Higher voltage and efficiency
 

Started this discussion somewhere else, but have wanted input from here on this for awhile. It's my opinion that higher voltage really is only more efficient for higher wattage.

Quote from Slimthelineman. But I have seen others say the same thing, and claim greater gains. I know this example is of different vehicles, just using his #'s as examples. Also there is the probability it's 2-pole to 4-pole motors.
Here's how I see it.

Wattage is wattage, in that example, the 6s setup is making much less wattage then the 2s setup. More then efficiency alone can compensate for, if the motor output was to be equal. The voltage / kv effect on efficiency really shouldn't play into it until the amps the motor is drawing is enough to push it out of it's efficiency range. At the Neu dyno sight, you can see that regardless of voltage, the efficiency is going to run between 85-92% http://neumotors.com/Site/Dyno_data.html But it doesn't really seem dependant on voltage, more so the motor. 600mah of a 6s would have to be 40% more efficient as a 2s setup using 3000mah for the motor output to be the same.

So, Until your wattage needs, takes your amp needs, out of the usable amp range of the system , I don't really buy the higher voltage is going to be that much more efficient. Using , say, a Neu 1412 and MMP, If I was only pulling 100 amp bursts on 2s, then cut the kv in half, and doubled the voltage, so 50amps on 4s, wattage, speed and performance being equal, I don't believe the setups would have a measurable difference in efficiency.

Am I wrong? How much efficiency can trully be gained from higher voltage? I would think for it to be noticeable you would have to see a 10% increase, and I just don't see how you could get that kind of gain, unless your original setup was just way off. What am I missing?

Well good morning sunshine!:lol: your up early lol. We should get some good info and opinions over here. Looking forward to it. I'm not really sure about the efficiency deal as I don't have the equipment to test for it but I have noticed that the current will drop on higher voltage/lower kv setups. That is really the main reason I go that route. The setups quoted were a prime example. The 4.5 vs 6.5 setup runs about 40 degrees cooler on 3s to the 2s. Working in Merced right now and it's been raining so the outdoor tracks are too wet for me right now but when I can I will get some amperage data for comparisons sake. Thinking your right about the efficiency deal but on the other hand with lower currents there would be less heat losses so maybe it's a balancing act between the two? Awaiting mr g to chime in, sure he has some insight on the topic.

For what it's worth, I will present my 2 cents on this subject.

I've often found my train of thought following along the same lines as what you've presented here.

High Voltage versus Low Voltage versus a given wattage of output(load)

This all has to do with resistence. Resistence is the root of all losses of efficiency.

Resistence in the wire, resistence in the fet's that do the switching, resistence in the Lipo Pack itself, and resistence in the motor winding.

Another thing that gets tossed into the equation is voltage drop.

For a given length of wire, you will see a measureable amount of voltage drop for a given load. The greater the load, and the smaller the wire, the greater the voltage drop you will see.

We try to make up for this by using larger wire that has less resistence. There's that word again. :yes:

If resistence makes for more voltage drop with the greater amp load, then it makes sense, by bumping up the overall voltage, we effectively drop the amp load and minimize voltage drop.

By dropping the amp load, we lose less. We lessen voltage drop and we lessen wasted energy spent on heat, because of resistive losses.

The cooler we can make something run, the more efficient it becomes.

But we push the limits. The limits are pushed with the low voltage setups AND the high voltage setups. I think this makes it harder to measure the differences.

I seen a good example with my lithium ion trolling motor setup. By eliminating a 7 foot 10Ga. length of wire, the motor would put out an extra 75 watts of power. This was the only change that was made.

Efficiency was also improved, because that 7 foot length of 10Ga. was noticably warm to the touch. Even though, it was "only" carrying 50 amps.

Well you know what they say,
Early to bed , early to rise,
I was between the sheets, but not her thighs :wink:

So is there really any viable way to test efficiency? Is heat the only effect to look for?

A couple graphs, these are my Rustler with a 1410 MMP , Badland 30 series tires. On 2s SPC 6000mah 30c 45 MPH 25/84 gearing
http://i531.photobucket.com/albums/d...dlandspics.jpg
On a 3s 3300 30c 45mph
http://i531.photobucket.com/albums/d...45mph3spic.jpg

Now if you average the bursts, as there always seems to be 1 or 2 that spike higher, it does seem that it is roughly 1/3 less amps, and the wattage is pretty close. Temps stay about the same also. Even on 2s, nothing is being pushed out of line. Does this mean the overall efficiency is equal?
When you look at the average draw it takes to drain a 2s 3200 mah in 5 minutes, you will almost be guaranteed some very high apm bursts. That would quite probably take that motor out of it's efficiency range, causing a big increase of motor temp. I know the Novak motors are quite bad about that. I would be interested in what kind of amps that pulls, I haven't logged mine yet. What kind of weight is the Jammin, and do you know what kind of speed it ran?

Wire and resistance, whew who's going to cover that ? Is the current all that really matter when figuring a wires resistance? IOW will the losses of 50 amps and 10 volts, be the same as 50 amps and 20 volts, given the same distance and wire?
The smallest wire I have in anything is the Castle 13g . Which I have always felt was a little lacking , though I have been assured from Castle that it is fine for the wattage and lengths we are running. But I've seen some of the new 1410 motors with wires over a foot long, that has to add some resistance. I've seen the 3800 draw 160 amps, take that with 16inches of 13g and you have to be losing something.

I This reminds me of my car stereo days. If I wanted to send a true 50 amps 7 ft I would have wanted at least 4ga. But I know distance has alot to do with it, and for 1 foot, I'm not sure there would be a noticeable difference in using 10g or 4g.

My jammin scrt10 iirc is about 6.5 lbs rtr. With a 2s 5000 40c and a 550 4.5 motor after a five minute heat is about 160 degrees. The 550 6.5 on 3s 3200 40c is about 110-120 after a five minute heat. Geared for the same 38-40 mph and both with the same rpm output (within about 1000 rpm of each other). Heat is a good measure of heat losses but idk if you could literaly equate it to efficiency. As soon as I get a chance I will log some amp draw from both setups on the track and see exactly how much less current the 3s 6.5 setup uses compared to the 2s 4.5 setup.

IMO, Heat is the main indication of efficiency losses.

It takes power to create that heat, so there goes some of your battery pack's power. It was used to make heat, not motor power.

Resistence isn't only found in the wiring, but also the other things that I mentioned.

Those others also produce heat as a result of losses, even your Lipo Pack.

If it gets hot, it is using energy to become heated.

Heat is energy.

I was thinking the same thing, the battteries can play a part in it too. If you are light on the trigger then there may be little difference but if you have a demanding track or a basher that like's showing brute power then there will be a lot of standing starts and out of control full throttling. That's when the system is under most stress and where a higher voltage system would show cooler temps (if not, then more grunt). So both systems are highly reliant on the batteries where you are heavy on the trigger and no punch control is used.

That's my understanding anyway:tongue:

I have been wondering when this conversation was going to start here, I got a few things to add. I'm not all that scientific, I go by feel and temps. I have played with higher voltage and lower kv for a few years now, I see the efficiency both in temps and runtime. I started with a full FLM'ed stampede, with a lehner basic XL1200/1200kv motor, a Quark 80 esc

[IMG]http://i187.photobucket.com/albums/x...icture2004.jpg[/IMG]

[IMG]http://i187.photobucket.com/albums/x...icture2003.jpg[/IMG]

I forgot what the weight was, but heavier than most Stadium trucks. I could run 60+ minutes running a 4s 3700 mah 20C flightpower lipo. Not sure exactly what the gearing was, but it did 45 on radar runs with masher 2k tires. Temps always at around air temps

Next i put the motor and esc into my RC10 gt conversion with 20/66 gearing on stadium truck tires. This time on a 5s 2500 25c flightpower lipo. It was nothing fancy, just a good basher. I could run as long as my mamba 4600 (CM36) powered T4 8000mah MA pack, roughly 30 minutes. Temps never above air temp geared to 35 mph, only problem was it spread the diff outputs to the point of the dogbones slipping

[IMG]http://i187.photobucket.com/albums/x...ture2002-3.jpg[/IMG]

The latest setup is using my associated T4 and running 4s lipo running a 23/87 gearing on ST tires, I expect to run over an hour on the same pack as the stampede, The 3700 4s 20C. will post pics when i really test it out.

Glad to see others looking at this

This whole discussion is rather simple. For a given performance, it takes X amount of power. If using higher voltage along with the right kv motor, and geared appropriately, less current will be drawn.

As far as efficiency, that's simple too. Efficiency as a percent can be determined by (input_power/output_power) * 100. So, obviously, the closer the input and output power are, the more efficient a system is. Unfortunately, wires, FETs, etc all have resistance. That resistance * current = voltage drop. Then, that voltage drop * current = power lost. So, reduce the current to reduce voltage drop, and therefore power lost. There are other losses (ESC FET switching, Motor magnetic flux, etc) that are going to happen no matter what setup you use, but reducing current, using a high quality motor (even a few % points in eff can make a BIG difference), and making sure the vehicle is in proper working order (no drivetrain drag, bearings ok, etc) are the main things we can do to make the efficiency as high as possible.

If you are moving up from a 2S to 3S setup, I doubt you are going to see much of a gain, maybe a couple percent, especially if the 2S setup is running with descent size wire to handle the amps that are being pulled. I think you would have to move up to a much higher voltage to see a 10 percent gain.
I've run stadium trucks and the temp differences between 2S and 3S geared for the same speed are night and day. Run the same setup on 4S and it can stay ice cold. I still dont think you would see a 10 percent difference even there, but thats my 2 cents.

Sure, simple :)
Brian, can I have your opinion on how much the overall system efficiency would change going from a 1000 watt setup on 3s to a 1000 watt setup on 6s, using a MMP 1512 type combo, adjusting kv for max motor efficiency. Also dropping the Mah of the battery in half.


Ha ha, crazy, I'm running one of those Quark 80's now. I have it in a Slash 2x with Jato arms. I run it with a Tacon 3930kv 3650 motor on a 2s 6000mah. Temps will normally be around 30-40 above ambient. This is a graph of it running 40mph, with Badland 30 series. I kill the battery in around 20 minutes. It's probably pretty close to the same weight as the above Pede.
http://i531.photobucket.com/albums/d.../slato3650.jpg
So here's what I don't get, I'm running 20 minutes on 6000mah going 40mph, you say
So according to this claim, if I was to run the same time , even at my lower speed, I would need 3 packs , or 18000mah on 2s the equivilant to a 9000mah 4s, or around 2.5 times more battery.

These are the kind of claims that confuse me. I just can't see where that kind of difference can really be had. I could see maybe the extra 5mph coming from the added efficiency of the voltage, but 5 mph and 2.5 times the runtime....

Shore! It's simple! :whistle: LOL

It is hard to fathom runtimes like posted above, but with lower current draw the same packs will run much longer. Not sure how long my rigs run. Typically get six minutes with a warm up and the five minute heat but using some numbers (600-700mah for the ebuggy) should be able to run hard for around 15-20 minutes on a 6s 2650 40c with my 1400 kv motor.

Nothing is simple but it's easy to remember that the less current you are using be it spikes or avg. The longer yo can run for a given capacity compared to a lv high current setup. By the way, great discussion so far. Lots of good stuff.

all depends of the scale of you use...... but if you use the correct config all this setups are perfect for race and win:

1/10 1500-2500gr .................... 2-3s
1/8 2500-4000gr ......................3-4s
big monsters 4000-6000gr .......... 3-6s
big scales 6000-10000gr .............4-10s

of course all depends of the circuit and the top speeds ...... if the top speed of your track is 25-30mph you dont need much voltaje because is innecesary ....... but if you need big top speeds is better if you use more voltaje........and

+voltaje and -mah = +speed and -temperature = same run time

I am comparing my Mamba Max 4600 CM36 verses a 1200kv Lehner basic XL

You need a noticable jump in voltage for the efficiency to really show, I did the 2s to 3s/same motor and saw not too much improvement. When i doubled and 2.5 times the voltage and lowered the kv, i saw mediate gains in efficiency and lower temps. I also gained by going bigger in motor, The Lehner XL basic motor was considered a MT motor when brushless rc was sorting itself out. At 1200kv, it's only pushing 22,000 RPM's on 5s lipo. I know that the motor is capable of 60,000 rpm, but why do you need ti run it like that? Runs fine and cool and like a 4 stroke nitro, torque out the wazoo

Too many variables to deduce anything. Is the overall query based on, "why all the hype over hv?" when your lv setups work fine?

IMO low voltage setups only work as well as they do now because all of the esc manufacturers have had time to work the bugs out. A simple oversight in gearing or maintenance can highlight the shortcomings in these setups and make them go poof.

Whether they are really more efficient or not in terms of usable mah, I believe, in the end will be negligible. However, the power flowing is used more efficiently as evidenced by component temperatures. The peace of mind of having cool components is enough reason for me to go hv. Ambient temp is way better than, "within acceptable temp range".

VERY Well Said Harold!

I aggree.

More so how much is hype, so I was shooting for proof of a real world difference, and at what point does it really start to help. I'm a firm believer in once your wattage needs gets to a certain point then Higher voltage is the way to go. 8-12s in a Baja, the way to go, 6s in a Rustler running 40mph, probably not going to matter over a 3s setup. In my Rustler, MMP 3800, temps are roughly the same on 2 or 3s, the system could care less. I know that's not a big jump, and maybe the kv of the motor must be changed for it to be accurate, but a 50% increase in voltage should give some notceable result, IMO. Here's an example
25/84 on 2s got me
speed= 45.8
Motor= 131
esc=112
ambient = 86

http://i531.photobucket.com/albums/d...dlandspics.jpg
on 3s
Speed = 48.6
motor= 135
ESC= 115
Ambient = 86
http://i531.photobucket.com/albums/d...adlandspic.jpg
Once you average the power used, the wattage was actually pretty close in the 2 setups.

I would love to see some examples like that comparing a true HV setup, if anyone has them.


This is a battery testing place, currently in the works, but he lists some wire and connector resistance specs, http://www.rclipotesting.com/Performance_Tips.php

I might be racing this year and plan to run the 4s powered T4 against the nitro ST's, if any show up. I got the ok from my local track to try it, he's cool, his answer to it was, "It would be like being told how much gas (nitro) to bring for a heat." The reason I tryed this was to see how much of the hype, was hype. I seen claims to and wanted to try it, got what i was after.

Hopefully in the next year i will try something more ambitious, I want to build the first 5-6s powered Mini e-revo with possibly an outrunner for a motor. Now that i'm running lowerkv and loving it, I want to explore outrunners and see if there is more efficiency to come

HV is good to have as long as the required amount of wattage is met for your setup. However you can only go so far as weight becomes an issue and throws out your gained efficiency.

One point here is, with the higher voltage setup, the amp load demand is less.

This allows you to consider smaller mah cells that make up the HV packs.

Instead of going from a 4S 5000mah pack and just adding 2 more cells, for a 6S 5000mah pack.

Perhaps going with a 6S 3000mah pack would help offset the weight penalty.

What exactly is your point against a 1/10 with 4s or a 1/8 with 6s? There are small car ESCs for 6s, there are appropriately-sized motors with <2000 kV and there are 30C LiPos with <3000 mAh.

I would recommend running the highest voltage which makes sense with the available components.

Yeah IMO for a comparison to be accurate, the batteries need have the same punch. IOW a 5000mah 2s would have to be compared to a 2500mah 4s, because if it's a 3000mah battery, how could it be determined what's coming from voltage, and what's coming from it just being a more powerful battery.

Tacon has a 2230 KV motor ,that is like my 3930kv motor I'm running in my Slash . I also have a pair of 5000mah 2s 20c batteries. So I'm thinking about picking one up, gearing for the same speed and testing it. Using the batteries as a 10000mah 2s setup, or a 5000mah 4s setup.
Does anyone think the KV's are far enough off to skew the results?
By data logging, radaring and temping, this setup, do you think some results can be had?
Is there a better way to test for results?

Thomas, I think his point, which is something I also want to know, is how much does the amp draw play into the need for high voltage. I think we all know that if you need 3000watts then a 2s is not going to cut it, but if you need 200, what is trully the advantage to 6s? Please if you have any input on this, state your case, hopefully with some hard data, but personal experience works also.

In my experience using the same kv motor on low and high voltages has negligable gains in anything but speed. To fully embrace it you must go down in kv and up in voltage at the same time. This is the only way I've found it to be worthwhile to use higer voltages. Not to beat a dead horse but the 4.5 ballistic is 5000kv and a amp hog I might add. The 6.5 ballistic is 3400kv and much better on three cells and makes the same rpm and power on the track but runs 30-40 degrees cooler iirc. That's the easiest comparison for now but my rc8 runs about the same temp margin cooler on 6s 1400kv vs. 4s 2050kv. Plus the 3&6s packs are smaller in capacity and weigh the same or less than a 2&4s pack of higher capacity. Hope this helps. Might be able to produce some data this weekend but we will see what I can come up with, I have a plan and a few rigs to try stuff with so maybe I can offer more than just my experiance.

My point exactly, with higher C-ratings, smaller packs can be used, Which adds to the efficiency thing. I have room to grow on this 1/10 project, The Lehner is good for 60,000 RPM, so 8s+ is possible with this setup. I figure two 1500 mah 4s packs will fit in a T4 tray, Just need an 8s capable esc small enough :diablo:

This is good, something i can contribute to with experience.

The cool part is, with all the talk of going up in voltage, I haven't gained weight on the setup. My 4s 3700 flightpower is still lighter than the 6 cell NIMH i used to run:mdr:

They earn their nickname of spinning toaster of death for good reason BUT, Honestly, I must have a special one, it's the old version HV4.5 before the 5mm shaft, but has the good rotor and endbell. I can run it 45mph on 4s in my E-revo, no problem unless you go for 50 ;) , or drop it in my Slash 4x4, on 2,3 or 4s, and never have it even start to get hot, above 140. People are going to want to shoot me for this, but speed for speed , voltage to voltage, in my Slash 4x4, I think my Novak runs cooler then my Castle 2400. I have much love for my 4.5, it's been around along time, in alot of vehicles, pushed beyond what it's little size should have allowed, and my toaster still spins up a mean piece of toasted ground.

But let's face it, with Novak's latest system being 4s limited, they have no business being talked about in a high voltage thread.

I totally agree. Further, to be completely fair, you would have to use different ESCs for the different voltages. The setup with lower voltage could do with FETs for lower voltage, but needs more of them (or higher current rating). However, it's not very practical. But I would argue that a 1/8 buggy on 3s probably needs an MMM, while it works with a smaller and lighter MMP at 6s.

I have not tested or measured different setups, but try to look at it from a scientific point of view:
Batteries: When going from 3s2p to 6s1p with the same cells, the amount of cells and the required power are constant. So the losses in the cells should be the same, both setups should see the same battery temperature. But in the wires and connectors, losses are higher because of P = R * I^2
Please tell me if you find a mistake in my calculations

ESC: More current through the FETs, wires etc. equals more losses. HV setup -> cooler ESC.

Motor: Double the voltage -> half the kV. Let's compare a 1509/2Y (1820 kV, 0.018 Ohm) to a 1509/1Y (3600 kV, 0.004 Ohm). From the specs, they should have roughly the same ohmic losses. This is because they have the same amount of copper, just different winding configuration (very much similar to the battery example above).

When theoretically comparing two setups, I assume both carry the same amount of energy (Wh), motor RPM is the same and the performance (W) is the same.

I'm afraid with this simple math, I can only explain the difference concerning wires, connectors and ESCs, not motors and batteries. I'm pretty sure the simplification holds true for the batteries, but not the motors (impedance etc. not taken into account).

Totally true Thomas, a HV setup makes the ESC cooler for sure, but theoritically it's the same for the motor and the battery (with the same runtime). For a low Kv motor, the windings wires are thinner (more turns in the same space). But it's maybe easier to package thinner wires (low Kv) in the motor than larger wires (high Kv)...
Some people noticed the some change on motor temp using low and high voltage setup, I didn't (Castle 2200Kv 4S versus leopard 1400Kv 6S with heat sink). But my ESC now operates at cooler temp.

I don't get it guys? The higher voltage setups run cooler, right? So to me and my simple brain just the absence of heat means that setup is more efficient. After all isn't heat just wasted energy that isn't turned into motion by the motor????? Doesn't that dictate that cooler HV setups are more efficient?????

yes, and yes. But we are into details, we want to know how much more efficient. And we want to weigh the benefits of increased efficiency against the cost of hv setups. We want data and hard numbers!!

If cost is a factor than hv goes out the window for many. It cost me 2x as much for my mamba xl in my e-maxx compared to the previous MMM.

I feel cool when I tell people it runs on 30 volts!:yes:

You are taking the problem from the wrong side.
The point is that HV voltage runs not necessarily cooler... For the motor it's pretty much the same, although the ESC is a little cooler with HV setup. So the gain in efficiency can sometimes be negligible...

Even so, an efficiency difference of even a minuscule 0.5% on a 400w average draw is a difference of 2 watts worth of heat. That doesn't sound like a lot, but take a hold of a 5w resistor dissipating 2w and you can get the idea.

No matter what, ANY heat on the motor, ESC, battery, wires, etc is lost power. All those have a given resistance. Current^2 * Resistance=power.
Higher voltage means less current, so less voltage drop and power loss on those components. This means more of your battery is getting to the ground.

Here's some numbers for ya:

If your system is pulling say 300w average, running that on 2s means 40.5A. That 300w on a 4s setup is only 20.3A. 300w average power is pretty typical for a smaller 4WD vehicle.

So, if you have a total resistance of 0.05 ohms for the motor, esc, and wires, that's a loss of 82W on 2s for a system efficiency of 72.67%. But on 4s, that's a loss of 20.6w for a system efficiency of 93%. Big difference there!

So, I really don't understand the argument and confusion here. It seems pretty simple to me. :neutral:

In terms of heat, ok, a single watt loss is always too much. But in terms of runtime ? Worth it to add 200$ in order to gain 10 sec on 15 minutes racetime ?
Low Kv brings also smoothness for some people who drive brushless motors like nitro engines (trigger ON-OFF). Due to the higher inductance, a low Kv motor has higher response time, some kind of intrinsic punch control...

I am sorry BrianG, your math is false ^^ This would be too easy to gain efficiency you don't think ? :lol:
The motor resistance gets higher as the Kv drops (more turns and smaller wires).
For example you need to double the number of turns to get the Kv divide by 2, that means for the same volume of copper (both motors have same weight), the wires section/surface is divided by 2, that means the resistance is multiplied by 4 :

R = resistivity * (lenth/surface)

So the R*I² loss is exactly the same for the two motor (I multiplied by 2 but R divided by 4). Someone explained this point above. And in reality it's not as simple as that. Sometimes it's easier to package thin copper wires in the can, sometimes not, so the resistance is also affected...

But there is a gain of efficiency in the ESC/Battery wires and connectors, and in the ESC. And this is this little gain you have to calculate, do you have some wires connector resistance values ? ESC is like 0.0003 Ohms...

Yes, Yes, Yes. This is exactly what I am curious about.

I'm gonna have to agree with nuz, something looks funny in your math there Brian, Unless I'm misreading what you are saying. Cause that would let you go from a 5000mah 2s, to a 2000mah 4s, and still have the same runtime.

I found a couple different motors I want to test, found a 6s motor that will put the RPMs within a thousand of my 2s motor, otherwise identical. I think that will give some hard #'s to look at.

Physics 101

Power (J or watts with a constant) = V (volts) * I (amps)

V = I * R (resistance)

For constant power as volts increase amps decrease at same ratio

Losses W = I * I * R (just rearrange above )

Hence losses decrease with square of amp decrease

This is why power transmission lines are ultra hv.

Pretty simple conceptually.

I'll keep on running hv then:yes:

He he yup. 500kv lines are pretty fun to build. Now if only I could run my rc8 on 500,000 v 250 mah packs. Lol. .00000001 amp spikes anyone? Sorry guys bored and hot right now. I am planning on ordering a v4 eagle tree and trying to contribute something more than reiterating ohms law and my experiences.

the 'efficiency' claim for HV is not necessarily for the motors, but the batteries and wire which have more or less fixed resistance characteristics. Like everyone said, the heat losses goes up quadratically with current, and by going with higher voltage for a given wattage it reduces both current draw and heat.

To be truly scientific, the equivalent watt-hour batteries and different motors (with similar total RPM) should be put in the same chassis. Castle's CMS36-4600 on 3s 2650mAh and the CMS-6900 on 2s 4000mAH seems to be a good matchup for ground RC.