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Apr 07, 2021, 07:59 PM
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Battery life with brushless ESC and motor


I've been trying to determine the battery life of 2200mAh 11.1V LiPo battery. I'm using an ESC with continuous output of 60A and an UBEC rating of 5.5V/4A with a voltage input from 6.4-16.8V. The motor is 20A with a 12-24V / 30-200W range and rated at 1000 rpm per volt. I thought the output current of the battery must exceed the output of the ESC. If the ESC is outputting 60A continuously the battery would have to operate at its max discharge rate of 66A (given by specs). I'm not too familiar with the functionality of ESCs. My question is how much current is being drawn from the battery?
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Apr 07, 2021, 08:32 PM
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The motor will draw whatever current it needs to drive the load it is attached to. Without knowing the load (propeller), it is impossible to answer your question. In any case, the best answer is to just measure the current. A wattmeter is an essential tool if you are planning to do anything involving electric motors, so the sooner you get one, the better.

Those 20A, 60A and 66A specs are a maximum rating the component is capable of if asked to do so. They don't mean that the component is continuously pushing out that much current.
If the motor is not connected to anything, the current will be an amp or two, to overcome bearing friction and other losses.
If you put on a big prop that needs 100A to turn, the motor will (try to) draw 100A and something will fail spectacularily.
Apr 07, 2021, 08:46 PM
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I'm using a Eastar 12-24V DIY Ship Motor Underwater Propeller Thruster Fit for RC Boat AUV Robot. How do calculate the load of the propeller?
Apr 07, 2021, 09:07 PM
Registered User
Ah, a water prop. That's outside of my knowledge, sorry. Rcgroups has a Boats section, maybe sombody there would know. Or maybe in the Modelling science section.

But as said, calculations are only as accurate as your assumptions of initial conditions. Better to just measure it.
Apr 07, 2021, 09:48 PM
If it flies, I can crash it.
rocketsled666's Avatar
The rules are basically the same. But the question as stated cannot be answered.

The parameters listed are all "maximum rated"/"do not exceed" values, not what is actually going to be required from the power system when it's in operation.

The ESC doesn't "output" anything, it just passes voltage and current from the battery to the motor. The prop diameter and blade count and blade pitch and motor RPM dictate how much current the motor will draw in operation.

If the current the motor will draw with the prop of choice exceeds the motor's maximum rating, the motor will be destroyed. If the current the motor will draw is greater than the ESC's maximum rating, the ESC will be destroyed. If the current the motor will draw is greater than the battery can deliver, the battery will be destroyed.

It all comes down to how much current the motor will draw. That's a function of the motor, the prop, and the operating voltage. It's not easy to figure out, the equations are complex and not at all "linear". There are on-line tools that can do the math for you, though. The one I've used is eCalc and I think it has a boat calculator (I've only used the airplane and multirotor calculators).
Apr 08, 2021, 10:47 AM
homo ludens modellisticus
Ron van Sommeren's Avatar
The 'full' story.
Even a seemingly small change/error in setup can result in quite an unexpected increase.
Therefore always measure current and power in a new or changed setup.
Because current and power drawn are proportional to
  • pitch
  • #blades
  • voltage² respectively voltage³
  • kv³
  • diameter⁴
So even a small change/error in setup can have huge effects.
Increase in current with one or two cells added, simple table - RCG

Expensive examples.
Going from 2s to 3s, a 50% increase, would (3/2)²=2.2 fold current, more than double, 120% extra.
A 25% increase in velocity konstant kv³ would (1.25)³=2 fold current, 100% extra.
A mere 10% increase in diameter would (11/10)⁴=1.1⁴=1.4 fold current, 40% extra.

Extreme examples.
Doubling voltage would 2²=4 fold current.
Doubling velocity konstant kv³ would 2³=8 fold current.
Doubling diameter would 2⁴=16 fold current.
Apr 08, 2021, 10:48 AM
homo ludens modellisticus
Ron van Sommeren's Avatar
Without a watt-meter you are in the dark.
Until something starts to glow
Vriendelijke groeten en wees voorzichtig, Ron
• Without a watt-meter you're in the dark ... until something starts to glow •
E-flight calculatorswatt-metersdiy motor tips&tricksCumulus MFC


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