# Calculating Amp Draw????



## tekrsq (Feb 28, 1999)

How would I calculate the amp draw an electric car would be putting on the battery pack? I know ever car, esc, motor, etc draws different amounts of current. I'm trying to see if there is a way to get a rough idea of runtime off a particular pack with a particular setup.


----------



## The Jet (Sep 25, 2001)

I believe this will work for you.
take the discharge time of your pack, ie. 420sec X 30amps =12600 then divide that number by the seconds of your race, ie. 300 plus what ever time you had left on the pack let's use 30 seconds. So now you have 12600 divided by 330= 38.181818, that means you averaged 38.181818 amps for that run.
Hope that works for ya.
Later, Bret


----------



## tekrsq (Feb 28, 1999)

That might work. What I was hoping for was maybe a way to hook a amp or volt meter to the motor (or where ever was needed on the car), open the throttle, and see what the draw would be. Can something like that work?


----------



## pepe (Sep 29, 2001)

The Jet said:


> I believe this will work for you.
> take the discharge time of your pack, ie. 420sec X 30amps =12600 then divide that number by the seconds of your race, ie. 300 plus what ever time you had left on the pack let's use 30 seconds. So now you have 12600 divided by 330= 38.181818, that means you averaged 38.181818 amps for that run.
> Hope that works for ya.
> Later, Bret


This is the best way to get avg amp draw for a run which is what I believe your looking for.


----------



## Nil (Dec 8, 2001)

tekrsq said:


> That might work. What I was hoping for was maybe a way to hook a amp or volt meter to the motor (or where ever was needed on the car), open the throttle, and see what the draw would be. Can something like that work?


Unless you have some way to load the motor similar to racing loads, no. A motor running free draws far fewer amps than one under load. If you could load it, you would still have problems:

1. Ammeters capable of handling the peak load from an RC are not cheap.
2. What load would you use to simulate "average" race conditions? Since RPM is _extremely_ variable during a race, so is amp draw. 

As other are saying, the best simulation of a race is . . . a race!


----------



## tekrsq (Feb 28, 1999)

The Jet said:


> I believe this will work for you.
> take the discharge time of your pack, ie. 420sec X 30amps =12600 then divide that number by the seconds of your race, ie. 300 plus what ever time you had left on the pack let's use 30 seconds. So now you have 12600 divided by 330= 38.181818, that means you averaged 38.181818 amps for that run.
> Hope that works for ya.
> Later, Bret


Ok, I'll try that. Is it typical for electric TCs to draw that many amps during a race?? I always thought my 30amp discharger was "extreme", but I guess not if they pull that much or higher.


----------



## David Root (Sep 5, 2002)

*Draw*

The best way is to run your car and see what is left in the pack after the race. You will find out in a hurry that different people and different styles require all kinds of different amps for a race.

Mod Oval probably draws or Uses the most amps if you are a good driver. 1/12 stock probably draws the least. Mod TC must be right behind oval. 

You can make your motor draw 70 amps by holding it still at full throttle, or it will take as little as 5 with no load. 

I THINK mod oval uses 30 to 45, 1/12 stock uses less than 20. These are average through out a race.

DynaMoHum has some pretty good ideas on this, but I have not heard from him lately. 

David Root


----------



## The Jet (Sep 25, 2001)

If you think 38 amps average is alot hows this. I dumped a 425 second battery in 240 seconds (a 4 minute race) that is 53.12 amp average    :thumbsup:


----------



## hankster (Jan 1, 1998)

Actually, it is most likely quite a bit less then 53 amps. At higher amp draws, your voltage is depressed a lot more, so you don't get the same amount of mAh hour out of your battery to the .9 cutoff.

An example of the old GP3000 cells is at http://www.teamrcv.com/modules.php?...ns&file=index&req=viewarticle&artid=51&page=1

There is shows 600 seconds of run time at 19 amps and 245 seconds at 40 amps. My somewhat educated guess is that you were actually averaging a little over 42 amps to dump that pack in 240 seconds.


----------



## The Jet (Sep 25, 2001)

OK Hank, now I'm confused even more so.
If you have a pack that is [email protected] amps and you dump it in 249 seconds, is'nt that a 46.16amp adverage instead of the 40 stated??? But now I see if I reverse it 249x40 = 9960 divide by 605 = 16.46, not 19. Wow this is too confusing...My head hurts now, LOL. Something does change, kinda exponentialy I guess.
So in summary, theres no cut and dry formula???

Thanks, Bret


----------



## David Root (Sep 5, 2002)

*Internal Resistance*

As internal resistance goes up, discharge rate in amps is limited. Anything more is converted into heat. Just try a pack with real high internal resistance, and you will see. The pack still dies, but what comes out of it in AMPS is not near as much as if it had less of a load.

Try it with AA batts. They are all rated at C5. That woulld be a 5 hour discharge to bring them dead (.9 volts/cell). Try to run them down in one hour. They will NOT put out rated capacity or even close. Now try to do it in 1/2 hour, they capacity goes down even more.


----------



## hankster (Jan 1, 1998)

At a battery is dischaged at a higher amp rate, the voltage is lower. Notice that the average voltage on a pack that is discharged at 20 amps is higher then if it is discharged at 30 amps. This is because the more current that a battery gives up, the lower the voltage at that current. 

Just as an example (these are not real numbers, just used as an example) if a battery pack is discharged at 20 amps, the voltage might be 7.0 volts. If the same battery is discharged at 40 amps, the voltage might be 6.7 volts. Since the voltage is lower at higher discharge rates, the battery will hit cutoff (or dump) sooner because the voltage is already .3 volts less during discharge.


----------



## erock1331 (Apr 1, 2002)

Ok gonna revive this old thread, cause I am trying to find which amp steps I should look at on the dyno for my stockers, currently using 18-26A.

With the stockers we use now I come off with about 110 on average for most tracks
So using Brets Calc:

330x35=11550
240 second race + 110 remaining = 350 
11550/350 = 33 amps

I have always heard stockers pull alot less than that which makes Hanks theory seem right. In the example above he took about 11 amps off Bret's calc (53 amps vs 42 amps = 11 amps diff) 

So should I take off about 11 amps (33A-11A = 22 amp average) ??


----------



## hankster (Jan 1, 1998)

For what it is worth, here is how I do it. For this to be anywhere close, you HAVE to discharge the pack immeditatly after you are done running. Waiting the 6 mintues or so to turn marshal then race will throw the readings way off.

You first have to know how many mAh the battery puts out during the discharge cycle. This can be done by cycling the pack. Number is in mAH, such as 3400.

 In this example I will use a "fake" pack with the following numbers.
Discharge rate = 35 amps
Seconds = 360
Capacity = 3300 mAh
Sec. left in pack after race = 110
You will have to substitute your own numbers above.

First thing I do is find out how many mAh is used per second during the discharge cycle
3300 / 360 = 9.1

Now we find out how many mAh are left in the pack after the race
110 * 9.1 = 1001

Find out how much of the battery in mAh was used during the race
3300 - 1001 = 2299

How many mAh per second were used during the race
2299 / 240 = 9.58

Now lets compare this number to the known (35 amp) discharge rate to get the final answer
9.58 / 9.1 * 35 = 36.85

So in my "fake" battery and race above, the motor drew 36.85 amps on average during the race. Of course the actual number will be slightly lower then this due to the time between the end of the race and the time you were actually able to discharge the pack. I hate guessing but I would say maybe 10% less then the figure. Which about the same as you come up with and I believe your numbers are correct.


----------



## tfrahm (Nov 14, 1998)

Here is a real world example from my local carpet oval 4-cell stock racing:

After the A-main:
4-cell GP3300's had 1160 mAh left, or 3:33 (213 seconds) at 20A discharge

Last T35 cycle (also at 20A)
2922 mAh, with 526 seconds of runtime 

"usage" calculations yield:
Motor Used : 1762 mAh in 4:02+5 (race time plus a "cool down lap"), or 247 seconds...
Which then calculates to 25.68 Avg Amp draw for race + cool down lap 
over the last several months, my average amp draw for 4-cell stock has been 26.88 Amps using this calculation...

On my TD45, I look at the 25 and 28 amp numbers....


----------



## erock1331 (Apr 1, 2002)

Thanks Guys !!


----------



## DynoMoHum (Sep 25, 2001)

Hanks theory is very likely correct if I understand it right... He seems to be saying that when you draw alot higher amps then the packs were rated/matched at, then your calculations aren't going to be right if you use the typical methood... well in fact anything short of testing your batteries capacity at what is very close to your race average is going to yeild less then accuate data...

I do belive it's common for stock oval racers to average at least 30 amps during a race... I'm not sure anyone racing these days ever goes as low as 20 amps, exept possibly for 8 minute 12th scale racers.

If your packs capacity/run time is calculated using 30 amp discharge rates, you should be able to take the tested capacity, subtract the run time remaining (when also done at 30 amps), then devide the number of seconds mAseconds by the number of seconds you raced... That should get you pretty close... using 35 amp discharg numbers will likely get you even closer when your averaging near 35 amps... Start averaging much over 35 amps, and you get back to the problem hank seemed to be pointing out...

There is another solution to all this... get yourself a eagle tree data logger, and collect the amp draw data as you run...


----------



## Fred B (Sep 26, 2001)

Just an FYI, in 12th roadcourse we draw about 20 amps average in stock and about 27 amps in mod. Using amp settings around the average will work well for oval but in roadcourse racing we see the entire RPM range (or amps if you think that way). 12th spikes up to 40 amps in stock and runs down to 10-15 amps at the end of the straight.

Mod roadcourse is an entirely different animal (in 12th) because you're only using part throttle on the infield (reduced voltage).

Dyno is correct that the best way to get what you're looking for is to use the Eagletree system.


----------

