Hi all, if my Conscendo has a 30-amp ESC, does that mean that as long as my LiPo can deliver 30 amps, I am OK?

If I have a 1500 mah LiPo that has a rating of 25c, my understanding is that it can deliver 37.5 amps continuously because the formula (I think) is 1.5 amp-hours x 25c = 37.5 amps. But is that correct? Maybe 37.5 is the burst/max amps, not continuous? But if my ESC is a 30 amp unit, does that means 30 amps is the max anyway that my motor can draw?

Thanks,

-Tom

ESC's are spec'd to be rated slightly higher than the motor/prop combination can deliver, so with an ARF like that where they pick the esc/motor/prop combo you're usually safe, but if you were to start tinkering with prop size it would be possible to over-amp or over heat a component.

I was once a high school Physics teacher so hopefully I will steer you on a righteous path. Just before we get to your questions, can I give you a little information, which is actually extremely important that you understand, if you are going to really get into electric power and you want to end up building your own models, and choosing suitable electric motors, esc's and batteries yourself ( rather than just buy RTF or ARF's with them already installed ) or else just using what someone else tells you they use. I am actually a bit unsure of your knowledge level. Apologies if you already know this !

Of the three components (motor, esc, battery), the current is drawn by the motor, the maximum value being determined by the motor's resistance and the battery's voltage. The esc "current rating" and the battery "current rating" do NOT determine the current flow in the circuit. Those ratings are just maximum safe values for those components, not to be exceeded. You have to make sure that the esc's current rating and the battery's current rating ( what you talked about in your post ) are high enough to cover the motor's demands i.e. at least as high as the max. current draw of the motor, but preferably a little higher to give you some safety margin. About 20 % higher works about right.

If you are setting up your own system from scratch, you always start with choosing the motor ( that's another topic ). Then you can select a suitable battery cell count / voltage to power the motor. Then you figure out the MAXIMUM current this motor will ever draw (which is highly dependent on propeller choice and throttle setting). Then you choose the esc current rating and the battery current rating to match.

An example: If a motor decides it wants ( draws ) a maximum of 40A, then you must use at least a 40 A rated esc and at least a 40 A rated battery ( continuous discharge rating, not burst rating ). But since you should factor in a safety margin, it's much better to use a ( say ) 50 A esc and a 50 A battery ( continuous rating ) with a 40 A motor.

In other words, the esc current rating and the battery current rating can be about the same, but should be about 20 % higher than the max. current the motor will ever draw.

Now to your questions ( bear in mind I am not sure if you have chosen the motor or it's what was factory installed. I don't know the Conscendo. But it makes no difference to what I have written. )

If I have a 1500 mah LiPo that has a rating of 25c, my understanding is that it can deliver 37.5 amps continuously because the formula (I think) is 1.5 amp-hours x 25c = 37.5 amps.

That is totally correct.

if my Conscendo has a 30-amp ESC, does that mean that as long as my LiPo can deliver 30 amps, I am OK?

If your esc was chosen correctly, the answer is yes.

But if my ESC is a 30 amp unit, does that means 30 amps is the max anyway that my motor can draw?

If your esc was chosen correctly, the answer is yes. ( Bearing in mind the 20 % thing. So the motor should really draw no more than say about 25 A. )

Geezus, I need to answer math questions after I wake up, you are correct of course.

lol. Thanks for the confirmation.

Tphage gave an excellent answer, but let's look at it slightly differently:

The 30 amp rating is the maximum the ESC can pass through to the motor. If the motor demands more (and the battery can provide), the ESC has a high likelihood to fail (ie "let the smoke out").
Your battery math is correct. If your ESC was, say 50 amp, then the limit to the motor would be the 37.5 maximum the battery can provide. While not great to push the battery to the max all the time, no big harm except possibly shorter long term life. You just may not be able to provide the motor with the maximum power it might be capable of producing.

"Oversizing" the battery "C" rating and ESC Amp rating above the amount the motor/prop will ever demand gives you a margin to allow things to run cooler and have a longer overall lifespan. But it doesn't give any boost in performance. And costs more. So some is good, lots is a waste. Your system has that accounted for, already.

Thanks everyone!

@Tphage, I wonder if you also have an answer to my question about how much cell voltages on lipo batteries can differ before it might be a problem:
Lipo balance: how much difference is acceptable?

I'd like to summarize the answer. Others have given great details.

Votts, measured at the motor, determine RPM. Nothing else matters for this.

Amp draw is determined by load.. The motor will draw as many amps as it needs to spin the prop you choose at the RPM that you've commanded with the battery voltage. Nothing else in the equation really matters for the question of how many amps will flow through your system. (one exception to that explained below)

If your amp draw goes above the motor's limits (usually due to using too bi a prop) it will overheat and fail.

If your amp draw exceeds the ESC's rating, it will overheat and fail. It might catch fire too, or maybe not.

If you amp draw exceeds what the battery can supply, your voltage will go down, which makes your motor RPM go down. That, in turn, makes your amp draw go down. C rating has very little to do with it. Internal resistance is the actual measurement that determines how many amps you can get out of your battery.

I guess I’ll throw my 2 cents in here as well. As Jester has explained, prop size ( load ) is what is going to determine how much current you will pull. The battery will provide up to 37 amps in your particular case however not for very long. As rule of thumb, I set up my electrics to never pull more then 1/2 of the C rating of the battery. Most of my electric flying has been with 10S 5000 mah packs with a 35C rating. Max amp draw is 70. My packs went 3 years before showing signs of wear. Another thing not mentioned is directing airflow over/through all your components to dissipate heat. This is an absolute must.