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Old 11-22-2007, 11:36 PM   #16
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Well the power of a Novak 3.5 is 620 watts.
P=IV (Power=Current*Voltage).
The motor would be about 90% efficient like the lrp’s.
90% of 690 is 620. So the motor uses up 690 watts.
P=IV (Power=Current*Voltage).
So 690= I * 7.2V
690 / 7.2= 95.833 A
(I'm not 100% sure this is right)
I imagine this would be the peak current.
Then work out the amount of current the lipo can handle and is it's more then the motors peak current it will handle it.
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Old 11-23-2007, 10:38 AM   #17
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But that calculation is at peak watts which is probably somewhere when the motor is running at the top end. Isn't it the bottom end (or any unusual high load) when the motor calls on for all the juice the battery can give (most inefficient range)? And this can lead to peak current draw anywhere from 150 amps and beyond?

When using that formula, isn't it better to calculate the amperage value with a voltage drop value (under load)? Then figuring in say anywhere from 5 to 6 volts would equate to a higher amp draw value.

So is brushless really not pulling out as much amps on average as we are speculating? Thus the reasoning for NiMH being able to handle some setups which seem pretty demanding?
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Old 11-23-2007, 11:32 AM   #18
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watts= voltage x current, so peak watts is exactley the same as peak amps in terms of RPM etc.
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Old 11-23-2007, 03:45 PM   #19
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How are you guys measuring or inferring the current flow? Do you have some sort of inductive pickup on the battery wire?
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I'm not really concerned the way you think.
Well now that's kind of harsh isn't it. You ask a question. A guy that actually knows, answers it, and you don't care what he thinks. How exactly are you getting your 12 amp average for a Mabuchi. Guessing out of thin air.

(It is interesting someone telling me how much I am on full throttle from a great distance. Usually that's something I can only report)


On this large track you gear the 3.5 motor to be in that high power band. Drew's math is in good order. That 95 amp value is representative for maximum accleration at full throttle (it is not peak current which may be double that or battery limited).

A motor does pull fewest amps at high RPM and increases amp drap all the way up to stall as load is applied to it. The following graph shows this for a stock motor. Stall speed is to the right and the line predicts an 80 amp pull on the start of the race. This data was extrapolated from data taken off a Turbo dyno. The Robitronic dyno which actually takes measurements near stall backs up this number. A test with a simple ammeter and a stalled stock truck backs up this number.
I doubt my 3.5 is pulling 150 amps except maybe very briefly at the start. I am able to maintain good corner speed. Traction is sufficient that traction roll is an issue to deal with this leads to high average amp draws as forward traction is also good.

So what evidence do I have of the high current. I use dean half plugs. If I plug the plug in backwards (not the wrong polarity but upside down) it will desolder after a few laps. (600F). And of course the calculations are accurate and have been repeated on a second battery.

My pan car pulls 38 amps average on the same track. Of course my stock truck on a sandy outdoor track pulls much less and has huge run time.

Note that peak power on an electric motor will occur just about in the middle of its RPM range or at about 1/2 of the amp draw at stall. For this stock motor peak power is at about 40-45 amps.
Attached Thumbnails
How much amps (continuous & peak) will a brushless system draw?-graph-rpm-vs-current-revised.jpg  

Last edited by John Stranahan; 11-23-2007 at 04:17 PM.
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Old 11-23-2007, 04:15 PM   #20
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Well now that's kind of harsh isn't it. You ask a question. A guy that actually knows, answers it, and you don't care what he thinks. How exactly are you getting your 12 amp average for a Mabuchi. Guessing out of thin air.
No more guessing than you are...the same math. Take the discharge and divide it by the time. I use 1000-1100mah for 5 minutes in an on-road 4wd sedan. Is there something more to your "system" than that?

It's not really "Drew's math". That's the formula that expresses the relationship between volt, amps and watts. Anyone who's taken a class in basic electronics has learned that one.

I'm just questioning your 45A average. It sounds fishy to me. Statistically, it doesn't sound realistic for a car running on a wide-open track and it doesn't line up with other data points that I've seen. I'm sorry that bothers you so, but there's not much I can really do about that.

Maybe there's more to it than you've explained (or that I see) here, but apparently only one of us is curious about that.

Last edited by Turbo Joe; 11-23-2007 at 04:30 PM.
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Old 11-23-2007, 04:35 PM   #21
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When using that formula, isn't it better to calculate the amperage value with a voltage drop value (under load)? Then figuring in say anywhere from 5 to 6 volts would equate to a higher amp draw value.
Yes probably, but then again the real voltage for a nimh battery would be over 8 volts.
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Old 11-23-2007, 04:46 PM   #22
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Well now that's kind of harsh isn't it. You ask a question. A guy that actually knows, answers it, and you don't care what he thinks. How exactly are you getting your 12 amp average for a Mabuchi. Guessing out of thin air.

(It is interesting someone telling me how much I am on full throttle from a great distance. Usually that's something I can only report)

John,

As a reader of almost everything you post on this thread I can tell you that really enjoy your reporting on the subjects you report on.......thank you.

Now on the other hand this guy Turbo Joe never ceases to amaze me with his attitude which is almost always rude and quite often borders on absolutely obnoxious. I have seen many threads he has almost destroyed with his horrible behaviour. Thank goodness I have learned to ignore anything he writes on any subject, I guess this is freedom of speech at it's worse!
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Old 11-23-2007, 05:01 PM   #23
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Who's got the horrible behavior? I'm pretty sure it isn't me. It's called "curiosity".

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John,

As a reader of almost everything you post on this thread I can tell you that really enjoy your reporting on the subjects you report on.......thank you.

Now on the other hand this guy Turbo Joe never ceases to amaze me with his attitude which is almost always rude and quite often borders on absolutely obnoxious. I have seen many threads he has almost destroyed with his horrible behaviour. Thank goodness I have learned to ignore anything he writes on any subject, I guess this is freedom of speech at it's worse!

Last edited by Turbo Joe; 11-23-2007 at 05:33 PM.
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Old 11-23-2007, 06:35 PM   #24
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What better way is there to check amp draw than to see what it takes to "refill" the pack. I know there is instrumentation that could give us the real answer but I doubt the average person has it available.
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Old 11-23-2007, 06:48 PM   #25
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Who's got the horrible behavior? I'm pretty sure it isn't me. It's called "curiosity".

Telling someone you don't care what they think is not "Curiosity".

As always you are trying to turn this around like you do on all the other threads when you act like this but not this time Joe.

You started it and I thought it wasn't very nice to treat a guy like John in that manner, period. You can now argue with yourself like always.
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Old 11-23-2007, 06:49 PM   #26
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Ohh..I see...I ask a question...get an answer...ask another question about the results and say it doesn't seem right to me, but that I may well be wrong -- and that's "flaming".

Glad you cleared that up for me Mr. Flack. And by the way, where did I tell anyone I "...didn't care what they thought"?

I know that it's possible for people to make some pretty egregious calculation errors on this stuff. I think (actually, I know...) even Mr. Stranahan has done so before on this very board, nobody else even noticed...LOL...heaven forbid anyone say anything. You know how dangerous questions are...

If you have a valid answer to a question or an accurate model, it will stand up to reasoned scrutiny or hold up when you substitute different values. But alas, it's all about my perceived attitude, rather than any of those "facts" you guys say you want.

I think it would be much more accurate to do some sort of "measurement" if you really wanted good information, rather than trying to guess at the color of the carpet from outside the house. Whether that's feasible or not is another question altogether, but you would actually see what's really happening, rather than just guessing at it by how much gas is left in the tank. You can guess at horsepower by the fuel consumed over a certain period of time, but lacking information as to the air/fuel ratio, it's a junk guess. That's an imperfect analogy, but I don't think it's too far off.

Look...as I've said several times before in this thread...I'm not convinced that I'm right. I'm just not convinced that you guys are, either.

Drew == you're calculating the amp draw of a 3.5 from a rating that Novak posted on their website. A rating that I bet you have no idea how they arrived at. You don't know the conditions. It says "average over 6 minutes." 6 minutes of what? There's a big problem when you use imperfect/incomplete information, boil it down and use it to draw conclusions.

John == I'm not saying your 45 amp number is "wrong". I'm saying that it should (and does to me) sound suspicious when you happen to arrive at a 45 amp number from what you say is a motor that draws 90 amps peak on a large track. You just couldn't. Maybe it's just a deal where my educational (a lot of number-crunching in my degree field) and racing background makes me look for stuff like that and give it a second look.

Maybe (probably) the true peaks are FAR higher than what you think...if that was the case, then 45A doesn't sound unreasonable. The GTB is rated at 540 Amps for some reason, right? The XBR is rated for 120A and that's not even a 7.5.

That...and if I'm understanding you correctly...you're measuring your draw relative to a discharge you ran one time, presumably when the battery was new. Why not just measure what you have to put back in and base your numbers off that? I would think that would introduce fewer errors into your numbers due to pack degradation and loss of capacity. This deal is already like a horse being nibbled to death by mice. The more additive errors you introduce, the farther from the real number you're going to be.

Last edited by Turbo Joe; 11-23-2007 at 08:47 PM.
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Old 11-23-2007, 09:36 PM   #27
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joe of loath - Peak watts does not equal peak amperage draw as shown by John Stranahan response below yours. His explanations and graphs clearly show this.


So if we know that a stock motor has an 80 amp draw at near stall rpm, what do you suppose a 13.5t is drawing near stall? How about for a 7.5t? Lastly a 3.5t & 7700kv? I would really like to know a good approximate stall amp draw window for those systems just to get a relative baseline. Does anybody have this data? With this data it would help us all to know exactly 'how much battery' to buy so it won't bottleneck and we won't be buying more than what we really need.


Turbo Joe - what if his car is geared taller, thus the need for drawing extra amperage at higher speeds? Same for aerodynamics and other inefficiencies. Lots of variables there to consider. I don't know either what loads his car setup is experiencing but by your argument it also does sound like he is drawing an usually high amount average amperage (that would also tax an NiMH to death?) I would like to know this too.


Drewdc90 - under high amperage draw I doubt an NiMH is anywhere near 8.4 volts. Doesn't battery labels usually rate cells at ~1.25v @ 30 amps... and guess what voltage drops as amps and heat (unfortuantely) go up. Depending on the need for acceleration I doubt you would be able to stay under 30 amp draws anyway when your always bursting. And at those figures 1.125v * 6 cells = 6.75 volts which is far from 8+ volts. I've speculated that the voltage drop could be dipping into the 5's under high amp draw with NiMH?


Thanks to every body for the helpful knowledge. Let's keep this thread going smoothly to benefit us all.

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Old 11-23-2007, 10:01 PM   #28
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I don't know what gear he's running, but I know my 7700 was geared really tall...high 4s final drive. I don't think you'd gear a 3.5 that tall, but the Castle guys basically told me ot keep going until the motor would get warm...it never really did.

I don't really know...or really think for that matter...that 45 amps is necessarily too high or too much. It just doesn't sound right mathematically given a ~90 amp peak. By my reckoning, just don't spend enough time at or near that 90A peak to offset the times below peak.

We had some of the most ballistic guys in the country at that track (revrace.com) and even the fast guys in 1/8th class were barely cracking 60 (measured on radar) on an even longer straight...and they were only WOT for maybe 1/3-1/2 of the straight...an electric car would only see peak loads for brief moments, then it would drop from there.
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Old 11-24-2007, 10:17 AM   #29
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I don't really know...or really think for that matter...that 45 amps is necessarily too high or too much. It just doesn't sound right mathematically given a ~90 amp peak. By my reckoning, just don't spend enough time at or near that 90A peak to offset the times below peak.
I'll repeat this again like HPI RS4 repeated it. Amperage at peak power is only about 1/2 amperage at stall. Expect a stall amperage about 180 amps from a 3.5. I can generate the entire set of motor curves from the specs of 10,200 kV with some trouble.

Amp draw is a function of traction. If you are getting 10 minute runtimes out of a touring car you don't have much traction. This is the case for me as well when the track is dusty.
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Old 11-24-2007, 10:47 AM   #30
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I'll repeat this again like HPI RS4 repeated it. Amperage at peak power is only about 1/2 amperage at stall. Expect a stall amperage about 180 amps from a 3.5. I can generate the entire set of motor curves from the specs of 10,200 kV with some trouble.

Amp draw is a function of traction. If you are getting 10 minute runtimes out of a touring car you don't have much traction. This is the case for me as well when the track is dusty.
John
That's the thing...you're never stalled when the car is moving. You can't have it both ways...a 70 MPH sprint from a dead stop at every turn?

The traction is another issue. On a race day, Rev would nearly pull your shoes off. Washed, vacuumed and VHT'd. I highly doubt that your track is superior to what Rev's on-road track was before it got turned into an off-road track. Not saying it's worse, but I really doubt you have something unique there.

My point was that a large track will lower the peak amp draws, not raise them, since the car will almost never be near stall.
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