Higher voltage and efficiency

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.

[Thomas]

Originally Posted by asheck

... IOW a 5000mah 2s would have to be compared to a 2500mah 4s, ...

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).

[nuz69]

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?????

[_paralyzed_]

Originally Posted by BIG-block

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!

[nuz69]

Originally Posted by BIG-block

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?????

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...

[BrianG]

Originally Posted by nuz69

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.

[nuz69]

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...