Max amps: is it dependent on how many cells?

[sumguy75]

I have a 2650kv 4068 motor that specs listed are max 18V &max 122A. I want to run it on a rig with a 120A Max10sct but don't want to overamp the esc.

Would running 3s have lower max amps than 4s? Seems like that would be ok but not sure.

 

[Dan B.]

The lower the voltage, the higher the amp draw is. If you’re borderline, then 4s would be the way to go if you can. 3s, all other factors being equal, will draw more current than 4s.

 

[sumguy75]

Wondered if I had it backwards, glad I asked!

 

[jkflow]

Careful, the math is correct, but keyword was 'if all else is equal'.
In your application, you would draw less amps on 3S and more on 4S because it's not simply equal. You get less rpm and less power as a result.
Obviously, also depends on gearing etc.

The 120A should be ok for your application, there is no hard cutoff at 120A and those 2A extra won't do it.

ESC/Motor should be matched closely. A 150A would probably be fine as well.

 

[Dan B.]

Careful, the math is correct, but keyword was 'if all else is equal'.
In your application, you would draw less amps on 3S and more on 4S because it's not simply equal. You get less rpm and less power as a result.
Obviously, also depends on gearing etc.

The 120A should be ok for your application, there is no hard cutoff at 120A and those 2A extra won't do it.

ESC/Motor should be matched closely. A 150A would probably be fine as well.

I’m not following. So a 2650kv motor on a 120amp ESC will pull fewer amps on 3s than 4s? I don’t understand why that would be. If he gears it for his application, even if changing gearing from 3s to 4s, shouldn’t 4s pull less amps? It has to, right? Unless there is some really out of whack gearing, or something I’m just not understanding.
Not arguing, I know you have the experience here, I’m just not understanding the physics of why this would be and would really appreciate an explanation.

 

[dure16]

I’m not following. So a 2650kv motor on a 120amp ESC will pull fewer amps on 3s than 4s? I don’t understand why that would be. If he gears it for his application, even if changing gearing from 3s to 4s, shouldn’t 4s pull less amps? It has to, right? Unless there is some really out of whack gearing, or something I’m just not understanding.
Not arguing, I know you have the experience here, I’m just not understanding the physics of why this would be and would really appreciate an explanation.

I think you’re assuming gear change for similar top speed. And @jkflow is assuming no gear change.

 

[Dan B.]

I think you’re assuming gear change for similar top speed. And @jkflow is assuming no gear change.

Maybe so. But just to be clear,
Take any rig with a 4s capable motor and ESC, and any sort of “sane” gearing. Don’t change the gearing between packs. Run it balls to the wall on 3s, and then in 4s. Read the logs. It pulls less amps on 4s, right?
Even if you change the gearing based on your goals, and then do it all again it will still pull fewer amps on 4s than 3s.
Or am I just losing my mind? Same setup, same load: lower voltage=higher amperage.

 

[dure16]

Take any rig with a 4s capable motor and ESC, and any sort of “sane” gearing. Don’t change the gearing between packs. Run it balls to the wall on 3s, and then in 4s. Read the logs. It pulls less amps on 4s, right?

No, I don’t think that’s correct. You “volt up, gear down” for better efficiency. Watts = volts x amps.

So, if you gear for similar speed, then you may be able to assume similar watts are needed. And since volts are higher, amps will be lower.

But in your example without changing gearing, the car will be traveling significantly faster on 4S, which will also increase the watts required. You’ll see at least as many amps through the system as you did on 3S, I think.

 

[jkflow]

RPM is the key here, motor won't give you as much power on lower rpm (3S).
The lower current is true if output power stays the same but at higher rpm you get more power, so to speak.
If you keep the gearing the same you will draw more current on 4S vs 3S.
The lower current is true if you use a lower kV motor on same gearing with same power to the road.
Hope that makes sense.

 

[Jerold]

The question in the title post: "Max amps: is it dependent on how many cells?"

From the batteries, perspective it can mean more voltage or more amps or both. It depends on the configuration. Cells in series give you voltage. Cells in parallel give you more current.

I want to run it on a rig with a 120A Max10sct but don't want to overamp the esc.

Yes you can be over current. It doesn't matter how many cells you have because the ESC will be the bottleneck. Your motor can try to draw 1,000 amps and if the ESC can't do it, you should be limiting the power. If your batteries can't give the current to the ESC, that will be limiting your ESC.

Most ESCs should have over current protection, meaning if the motor goes crazy it will limit the current. It may cut out, give you warnings or beeps. I say most because I'm sure there are some that don't do that.

It's not to say that you can't grenade your ESC, but it should at least complain first.

BTW you'll be fine. 122 Amp vs 120Amp is in the noise, it's less than 1% difference. That spec is also continous current, not burst current. If they didn't build in 1% margin it was going to break anyway.

https://www.hobbytown.com/hobbywing...-sensorless-brushless-esc-hwa30102601/p507477

 

[Sandracer_NL]

AFAIK the max motor amps specs are not that relevant for comparing it with esc amp specs. The max motor amps are is the amount of amps at which the motor can safely operate (let’s say run an hour or so without overheating).
The motor can draw more amps than the max motor amps stated in the specs.
Watt = Amps x Voltage. If the specs state the power (Watt) of the motor you can get an indication of the amps given the voltage you will be using.

 

[Neelis]

RPM is the key here, motor won't give you as much power on lower rpm (3S).
The lower current is true if output power stays the same but at higher rpm you get more power, so to speak.
If you keep the gearing the same you will draw more current on 4S vs 3S.
The lower current is true if you use a lower kV motor on same gearing with same power to the road.
Hope that makes sense.

I disagree on this partly, actually mostly.

It is all about load and load is defined by all kinds of factors like the type of surface you are driving on, are you on top speed or accelerating, wind/air resistance at high speed, weight of the car and RPM's.

If you take an RC car that is heavy, with big grippy wheels, punch setting to max and you accelerate at maximum power, driving on long grass, you draw way more amps then at max RPM on smooth tarmac at top speed.

An electric engine has an X watt power that is can handle, let's say 2000 watts. That is about 159 Amps at 3s and 119 amps at 4s.

RPM's alone are not key here, is the load that is key here.
You don't get more power at higher RPM, that counts for petrol engine to some degree, but not for e-motors.

@sumguy75

I guess you have a Surpass Hobby motor?
The 4068 2650kv motor is rated at 2200 watts. That would be 174 amps at 3S and 131 amps at 4s. Not sure your ESC is going to like that.
If the 122A is the leading number, you will not draw more than 1537 watts.
Maybe motor experts can say something about this?

 

[jkflow]

I disagree on this partly, actually mostly.

It is all about load and load is defined by all kinds of factors like the type of surface you are driving on, are you on top speed or accelerating, wind/air resistance at high speed, weight of the car and RPM's.

You are actually agreeing.

You are saying the load matters, which was my point.
This specific motor runs at 2650*3S*3.8V = 30000 rpm while 4S = 41000rpm.
Higher speed = higher resistance = more power required

Current will be higher on any surface on 4S as all things are not equal, i.e. rpm.
You gear down to the same rpm and yes, current would be lower.

 

[Dan B.]

Although I appreciate all the technical info and explanations, I believe we’re straying a bit. Not comparing load, setup, conditions where run, gearing, etc. NONE of that matters in relation to the OP question.
Set up your truck however you want. It will pull fewer amps on 4s than it will on 3s. Period. The end.

 

[Neelis]

Although I appreciate all the technical info and explanations, I believe we’re straying a bit. Not comparing load, setup, conditions where run, gearing, etc. NONE of that matters in relation to the OP question.
Set up your truck however you want. It will pull fewer amps on 4s than it will on 3s. Period. The end.

You must be an engineer!

 

[Dan B.]

You must be an engineer!

Do you disagree with my statement?

 

[Neelis]

You are actually agreeing.

You are saying the load matters, which was my point.
This specific motor runs at 2650*3S*3.8V = 30000 rpm while 4S = 41000rpm.
Higher speed = higher resistance = more power required

Current will be higher on any surface on 4S as all things are not equal, i.e. rpm.
You gear down to the same rpm and yes, current would be lower.

That is not what I am saying and I don't agree with that.

If we leave the load out of the picture, the motor can deliver an X amount of watt maximum, that is a fixed value.
To reach that maximum you need an X amount of current and that is related to the voltage delivered by the battery. Voltage is defined by the battery and the end result is more amps at a lower voltage.

Do you disagree with my statement?

I agree with you that we should focus on the question of the topic starter and I think that is what we are doing.
And I also agree with you that it will pull more amps on 3s then on 4s.

jkflow is saying that the system pulls less amps on 3s then on 4s and I disagree with that and that is what we are discussing, so I would say that it relates to the topic starters question.

 

[Hector_Fisher]

To answer the OP question you have to examine electrical load, no getting around it. Amps is a symptom of load, which is governed by gearing, weight of the vehicle, and motor size/quality.

If we assume in OP question that in switching to 4s the gearing is changed to match the same top speed as on 3s, then watts would likely go down. However, it's more complicated if just switching straight from 3s to 4s.

In my experience, swapping up in lipo cells without changing gearing ends up lowering amperage, but not due to anything electrically. Rather, you've effectively gearing up by 50% and you will now spend less time on throttle because of it, effectively being more "power efficient" due to lower load requested from the motor. At least I've deduced that from the lower temps I've seen. Just my two cents.

 

[Jerold]

AFAIK the max motor amps specs are not that relevant for comparing it with esc amp specs. The max motor amps are is the amount of amps at which the motor can safely operate (let’s say run an hour or so without overheating).
The motor can draw more amps than the max motor amps stated in the specs.
Watt = Amps x Voltage. If the specs state the power (Watt) of the motor you can get an indication of the amps given the voltage you will be using.

Not sure I follow this. It sounds like you are saying the ESC is supposed to limit the current for the motor, which should not be the case. . .or at least it shouldn't be. But I've seen weirder things in the RC world. There seems to be some terms in the physical/engineering world got redefined to something completely different for RCs.

The motor is a load, the ESC is a source. The load can only get as many amps as it's given from the source. Normally the max current for a load means the maximum it will draw and you would spec your source for a bit more than that so you don't starve it. If I've got a 1500W hair dryer, I shouldn't expect it to work on a 1000W circuit. It would pop the breaker.

There is a lot of marketing terms thrown around here. We have two specs, maximum continuous current and burst current, which don't exist in the real world. The 120A is maximum continuous current. The burst current happens for a very short magical undefined amount time (sarcasm) when the system is suddenly loaded up and you get a current spike.

If you take the motor at max voltage at max rpm, with no load it should draw no more than the maximum continuous current on the spec'd.

With that said if you buy and industrial DC Brushless motor they usually spec it in terms of power (Watts) and they give you torque curves. There is a maximum voltage but typically amps are not stated. Which means you would draw more amps at lower voltage to utilize the full power. You can run it at lower current, but you won't get the same torque.

I would take th 120A as your goal and make sure the "burst" current of both roughly align. If you are unsure get a bigger ESC.

 

[Sandracer_NL]

Not sure I follow this. It sounds like you are saying the ESC is supposed to limit the current for the motor, which should not be the case. . .or at least it shouldn't be. But I've seen weirder things in the RC world. There seems to be some terms in the physical/engineering world got redefined to something completely different for RCs.

The motor is a load, the ESC is a source. The load can only get as many amps as it's given from the source. Normally the max current for a load means the maximum it will draw and you would spec your source for a bit more than that so you don't starve it. If I've got a 1500W hair dryer, I shouldn't expect it to work on a 1000W circuit. It would pop the breaker.

There is a lot of marketing terms thrown around here. We have two specs, maximum continuous current and burst current, which don't exist in the real world. The 120A is maximum continuous current. The burst current happens for a very short magical undefined amount time (sarcasm) when the system is suddenly loaded up and you get a current spike.

If you take the motor at max voltage at max rpm, with no load it should draw no more than the maximum continuous current on the spec'd.

With that said if you buy and industrial DC Brushless motor they usually spec it in terms of power (Watts) and they give you torque curves. There is a maximum voltage but typically amps are not stated. Which means you would draw more amps at lower voltage to utilize the full power. You can run it at lower current, but you won't get the same torque.

I would take th 120A as your goal and make sure the "burst" current of both roughly align. If you are unsure get a bigger ESC.

The motor is in the lead; it pulls whatever amps it needs (Watt=amps x volts) from the esc, which in turn pulls them from the lipo’s. A 150a esc doesn’t limit that at 150a, the 150a is an indication of what it ‘safely’ can deliver for a certain amount of time.
If your lipo’s can’t deliver the amount of amps the motor is asking for, your lipo’s will go bad (puffing). If your lipo’s can deliver the amps, but it’s above the esc specs, your esc will overheat and either go on thermal shutdown or burns down .
Based on the motor power specs (Watts) you can calculate, based on on how many volts you will be running the car (3s, 4s etc.), an indication of the esc amp specs you should be looking for.