3077 12fet controller.

1) Are the extra volts over and above the 47V (needed to get to the 45kph limit) simply wasted ?
2) Do the (above) extra volts somehow reduce the amps needed to maintain 45 kph ?
3) Am I potentially wasting amps if I'm struggling along at 40kph, 42V and electric throttle wide open ?
4) If I were to improve my battery pack to improve range, what should I do ? Go to 14S or 8P
5) The 29E batteries are great value but I'm a little concerned about the conséquences of exceeding the recommended continuous 1C output. I will never exceed 2C but the continuous drain will be above 1C and maybe in excess of 1.5C. Any advise - apart from changing out the 29E batteries ?
6) Is the the lack of a stable volt plateau on the 29E battery somehow wasting its own ulitimate capacity by initially supplying too much V and not enough V at the end ?

I'm sure there are some very intelligent well informed people out there that can help me.

Mike

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biohazardman   100 kW

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Re: Question - more volts or more amps ?

Post by biohazardman » Feb 09 2015 4:58am

This is the way I see it
1- If your motor, controller and battery pack are not overly hot you are not wasting much of anything. But you pay in energy consumption at speeds much over 20MPH because of aerodynamic drag and pay again in rapid acceleration from a stop or just accelerating quickly when you are already moving.
2- I noticed a slight decrease in throttle was needed to maintain speed when I went from 36V to 48V so maybe a little less amperage is needed but it’s easy to go faster and accelerate more quickly when you can. I managed no improvement in my initial miles traveled with the voltage increase because I traveled faster. I did a test once and kept the speed down and did manage to get more miles out of the pack though so more battery is more miles as far as the number of cells goes.
3- Most motors are at near peak efficiency at near the top of their rated RPMs.
4- To improve range you parallel if your speed is already acceptable that is what you want to do.
5- Another reason to go with more cells in parallel.
6- If you have more than you need nothing is wasted unless you are heavy on the throttle. Not enough means you are at LVC and so long as the battery voltage is not much below specification of your cells there is no problem.

The only troubles I can see is that you may experience a bit shorter battery lifespan because of higher than rated draw. If the cells don't like it they will heat up excessively and tell you so. Once you add more cells in parallel that will be less of a problem if you have one.
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riba2233   1 MW

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Re: Question - more volts or more amps ?

Post by riba2233 » Feb 09 2015 5:36am

http://www.samsungsdi.com/lithium-ion-battery/e-bike

Samsung 29E are actually 2C continuous and 3C burst, so don't worry.

Also, you are not wasting amps/volts, controller acts as s buck converter. So, if you have the same motor current, at higher battery volts the battery current will be lower, and at lower battery volts the battery current will be higher.
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lester12483   10 kW

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Re: Question - more volts or more amps ?

Post by lester12483 » Feb 09 2015 5:55am

A simple answer to your question of range is that you battery AH and weight of the bike determine range. Speed and level terrain are also factors.

Think of it this way. volts x amps = watts.

I find 48V 20AH gives you the best of both worlds, speed and distance. You can easily get 40k on a pack like that going 20kph.
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dogman dan   100 GW

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Re: Question - more volts or more amps ?

Post by dogman dan » Feb 09 2015 12:16pm

If you just read that, I'm trying again.

No, if your go slower, both the 36v and 48v pack will still use the same watt hours per mile, to go slower.

There may be measurable differences, one voltage more efficient than the other. but the difference will be not measured in miles of range. More like a few feet more for one than the other.

bottom line. Both are pretty dang efficient, and not one enough more than the other to matter a hell of a lot. The real reason to run 48v is so when you want to, you can go faster. Or so when you need it, you can use more watts to leave a stop sign faster.

But either one at 20 kph, you won't be able to tell any difference in range at all.

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Drunkskunk   100 GW

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Re: Question - more volts or more amps ?

Post by Drunkskunk » Feb 09 2015 12:33pm

A battery with 10 volts at 100 amp hours, or 100 volts at 10 amp hours will each have 1000 watt hours. A watt is is a watt, the motor doesn't care how it gets them, and it's power and efficiency are based on the watts it uses. So it makes no difference how you feet it power.

Almost.

Most motors perform better when given more volts. If you raise the voltage but don't lower the amp limit on the controller, you can pass more watts to the motor, which can cause you to ride less efficiently. Call this the YEE HAW law or Ebikes. If you have more power, you'll use it.

Just adding more Amp hour capacity to your battery won't change it's performance much. there is a small effect as raising a batteries capacity lowers the load on the cells, so they can operate under less stress with less voltage sag. on many batteries, that's something you won't even notice unless you're logging all of your performance
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dogman dan   100 GW

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Re: Question - more volts or more amps ?

Post by dogman dan » Feb 09 2015 1:01pm

Definitely tend to use more yee haw, if you have it. Slowing down some way is how to get more range from the pack you have now.

Three speed, or 5 speed switch can help. as can things that limit amps, like a DP Cycleanalyst.

But no yee haw, when you slow down. :cry:

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Drunkskunk   100 GW

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Re: Question - more volts or more amps ?

Post by Drunkskunk » Feb 09 2015 1:55pm

Nelson37 wrote:I most likely mis-understood something I read here.

What I thought I understood was that increased voltage was either better/more efficient/cheaper or offered some advantage over increased amperage in order to get a given wattage output. I forget the context but the message was "raise the volts, not the amps". This may have only related to a specific circumstance or aspect of the system.

I do understand that a given motor will spin faster with increased voltage, along with increased wind resistance. Perhaps the increased speed was the only factor being considered. To me, "perform better" could mean last longer, go faster, go farther (use less power or run more efficiently), or run quieter, or any combination of these.

My main concern is range rather than speed, so just going faster is not necessarily a good thing for me.
This is a case where 2 seemingly conflicting answers are true, depending on context. Yes, higher voltage is more efficient. and Yes, higher voltage will gain you little and might be less efficient.
The difference is in how we apply the tech.

High voltage is more efficient in theory. Motors operate more efficiently at high speeds and at higher voltage levels by a little. Power is lost through heating less. Since the resistance stays the same, but the current needed for each watt is lower, there is less waste heat made.
But it's a small effect, and there really is very little difference between an Ebike motor operating at 24 volts or 72 volts. You need much higher voltages before you start seeing any appreciable efficiency gains.
But there is also a problem with raising the voltage too high. The electronics needed to control higher voltage aren't as efficient. For example, A 4110 FET used in 72 volt controllers has twice the internal resistance as a 3770 FET used in a 36 and 48 volt controller. And if you go for an even higher voltage range controller, the FETs get even worse. It's only a small loss in over all efficiency, but it serves to wipe out some of the gains of higher voltages.
Also consider that some of the gains of higher voltages come from designing a motor that spins faster to do the same job. But with an ebike, raising the voltage doesn't change how fast the motor spins at a given speed, only how fast it can spin at it's top speed. So no efficiency gains will happen when using the same motor.

Its all about context.
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dusan   100 W

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Re: Question - more volts or more amps ?

Post by dusan » Feb 09 2015 5:51pm

As we know, more voltage means more rpm. From electricity perspective, it's more efficient as well. But, you are human, if you pedal and you want to help motor or otherwise, you both have to find proper cadence.
And that's why lower voltage and high current could be more efficient at the end.

John in CR   100 GW

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Re: Question - more volts or more amps ?

Post by John in CR » Feb 09 2015 6:52pm

Higher A (higher current) has lower efficiency, because higher current makes more heat from the resistance in copper.

Higher Ah (Amp hours) is more battery capacity and will definitely increase RANGE as you put it. As stated by others, slowing down also increases range, and that's because wind resistance increases geometrically with speed, and above 15-20mph wind resistance is the dominant factor.

Higher V (voltage) with everything else constant is sure to decrease range. It may increase peak efficiency, but we spend so little time at peak efficiency, that I'd have to see one example of improved real world efficiency to believe it. Where is gets interesting is when you increase the voltage and decrease the gearing. Then for the same speed (same power at the wheel), current is lower, making real world efficiency higher. Because heat losses in the copper increase by the square of current, it's quite easy to achieve increased performance and efficiency (more acceleration and greater range) with an increase in voltage and gearing down. That increase in efficiency due to decreased current for the same power and speed has a further increase in efficiency, because there is less heat in the copper and lower temps means less copper loss because copper's resistance increases with temperature. This benefit is seldom discussed, but it can be significant since copper losses can increase by over 40% for the same power out from ambient temp to the stressful temps some members run their motors.

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dogman dan   100 GW

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Re: Question - more volts or more amps ?

Post by dogman dan » Feb 10 2015 8:28am

Never question it when John speaks. He's set me straight the best he can. Smaller wheels rule with hubmotors, or lower gearing with mid drive. Lots of people screw up riding mid drive bikes by selecting way too high a gear all the time. There is low hanging fruit out there for better efficiency that often doesn't get picked.

My point though, was that the difference in efficiency between two volts and amps choices that are close to each other is small. Not much change means not much difference. All else same, same motor, same wheel size, only variable the volts and amps to have the same watts.

And I was assuming we were talking about cruising, not stop and go riding. In stop and go riding, like city urban riding, you need that gearing advantage. 26" wheel and direct drive is geared mighty tall for stop and go.

Assuming it's more like non stop cruise, 2000w by running 36v 50 amps though suitable gage wire will not be so much less efficient than running 48v 30 amps, that you will see miles more range running 48v. Both, if ridden the same speed, will go damn close to the same distance. But 48v should be measurably more efficient. It won't be measured in miles though.

This idea that it doesn't matter much applies to bikes, running 1-2000w and going 30 mph or so. I don't have the on the road experience to say if it still applies to faster, more high powered vehicles. At some point, as the gap between one choice and another widens, it could be significantly different. And you start getting into hard to pin it down, with other differences in play, like gearing, or speed.

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cal3thousand   1.21 GW

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Re: Question - more volts or more amps ?

Post by cal3thousand » Feb 10 2015 1:22pm

I have to agree with everything that John and Dogman have said. But since you are not building from the ground up and motor and bike choices are locked, let's look at just the battery.

Going to 14S on a 7P battery means 7 more cells while going to 8P on a 13S battery means 13 more cells. To me, it's a no-brainer to go for 13 more cells since that is nearly twice the Wh you get from just 7 more cells.

If you DID want to have a higher top speed as well, you could tune the controller to output a 110% or 120% setting, which would increase current, but within a range that your wiring should be able to handle. Additionally, you could upgrade the portion of wiring outside of the motor to higher gauge (lower resistance) and offset any extra heat the would occur from the increased current.

Using a 3 speed switch, 100% setting will keep you at the speeds your bike achieves currently, but for a longer period. It will take longer before you dip down below 45kph. And with a flip of a switch, you could have higher speed for the occasional burst.
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Mike Allred   1 mW

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Re: Question - more volts or more amps ?

Post by Mike Allred » Feb 10 2015 4:02pm

Thanks for your contributions. I’ve drawn the following conclusions from your experiences.
1) All other factors held equal, increasing the volts will give more range or more speed or faster acceleration (torque) and potentially a bit of all. The relationship of volts and speed is theoretically linear. The relationship of volts to torque and range is less clear but may also be close to linear.
2) The relationship of amps and torque is almost linear and therefore (perhaps) the most efficient way to increase hill climbing and acceleration (which is less important), and more obviously, range.
I note that no-one proposed changing to an 8T motor. I wonder if that would be more efficient but I have doubts. The 8T motor turns very fast. The 8T on 48V has highest efficiency above 400rpm, therefore well above 50kph (26” wheels). The 10T motor on 36V is most efficient at 257rpm and I assume that on 48V, it’s most efficient at 336 rpm (257 * 13P / 10P ), therefore 42kph. Does that sound right ? The major advantage of the 10T, I think, is that I can maximise volts and minimise amps, so the 10T (vs 8T) and controller will run cooler. The torque of the 10T is probably more efficient in stop and go situations.
One problem is that I can increase volts by adding 7 cells. To increase amps, I have to add 13 cells – it’s a bigger jump in weight and volume! Of course it’s a no-brainer if range was the only consideration and consequence, and if it was, I’d add 500 cells. Interestingly, if the amp discharge per cell is reduced by 28% (7Ah to 5Ah), the voltage increases by 3% from 3.2V to 3.3V due to reduced sag. Therefore, an increase from 7P to 8P (14%) should increase the voltage (and watts) by 1.5%. That’s not much but it’s free and interesting nonetheless. Some will be lost by the 650g weight increase.
Sadly, my range will be less than planned. I’d like to see 40km nut I think it may be only 20-30km. I wonder if someone could indicate from experience how many watts are needed to maintain 45kph on the flat.
Mike

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cal3thousand   1.21 GW

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Re: Question - more volts or more amps ?

Post by cal3thousand » Feb 10 2015 5:07pm

Mike Allred wrote: I wonder if someone could indicate from experience how many watts are needed to maintain 45kph on the flat.
Mike
Use the simulator to figure that stuff out:

Grin ebike simulator

But to answer your question directly it is about 21Wh/km at 45kph on a normal upright bike which on your current pack would yield about 50km. (That's if you were just cruising and no stop/go and ideal conditions etc)
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