How much amps (continuous & peak) will a brushless system draw?
I'm going to jump on the brushless bandwagon soon and I need some energy, battery and brushless information.
I've been researching a lot and I came upon some questions which I can't find the answer for. Looking at this chart here I had questions about pairing up the proper batteries with say either one of these setups.
First of all if I look at the watts rating for the motors. Is this a peak figure at it's most efficient point in the upper rpms and most likely when amp draw isn't at it's maximum? Am I correct on this statement?
Next is how will I know what the typical burst amp draws are from a brushless system? Can we calculate this information from this chart alone?
I've looked around and read that some NiMH cells have a maximum rate of 40 amp discharge. Is this 40A continuous? If that's continuous what are it's bursts ratings? Exactly how long is a burst per say?
I've also read that some brushless systems can be amp hungry with constant amp draws over 50 maybe 100 and 100+ amp bursts peak? Can NiMH even hold up under that condition?
Lastly I just want to make sure that my NiMH isn't bottle necking the system. So how do I know w/o spending money to experiment with LiPo on which systems will greatly benefit from LiPo (or tax the NiMH limitations) by looking at the reference specifications? Also in the future I'd want to make sure I'd pair up the proper LiPo discharge capacity as well.
I wish manufactures would post typical discharge rates on typical loads so it would be easier to know and setup.
In 4wd sedan, my sintered rotor 13.5 averages about 25-28 amps for a 5 minute race...
We ran a 'spec' sedan class with bonded rotor 4300 (10.5) brushless, and mine averaged about the same as the 13.5, or maybe a little less (you can't gear the bonded rotor too hard or it dies, so the amp draw is less than it would be for a sintered 10.5).
In dirt oval, my 6.5R Truck averages 28-35 amps (depending on how tacky the track is) for a 5 minute race...
Does that help?
"If you cannot win, make the one ahead of you break the record."
Biff Racing Team #420 (Ah... The "good old days"...)
The local indoor Offroad and Dirt Oval Track: The RC Race Barn
tfrahm thanks for the input and it sort of does help.
I was really looking for specific answers to my initial questions originally posted.
Basically it boils down to this, how would one know when NiMH becomes the limiting factor? For instance I've heard that to get the full benefit of 3.5R/7700MM one would need to go the route of LiPo, perferably with a discharge rating of 100A continuous and higher burst rates.
Well... what about the other less powerful setups? How does NiMH handle the draw? Is NiMH just as good as the LiPo even on the slowest brushless systems? Does NiMH handle the amp draw intensity well or poorly? Where do you draw the line?
And though you stated an average amp draw, I've read systems can draw burst amps really high. Sure the NiMH I've read are rated at 30-40A (is this continuous?, still would like to know and what it's bursts are), but I wanna know when I will see a noticeable performance benefit with LiPo paired with the slower systems (i.e. acceleration is where it draws the most) because it's too expensive for me to just experimentally 'try out'.
My touring car on a long outdoor track averages about 45 amps. Yes the battery can certainly become the limiting factor by overheating. This is not so much of a problem in cooler climates but it is in Houston. 140F is as hot as you should run a LiPo battery. Above this they will get some puffiness as the solvent inside becomes vapor.
A coreless motor is less efficient than a cored motor. If the motor can has just wire inside then it is coreless (Mamba, Trinity). This will require more of your battery with 3.5 motors. They may overheat the battery. I have had good luck with FMA 4800 mA-h Revolution LiPo's which run about 20 degrees cooler on discharge than other brands I have tried. Unfortunately they are a bit long and require you to redo the leads to fit most cars. They also happen to have very good discharge voltage which tends to equalize the performance to the high capacity NiMH when using less powerful motors.
I should note that in summer the NiMH overheat as well. It is common for them to vent a small ammount each run. One problem is that they are very hot off the charger in summer. When they vent they puke a small amount of the electrolyte out the vent. This crystallizes. After many repetitions the vent can become clogged. This is responsible for some NiMH explosions. In cooler weather they don't overheat on charging.
I have found a very durable hot weather system. Sphere Comp TC edition. FMA 4900 mAH Revolution LiPo, either a Novak 3.5R in the pan car or LRP X11 3.5 in the touring car.
20C discharge is about right. You can never achieve or exceed this in houston without seriously overheating the pack.
Peak amp draws on startup are up to 80 amps for a brushed stock motor, more with mods. This is of little concern with either battery type in the size battery that we commonly buy.
Last edited by John Stranahan; 11-22-2007 at 01:34 PM.
Average Amp Draw-You measure a packs capacity (Turbo 35). You run the pack. You measure whats left over in the pack. Then you can calculate your average amp draw by amp-seconds used divided by seconds in the run. Details on request.
Averages being...well..."averages", that sounds way too high. Especially on a longer track, where you wouldn't have the motor loaded as hard or drawing as much current.
To have 45 as the mean over 5 minutes, you must have some huge draws to offset the very much lower to near-zero draws you would expect during the majority of the run.
It sounds suspicious statistically to me when your "average amp draw" is half your pack's rating. Sometimes, one can construct statistical models that discover exactly what one believes it should discover.
BTW -- I'm not totally convinced that I'm correct, just that your 45A average number seems unrealistic to me. Using that same methodology, my Mabuchi motor draws an average of 12A over 5 minutes...does that fit? Does it follow that a 3.5 brushless is only using 3.75 times as much power as a Mabuchi over 5 minutes?
on a 6.5 after 5 minutes i drain about 1800mah, so average about 21amps
So...your 6.5 draws less than twice the power of my (apparently very powerful) Mabuchi in my T2. Is that on or off-road? I would expect that on-road would draw many more amps on average. I only use 1500-1700 for 5 minutes in my ZX-5 with a 7.5.
It just doesn't fit...to me at least.
Again, I'm not calling BS or anything. I just think there must be some other factor missing. Or maybe I'm just missing something. Could be either one, honestly.
Yeah something doesn't sound right to me. It seems that all brushless systems, even the slowest ones are rated at higher watts vs their brushed counterparts (reference specifications). I'm not sure if this is an accurate measure of energy consumption, but brushless systems in general (from what I've been reading) sure do seem amp hungry (or do brushed motors eat up the same amount of energy over time?). All these power hungry brushless systems is starting to make me wonder now... where exactly is the "efficiency" benefits coming from again?
Btw a 6.5r is rated at 390 watts. That's ~54A averaged out?
Somebody elsewhere said a simple way to determine the need for LiPo is that if after a run if your NiMH pack gets too hot to hold then you would probably most likely see performance gains from LiPo.
Well exactly how hot is 'too hot' for NiMH? Some people have different pain thresholds.
In a touring car I geared for top end w/ a 10t double I have on rare occasions peaked the motor and battery temperatures at ~190F. That's probably not good. What's that maximum recommended temperature for NiMH? Does overheating lead to shorter capacity (venting), lower punch (increased IR) and overall less life cycle?
I'm thinking the motor heated up from the increase in amperage draw from the taller gearing during the constant acceleration which in turn also placed that amperage demand on the NiMH cells. So I can see from this a LiPo cell with it's higher amp discharge capability (and lower IR?) will allow cooler running temperature of the battery itself, yet the motor will still cook (or will the higher voltage help the motor here as well from baking). Is this another benefit to LiPo where it will provide the ability to gear up over NiMH chemistry?
Here are some more numbers. My touring car dumps a 4000 mA-h amp maxAmps pack in a 300 second (5 minute) race. The pack tested 3900 mA-h on a turbo thirty. The testing goes something like this. I put the battery on the Turbo thirty and set it to discharge at 20 amperes. It discharges to a safe voltage in 702 seconds. The capacity in simplest terms is 20 Amperes x 702 seconds x or 14040 Amp-seconds (this is also 3900 mA-h with different units). Now the car just barely crossed the finished line and stopped, dumped, so the total run time was 5 minutes or 300 seconds. I divide 14040 amp-seconds by 300 seconds and that is my average amp draw. 46.8 amperes. I solve my little dumping problem by getting new FMA revolution packs which measure 5000 mA-h. Yes occasionally on brakes the pull is 3-4 amperes or so just to run the electronics, but occasionally on full throttle it is more like over 90 amperes.
Now the second question is why is this high. Well a 3.5 turn motor has huge wire. When you add up the multiple wires in a connection, it is over an 1/8 inch thick. It can draw some serious amperage. You would think since the car is going fast and the motor is turning fast on the average that the amp draw would be low, but this is a powerful mod motor. Instead you are getting close to 50 mph on each run down the straight. You are pushing a lot of air at high power. You burn more amps accelerating hard out of the corners. Some brushless speed controls cannot withstand this at the moment. My numbers are not unique. This large number and the related heat are why Europe went to 5 cell mod.
Cetainly the less traction you have the fewer amps you will pull. You will pull fewer amps with less powerful motors.
In August heat a NiMH pack comes off the charger at 140 F most the time in Houston. (a temperature cuttof is much desired). It is very hot when it comes off the track. This makes the pack short lived and prone to lose a lot of run time with each run. A LiPo is a better deal here.
Here is a pic of the wires coming out of each coil of the Novak 3.5 R brushless motor. Note the size of each wire bundle and imagine trying to solder this to a comm. Brushed motors on average will draw less on this track as the comm is just not able to tranmit this much amperage without becoming desoldered. We have plenty of experience with this.
Last edited by John Stranahan; 11-22-2007 at 07:29 PM.
I'm not really concerned the way you think. I'm sure that at times, the draw may well be double that 45A. It's just that if the peak is 90A, then statistically, when you take the time @WOT into account (I would be shocked if you were WOT and/or going anywhere near 50 for anywhere near 50%), it would be highly unlikely you would have a 45A average.
I ran a 7700 with a 4800 Orion at Revelation, it was fast as hell, but I wasn't anywhere near WOT, even on that track for more than a few brief moments here and there. Totally dialed and VHT'd 200' straight...national-caliber 1/8th track (R.I.P.). I never came close to dumping a 4800 in 5 minutes. I could go close to 10.
It just seems too convenient and out of scale when you consider that motors with far less power output (13.5 or Mabuchi) use (with your methodology) 12-16 amps average.
I don't know...it just doesn't sound right to me...but I've been wrong before.
I think everybody is putting to much thought into this. I've ran a 3.5r with 3800's and 4200's in TC with no overheating problems. I also run a MM7700 in my offroad RB5 with orion 3200/4800 lipo with no problems at all.
Also be careful comparing brushless motor numbers on paper. Example: Novak 3.5r is rated at 10,500KV and a MM7700 is rated at 7700kv, so on paper the Novak should be faster, but I can tell you from my own experience the MM7700 is a much faster motor.