niznai

Resistance is not "one factor". It is one of only two constant measures of the system you're looking at. The other is voltage (whatever voltage you choose). Once chosen, these two remain constant regardless of everything else. All else is derived from these two. Don't confuse yourself calculating this that and the other before you decide what the values for these two are. Power is just one derived parameter and it will vary when voltage or resistance change (as will all others).

Sure, there is more complication than that but in principle this simplification is close enough. Under load, a lot of things change and in response there are serious deviations from the above (simplified) picture, but you can use the simplification to put some constraints on how much power you need from your battery when running motor x in application y (under ideal conditions - again a caveat here).

If companies on the market today would give you a nice printout of their motor's performance (current vs voltage at various loads) you could easily find the desired regime for your motor, hence gearing, and battery you need. Unfortunately brushless has not reached that point. Moreover, with programmable speedos that change timing on the fly it is even harder to decide all that and gearing (and choosing batteries and motors) becomes more of a black art where you depend on information from the company whose ESC you buy (that is the starting point in building a BL setup as far as I see it, not the motor). This situation is not very pleasant (for me at least) and I have tried to obtain data from manufacturers but based on the responses I get I would hazard a guess they don't have it (they don't dyno their motors).

As for the C rating of the battery I am not sure what various manufacturers want to convince us it means, but I am pretty sure there is no standard applied across the board and as such the ratings mean almost nothing when compared to each other. Unless each manufacturer gives comprehensive detail of what they measure and under what conditions, it will stay that way.

Unfortunate, but that's the way it is. The more complicated the technology, the more we depend on what the manufacturers decide to tell us.

One last note relating to the quote above, similar power with different voltage is another expression that will confuse many. Electrical power available to the motor won't happen. What you are talking about there is mechanical power which means a fraction of the electrical power available to the motor. This means you are shooting to get to use the motor at lower voltage but hopefully at better efficiency (equal to saying less loss by heat and other means). This is again not very likely with sensored brushless and timing variations available from the speedo. I don't know this for sure, but any intelligent sensored ESC manufacturer (or even those with half a brain) today will aim to develop a software that keeps the motor at best efficiency at all times (transitory loads being the hardest challenge). Variable timing does exactly that by looking at various parameters on the fly (I would guess current, temperature, revs, reverse EMF spikes, possibly others) and adjusting the advance of the command pulses in the rotor so you see, there is little chance you'll get your wish (i.e. equal power at lower voltage, all else being the same). Gearing by definition only changes the efficiency of your transmission (i.e. shifts around the point where your transmission is at peak efficiency), that's all. This can make a difference (as everybody already knows) but not in the order you are aiming for.

Or who knows, maybe I am totally wrong in my assumptions?

kufman

iTrader: (6)

Modern brushless motors convert electrical power to mechanical power rather efficiently (>80% in most cases) so it isn't really a fraction... it is most of the electrical power.

Dave H

I’d like to add to this too if I may. What you say is correct, a 6S 5ah with the appropriate motor and gearing changes can provide the same power for a longer time than a 4S 5ah with it’s motor and gearing. It’s 50% more energy to start with.

I think a better comparison to display the improved efficiency of high voltage would be a comparison of a 4S 5ah with a 6S 3.333ah, or 5S 4ah, again each with appropriate motor/gearing. Then the only variable is voltage, essentially the same size and energy battery. This will provide some benefits in terms of run times and reduced temps. Not as much run time as the +50% example, but an improvement just the same. Essentially equal temp improvement.

The key of course is the change in gearing and motor. You recognize this, the reason I’m piping in is I see it frequently misunderstood by others. Just going to higher voltage alone will reduce run time and increase temps. To some extent analogous to your initial comments on motor power. Maximum power is only going to be achieved at maximum voltage. At least reliably for any practical length of time.

Complicated, you bet. One must always be careful to not focus on only one aspect/variable of a system, everything is interdependent. For what it’s worth, the standard 1/10 scale format we have didn’t just appear, everything from 4 to 7 round cells, motors from 380 to 550, at the very least that I am aware of, were tried to get where we are (were?). The 1/8 scale guys are working through the same application development process now. Just like the single cell lipo 1/12 scale guys to a lesser extent.

jmcvicker

iTrader: (2)

Agreed - with 1/8 you have anywhere from 3S to 6S and motors from 1300KV to 3000+KV. ROAR appears to be allowing for a standard of 4S for 1/8 scale - though some changes may end up happening too. It'll mainly be based on electric scale race-length.

I did buy the wrong motor to start 1/8 electric buggy racing. A Castle 2650KV motor. Should have gotten a 1900-2000KV motor with 4S1P or even 1700KV with 5S1P. The 2650 is quite fun but just isn't "right" based on this year's move to seeing folks go to lower KV and higher voltage.

Now, for 10minute races - 3S1P with a 2650KV is fine - so that's still an option for electric-only racing or qualifying in nitro club racing. But try to run a full 20-30 minute main with nitros and skipping battery changes would require much more stored energy and smaller KV motor. One battery change is still typically "more time" than three normal nitro fuel stops.

niznai

Sure, efficiency has gone up a notch, my point is that there is some loss (and there will always be). 80% is still a fraction of 100%.

But the more important point is that the efficiency is measured under ideal conditions (if not calculated). I would like to know what these conditions are, because this would give me very important information about the motor when I try to make a decision which to buy. Unfortunately, there is no such info available to customers and we have to take it for gospel from the manufacturer. As I said, not an ideal situation.

kufman

iTrader: (6)

True, we don't have real in car data supporting efficiency numbers. The plane guys are way ahead of us on this one and they should be. It is much easier to take data with the motor spinning at a constant speed for long periods of time. Us car guys have basically done everything by trial and error and I think we have gotten to a pretty good place. I wouldn't have imagined running 15 minute mains even 3 or 4 years ago. Now, we do it every weekend. Pretty cool