Diminishing Returns?
 

It seems over the last year, the C rating has almosted doubled from 25c to the newly released 50c. It seems for 17.5 anything over 30c-35c is a waste. For most of the population racing 17.5/13.5/10.5 what is the most battery you will need? The second question and probaly the more important one is at what point will we need new connector/wiring options to handle the load?

Most cars with motors under 10.5 don't pull over 2 times the battery's C rating continous. Most peak out at around 35 to 40 Amps continous with a max spike of around 60 to 70 amps. A 25C to 35C battery is plenty able to provide 50 and 70 peak amps respectively. Also peak current draw occurs from pulling full throttle at a dead stop.

I suppose you meant the highest current draw is under hardest acceleration.
It is true that is one of the highest loads, but I think at least for brushed motors braking was the highest drain and the reason some brushed escs were specced to hold insane currents (like 600A on a GM V12xc).
I am not sure if this holds true for brushless, but it is a real possibility.

There is a diminishing return curve in terms of acceleration torque and peak mechanical power but I'd need some motor resistance data to generate a typical plot from a 2s Lipo voltage source with variable internal resistance.

Braking can produce a high current, but I think reversing speed controls are put under maximum current stress, and require internal self-protection for the transistors to prevent thermal failure.

Think of the motor resistance Rm and series resistance Rs in the battery, wires, and ESC transistors as one resistor in the motor circuit loop Req = Rs + Rm.

The current in this loop is the PWM voltage minus the reverse voltage in the motor. At high rpm, note Vbe ~= Vpwm but with opposite polarity:

Im = (Vpwm - Vbe)/(Rs + Rm)

thus maximum starting current flows at zero rpm when Vbe = 0 and Vpwm = Vbat.

Is = (Vbat - 0)/(Rs + Rm)

If one reverses the motor at high rpm then the reversing current is doubled:

Im (peak reversing) = (Vbat + ~Vbat)/(Rs + Rm)

so the current in a reversing control can approach double the rated starting current, and this is why a reversing ESC has an internal self-limit if well made. For a car or robot, there is significant inertia at high rpm, so the high current can flow for an extended period of time.

A high C rated battery should have lower internal Rs. For a given motor, once warm, Rm is roughly constant. Taking the peak mechanical power as the metric and assuming a brush motor model:

Pmpk = (1/4)*(Vbat^2)/(Rs + Rm)

where the power increases as the ratio of Rs/Rm approaches zero. This means for a higher turn, higher Rm motor the lower battery resistance Rs is less important than for a lower turn, reduced Rm motor.

The way I study the effect is to plot the curve in Numerit Pro Evaluation Edition using realistic value of Rm for a motor, plug in Vbat = 8.2{V} 2s Lipo, and an array of Rs from 0 up to some value approximating a realistic range for batteries.

A plot of peak shaft power Pmpk in Watts versus internal battery resistance Rs then approximates the analysis of diminishing returns.

The problem with the C rating is that as you suspect, the less powerful motors will not get anywhere near the drain the battery is rated for. However, the higher C rated batteries are often high quality cells that do hold a higher voltage for a longer period. Sometimes this means that the new cell IS better, but not because it has a better C rating, at least for your application

Yeah, what he said.............or to make it simple, put the biggest mAh battery in that fits your car because they weigh more and it makes it easier to balance your car out. That and the voltage will be at a higher rate at the same time during a discharge.

The problem with 'C' ratings is that they are total bollocks :rolleyes:

There is no standard for testing the ratings so each manufacturer can adjust things to claim whatever they like.

The important factor is the packs internal resistance, but I belive only SMC give a printout with that on it.

Skiddins

I didn't think you (or the initial post) were considering a reversible ESC. Higher performance ESCs don't have reverse.

In your explanation I am not sure what Vbe is, but in principle you are right. On the other hand, good ESCs don't switch directly to reverse to avoid the problem you elaborate. They have a delay function which introduces a few seconds (on the V12R it is 3seconds) delay before you can go into reverse after applying the brake.

But the way I read it, the initial question is where is the expense of buying higher C rating batteries no longer justified for the purpose intended (i.e. what is the maximum C rating useable in R/C cars before wires become a limiting factor). I think (form personal experience) the wires can easily hold higher currents than any ESC on the market today can deliver, so I don't worry too much about it. Again from personal experience I don't think we use currents higher than 100A under normal conditions (save for accidents, etc).

That being said, there are other problems with pulsating high currents. Your theoretical explanation does not take into account the behaviour of the winding which is a coil and as such has a variable impedance depending on the frequency of the current (including the parasitic Emf). Parasitic reverse voltage spikes are managed by various methods (such as Schottky diodes, but again only on non-reversible ESCs) so hopefully some of that effect is alleviated but I wouldn't be able to put a figure on how efficiently.

And yes, on top of everything there is a lot of doubt regarding the C ratings manufacturers put on their batteries.

Actually there is a standard, but it is up to each manufacturer to test according to the standard. There is no regulatory commission for the standard so there is no real enforcement.

I have never seen a standard , got a link to it?

Is it worth the cost? Definitely not in stock racing

I missed the part about beefing up wires and connectors.

Summarizing what I see above, if the battery is actually higher C with lower internal resistance Rs, then:

1. you get a bit more peak power on the infield with each reduction of Rs;
2. your battery lasts more time relative to a lower C rated battery;
3. you stay high on the voltage curve for a bit longer (delays power fade)

and if you want to get more power from a certain voltage, say 6 cell NiMH or 2s Lipo, then you need to reduce internal resitance and beef up ampacity rating of motor circuit components, based on circuit theory and the air gap physics. To get more run time you need higher capacity ratings.

Is it worth the cost in stock racing? There are no $ signs in the equations of natural law, so that is a question open to moral debate.

Due to the high internal resistance of lower power stock motors, the benefit of a high C low Rs battery is a much longer run time, but not that much more peak power, if my intuition is right.

Final Thought: The current limit for the foreseeable future is 8.4{V}/Rm, where Rs = 0. For smaller motors the current is 150 amps and under, not going to get higher in the near future. For large brushless motors on the Sentry Dyno Thread it appears some motors could draw 300 amps starting current, but the ESCs may be limiting current to 220 amps maximum. Just some numbers I recall from studying the Dyno data published by John Stranahan.

you made laugh, (outloud)

and yea, what you said.

Well speaking from experience there is a noticeable difference between 35c and 40c batteries running a 13.5 in a T4. I have run Reedy 5000mah 35C against Thunderpower 40c batteries and you can tell a difference. The problem as posted above is there is no standards that are adhered by manufacturers for the actual c testing. It would be interesting to see the numbers between 5000mah 40c SMC batteries and 5000mah 40c Thunderpower to see what the actual rates are...

There guys here:
http://www.iec.ch/

This is that standards group we used when I worked for Tenergy. Of course if you want a copy of the standard you have to be a member and pay for it.

There is also an old saying that goes " I love standards, there are so many to choose from" That being said I'd be willing to bet that there are enough standards for testing around that if you were to test cells using different standards you'd get vastly different results.

yes its true that the companies are making higher c ratings. what does the mean to higher wind motors. just more low end punch and longer run times. the pull that the motor uses during a race varies from stock to mod. a mod definatly pulls more. in stock mah doesnt matter bcuz you are only using 1000mah in a six min race. when i ran my packs for 6 min after being fully charged before the race and i put them back on the charger the amount of mah that went back into the pack was never ever 1000 it was more like 850-900. i use a multiplex charger so it tells me what goes back into the pack after a run. so the mah amount really only equates to longer practices times. i ran a gtb with 13.5 novak for 2 years and this is what i saw every time. but the difference i saw was when i went to a higher c rating the punch definatly increased. i was able to clear jumps and pass people easier. what it comes down to is money. i know that companies are saying they have a "?c" rated battery when it is really not. its has become soley about marketing and the almighty dollar. the only company that i have seen that is close or even right on about their ratings is SMC. trinity is not even close on some of their packs and integy was never right( not tenergy but integy)
the bottom line is yes a 40c battery will help a 17.5 tremendously. if you want it to run a long time get a higher mah pack.