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Old 03-10-2013, 11:44 PM   #1
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Default explanation of electrical current needed, *smart people look*

after doing some research about ohms law on google i am wondering how it all applies to rc cars, when you have a 120 amp esc does that mean it is the total ammount of amps that can pass through it at any given time? and for an esc with a lower amp rating does that mean it just has more resistance? im all very confused by this. any help or clarification is appriciated
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Old 03-11-2013, 12:29 AM   #2
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ESC's usually have two current ratings listed (continuous and burst)

Generally speaking they are referring to the components used in their construction (capacitors, transistors etc....)

Select a motor appropriate for your task (motors also have similar ratings) then choose an ESC that will handle the expected load, best to allow some leeway

You are correct in that a higher rated ESC will generally have a lower resistance

An Example might be ..........

100 amp burst rated motor (740 watts runnning 2s lipo)
120 amp continuous rated ESC
200 amp continous rated battery (5000mah 40c)


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Old 04-12-2013, 11:51 PM   #3
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i'm also looking for on resistance for 17.5 blinky class racing. does the difference between .003 and .006 make more of a difference or can a low end revention or juststock be just as good as a high end esc.
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Old 04-13-2013, 07:14 AM   #4
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So it bascically means that the higher the amp rating the higher the c rating lipo it can handle?
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Old 04-13-2013, 07:41 AM   #5
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Originally Posted by 2dialed View Post
i'm also looking for on resistance for 17.5 blinky class racing. does the difference between .003 and .006 make more of a difference or can a low end revention or juststock be just as good as a high end esc.
Assuming the values are comparable, say from the same manufacturer, then yes theoretically the lower resistance esc would have less power loss. How much practical difference that makes on track might be debatable though. Possibly worthwhile in oval racing, less so in offroad, not sure.
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Old 04-13-2013, 07:42 AM   #6
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So it bascically means that the higher the amp rating the higher the c rating lipo it can handle?
Not really, at least for most RC cars. It’s more about how hot of a motor it can support (low turns or high kv).

What is needed is: Battery amps > ESC amps > Motor amps

Having a higher C or larger amp battery generally isn’t a problem for a RC car. (In specific cases it can be an issue for flight RC, as they can run more constant high throttle with more limited battery and esc sizes than cars typically have)

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Old 04-14-2013, 01:39 PM   #7
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Originally Posted by Dave H View Post
Not really, at least for most RC cars. Itís more about how hot of a motor it can support (low turns or high kv).

What is needed is: Battery amps > ESC amps > Motor amps

Having a higher C or larger amp battery generally isnít a problem for a RC car. (In specific cases it can be an issue for flight RC, as they can run more constant high throttle with more limited battery and esc sizes than cars typically have)
Correct, in cars, it doesn't matter what C rating a battery has(as far as wondering if a C rating is too high for an application), the ESC will only move as much current as the motor can draw. The motor, gearing & any possible drag in the drivetrain are the big factors in determining how much current you're dealing with. Using a higher C-Rating battery will not change any of that, the C rating only implies what the battery is capable of handling, just like the current rating of the ESC. As long as you have one rated high enough for your motor, gearing, car, etc., you'll be just fine....
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Old 04-15-2013, 09:24 AM   #8
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I have used the hobbywing justock 45a esc (apparently really good esc for blinky stock) and a reventon pro 120a esc and noticed no difference.

The amp rating on ESC's tell you how much current it is happy with passing through before things overheat and fail. The WATT rating on you motor determines how much current it will draw.

Lets say your 17.5 motor is rated at 250 watts. It will pull almost 30a at full charge and 39 when the battery is almost dead. So the justock 45a and a 5000mah 10c battery would cover that with a little room to spare for efficiency losses.

C rating isn't really as important as how long your battery will go before voltage drops off. Less decline in voltage will make you go faster toward the end of a race.
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Old 04-15-2013, 11:18 AM   #9
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Everything about an ESC is largely dependant on the load and the resistance it presents to the circuit.

If your motor maxes out at 80A draw (say [email protected] Avg), that's all the ESC is ever going to give it, and that's all your battery will ever have asked of it.

Obviously, motors will draw surge currents during acceleration, as they are now being tasked with generating a significantly larger magnetic field.

Match your motor's surge current ratings to your ESC's surge current ratings. As long as the motor doesn't exceed your ESC's capabilities, you'll never have an issue. Just make sure your batteries can keep up at the end of it all.

Edit: One thing you'll see typically on motors is a maximum watt count. To work that into Amps, it's Watts/Volts=Amps. So a 800w motor is only good for [email protected] Nominal.

Here's some heat numbers for you, too!

Say you've got a motor resistance of 0.8 Ohms. That's 81.6W of heat thrown off. Now here's something neat. Double the voltage, assuming a 4S pack. Now we're only using [email protected], throwing off a measly 40.8W of heat. This is why you'll hear people talking about HV setups being more efficient. Because they are!
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Old 04-15-2013, 12:52 PM   #10
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Originally Posted by 2dialed View Post
i'm also looking for on resistance for 17.5 blinky class racing. does the difference between .003 and .006 make more of a difference or can a low end revention or juststock be just as good as a high end esc.
Out of curiosity I looked into this a little more, see if I could provide a rough estimate of the power difference. Those ESC resistance numbers seemed fairly high, so I used the popular Tekin RS and RS Pro ratings of 0.0003 and 0.00015 ohms for brushless operation.

Note from the typical 17.5 dyno chart amps can vary from around 100A at low RPM to around 25A at high RPM. Have to pick a battery voltage, lets assume a nearly full charged 2S can maintain 8V under the 25 amp load and 7.4V under the 100amp load to the ESC (Starting voltage of 8.2V, IR of 8 milli ohms).

At the 100A load, the delta of the voltage drop is 0.015V, the power delta is about 0.41%.

At the 25A load, the delta of the voltage drop is 0.00375V, the power delta is about 0.094%.

If the ESC resistance numbers were as high as the numbers you mention, the power deltas would be about 9% and 1.9% respectively.

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Old 04-15-2013, 01:01 PM   #11
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Originally Posted by neospud View Post
I have used the hobbywing justock 45a esc (apparently really good esc for blinky stock) and a reventon pro 120a esc and noticed no difference.

The amp rating on ESC's tell you how much current it is happy with passing through before things overheat and fail. The WATT rating on you motor determines how much current it will draw.

Lets say your 17.5 motor is rated at 250 watts. It will pull almost 30a at full charge and 39 when the battery is almost dead. So the justock 45a and a 5000mah 10c battery would cover that with a little room to spare for efficiency losses.

C rating isn't really as important as how long your battery will go before voltage drops off. Less decline in voltage will make you go faster toward the end of a race.
Amps do not go up as the battery discharges. Voltage, amps, and power all drop off as the battery discharges.

A truly higher C battery of same capacity will maintain a higher voltage under load throughout the run. Or it can be looked at in terms of IR, a lower IR battery will provide same.
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Old 04-15-2013, 01:05 PM   #12
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Originally Posted by WindDrake View Post
Everything about an ESC is largely dependant on the load and the resistance it presents to the circuit.

If your motor maxes out at 80A draw (say [email protected] Avg), that's all the ESC is ever going to give it, and that's all your battery will ever have asked of it.

Obviously, motors will draw surge currents during acceleration, as they are now being tasked with generating a significantly larger magnetic field.

Match your motor's surge current ratings to your ESC's surge current ratings. As long as the motor doesn't exceed your ESC's capabilities, you'll never have an issue. Just make sure your batteries can keep up at the end of it all.

Edit: One thing you'll see typically on motors is a maximum watt count. To work that into Amps, it's Watts/Volts=Amps. So a 800w motor is only good for [email protected] Nominal.

Here's some heat numbers for you, too!

Say you've got a motor resistance of 0.8 Ohms. That's 81.6W of heat thrown off. Now here's something neat. Double the voltage, assuming a 4S pack. Now we're only using [email protected], throwing off a measly 40.8W of heat. This is why you'll hear people talking about HV setups being more efficient. Because they are!
Only true if the load is changed, typically meaning a motor change. Or at the very least significant gearing change. This is a frequently overlooked point when discussing the practical use of higher voltage to gain improved efficiency.
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Old 04-15-2013, 02:20 PM   #13
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Only true if the load is changed, typically meaning a motor change. Or at the very least significant gearing change. This is a frequently overlooked point when discussing the practical use of higher voltage to gain improved efficiency.
^ This is true. I suppose I should start warning people about running numbers against an unloaded motor.
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Old 04-15-2013, 03:31 PM   #14
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Oops, nevermimd
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Old 04-15-2013, 07:12 PM   #15
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Amps do not go up as the battery discharges. Voltage, amps, and power all drop off as the battery discharges.

A truly higher C battery of same capacity will maintain a higher voltage under load throughout the run. Or it can be looked at in terms of IR, a lower IR battery will provide same.
What amps don't change? Are you saying a 250 watt motor running on a 8.4v battery and another on a 6.0v battery pull the same amps? Or are you saying the battery can supply less than when fully charged? Just because a battery isn't at 100% capacity doesn't mean it can't still cope with demand does it?
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