# How do I calculate the current used by a BLDC motor under a load?

I am working on a robotics project where we are currently designing a battery. This battery is specific for a motor driver and two BLDC motors. I would like to know how much current will be drawn from the battery if we drive the robot at some speed. More specifically, How do you calculate the speed of a BLDC motor under some load, and the current needed for that speed. Our design has rear wheel drive as well as the motors connected to gear system that turns the wheels.

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• You need to know your load and speed to derive the mechanical power required. You’ll probably need to consider F =MA. There will be various losses, both mechanical and electrical, but the electrical power required will be a relation to the mechanical power required. Commented Jul 18, 2023 at 14:05

You need to know the mechanical power (P=2Pi*M*n) the motors have to provide in order to do their job. The mechanical power is equal to the electrical power (P=U*I) your battery has to deliver.
Knowing your battery voltage you can then solve 2Pi*M*n=U*I for the current and you have your answer. Of course, this doesn't account for any losses or the battery voltage not being exactly constant. But it gives you a rough estimate.