Max amps: is it dependent on how many cells?

[Hector_Fisher]

I think the math is straight up but I think the concept is lost. These are Si Units and you can see the ugly math on Wikipedia if you like, but this is the one liner.

Voltage is the amount of potential something has. Like a bucket pouring water in to a pipe. The higher the bucket the more potential it has.
Current is like the size of the pipe. If you have a giant pipe it will drain the potential quickly.
Power is just multiplying the two. Says how much power can you have from your bucket and pipe.

High potential with skinny pipe is the same as low potential with fat pipe. How much water can you get down there at one time?

So
5V*1A is 5W
5V*.5A is 2.5W
10V*.05A is 5W

So you can have higher voltage and lower current, which is generally more efficient.
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You can take power and dice it up and compare it to other things like MPH or RPM to optimize what you want

It's a bizarre thing, I know.

Considering all else is the same between the runs that @dure16 did, i think a better metaphor is: voltage is like water pressure in a pipe, and amps is the amount of flow in a pipe. Wire gauge would be equivalent to pipe diameter. Amps are the indication of how much load you are applying to your system -- a measured or observed value based on the characteristics of your truck's build.

I think what we need now is more data to fill out the sample size and better define what's going on. A sample size of 1 is not nearly reliable enough to draw deep conclusions. My MT410 is set up to be able to run the 3s to 6s packs that I have, but I can't quite reach full throttle without it flipping backwards and taking flight. Maybe i'll slap on some hoons/GRPs and see what I can come up with, or swap in the mmx into my SCT410 and try to get some data.

 

[rotauq]

The problem is that it's much to easy to make an wrong comparison. I dove into my old logsheet of very consistant speedruns. Here are some numbers:

4S (268A) vs 8S (517.7A) on the same gearing, but different speeds

3S (634.6) vs 8S (275A) different gearing (and everything else), but simular speeds

Not even naming all factors but the higher the speed the higher the load. So regardsless of what voltage you use, amps will also increase quickly when you go faster.
It's extremely hard to get a true comparison between voltage and amp without changing load. And even voltage is not constant...

5 runs with simular peak load(P_max) on 8S. Voltage drop 22.1-27.1v (delta of 23%) 365-431.7A (delta of 17%)

 

[Dan B.]

Y’all are killing me, smalls. Coming up with every “what if” and “what happens when” scenario under the sun.
Here is a fact. It’s physics, the truth is incontrovertible and science is on your side:
Set up your rig the way you wish. If you do it correctly, then you will pull fewer amps under load the higher the voltage you supply. If that proves to not be the case, something you have done in setup (gearing, crappy batteries, etc.) or implementation (too hot on the trigger finger for speed running, stupid heavy oversized wheel setups for bashing, etc.) has been done so terribly wrong that it has resulted in terrible voltage sag or applied amp spike/demand loads that the ESC just can’t keep up with it. Amp spikes due to silly expectations or really bad setups can and will happen at any voltage.
But, for the love of god, it must be understood that when all factors are equal, then higher supply voltage results in fewer amps demanded by the power consuming component (the motor) in the equation.
This fact just simply can not be argued with. It’s truth. It’s physics.

 

[rotauq]

Y’all are killing me, smalls. Coming up with every “what if” and “what happens when” scenario under the sun.
Here is a fact. It’s physics, the truth is incontrovertible and science is on your side:
Set up your rig the way you wish. If you do it correctly, then you will pull fewer amps under load the higher the voltage you supply. If that proves to not be the case, something you have done in setup (gearing, crappy batteries, etc.) or implementation (too hot on the trigger finger for speed running, stupid heavy oversized wheel setups for bashing, etc.) has been done so terribly wrong that it has resulted in terrible voltage sag or applied amp spike/demand loads that the ESC just can’t keep up with it. Amp spikes due to silly expectations or really bad setups can and will happen at any voltage.
But, for the love of god, it must be understood that when all factors are equal, then higher supply voltage results in fewer amps demanded by the power consuming component (the motor) in the equation.
This fact just simply can not be argued with. It’s truth. It’s physics.

I do agree (in theory), Dan

But still theory is not real practice... have your rig perfectly optimized for 4S and pop in a 3S without any changes, it will be geared too low that the motor will not reach it's potential. Thus amps will hardly be higher. Pop in a 6S and it will be overgeared and burn massive amps due low efficiency.
Thats what you can read from Dure16's test...

 

[Dan B.]

I do agree (in theory), Dan

But still theory is not real practice... have your rig perfectly optimized for 4S and pop in a 3S without any changes, it will be geared too low that the motor will not reach it's potential. Thus amps will hardly be higher. Pop in a 6S and it will be overgeared and burn massive amps due low efficiency.
Thats what you can read from Dure16's test...

I’d suspect bad voltage sag on 6s. It would have been really under-geared as opposed to 4s though, right? Not over-geared. I have no explanation. I wish @dure16 would have posted the logs.
Interesting.

 

[dure16]

I wish @dure16 would have posted the logs.

Ask and you shall receive.

 

[Dan B.]

Ask and you shall receive.

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Thank you! So much for my theory of voltage sag on 6s. I’m wondering if, for the sake of continuity, using Perfect Pass or the Noble launch control could lend a more even playing field to the test?
I hate when I can’t explain results that should be a foregone conclusion.
And at this point I don’t feel that @jkflow owes me a beer at all. I’m a little stumped at this.

 

[dure16]

Thank you! So much for my theory of voltage sag on 6s. I’m wondering if, for the sake of continuity, using Perfect Pass or the Noble launch control could lend a more even playing field to the test?
I hate when I can’t explain results that should be a foregone conclusion.
And at this point I don’t feel that @jkflow owes me a beer at all. I’m a little stumped at this.

I really wasn't taxing the system during the speed runs, so I'm not sure launch control would've made much of a difference.

I'm tempted to do another test geared much higher. And with higher-quality batteries. Not sure when I'll get to it though.

 

[rotauq]

I made my hypothetical comparison before i referred to dure16 results.

I'd say gearing was optimized for 6S+. So both 3S and 4S were undergeared and even 6S to a degree. You can tell from the motor efficiency. Lazy calc max rpm/min voltage all runs end up at 1536+ kv. Once load kicks in not only voltage but measured kv (as in efficiency) will drop. On my speedrunsheet you can see real low numbers on the last column, whiles the 1550kv motor hardly breaks a sweat on all runs.

The higher (but still pretty low) amps on the 6S run are just because of drag/air resistance/ect due to speed.

 

[Jerold]

Great data logs!

Notice the peak power only occurs at the end of the acceleration and presumably terminal speed.

Which makes 100% sense. It takes a lot of power to accelerates something to speed, than maintain the speed. There is a bunch of physics that explains all that and I won't get into too much.

Your wheels give you mechanical power via the Motor/gears etc.. The mechanical power can be measured in terms of a directional force. It depends (and changes all the time) on traction, friction and loading. In a nut shell. Force/Mass = Acceleration. (f/m=a). Since Mass is constant, only Force (your throttle input) can change. The faster you want to accelerate (change velocity) the more force (power) you have to use.

Once you're at speed the change in velocity is ~0 and the force required is only the friction of the system.

So if you have the space, you can spread the acceleration over a longer distance and keep the amps down. Make your power curve less steep and move your terminal velocity to the right. Since the amps are down it means you can gear up. But if you get throttle happy, you will ask too much of your Motory/ESC and go over current.

Hopefully that makes sense.

 

[rotauq]

Notice the peak power only occurs at the end of the acceleration and presumably terminal speed.

Terminal speed is about 2 seconds after the amp peak. Red line(rpm) stops at ~67.5s (6S log) This is due recovery of battery voltage & motor saturation

Downforce at speed also adds to mass and friction. It's not a constant but actually the speed wall

This log of a battery stress test (with a little help of NB4 channel delay ~perfect pass) shows a nearly perfect amp line.

 

[Jerold]

Terminal speed is about 2 seconds after the amp peak. Red line(rpm) stops at ~67.5s (6S log) This is due recovery of battery voltage & motor saturation

Downforce at speed also adds to mass and friction. It's not a constant but actually the speed wall

This log of a battery stress test (with a little help of NB4 channel delay ~perfect pass) shows a nearly perfect amp line.

View attachment 328568

I think you've got it.

As you noticed I greatly simplified the concept, but yes friction, cd/downforce do change with the conditions. Your surface also changes, the loading on the motor changes too.

Looking at the voltage of all the runs, it doesn't look horrible. However, less V means less RPM