LiPo discharge control

Question

How to control the amount of current I drain from a lipo? U=I*R and I know R but how do I control the values of U and I? If the battery has 1000mAh, 10C and 10 V (I made this up), does the battery constantly drain 10 A at 10 V and I have to increase the resistance to lower the current?

Answer 1

The speed of a DC motor is proportional to the voltage applied to it. Current draw is proportional to torque. If you connect the motor directly to the battery then it will try to spin up to whatever voltage the battery puts out, and the only way to control current will be to vary the load (torque).

If you could somehow lower the motor's voltage then it would spin slower. Since most loads require less torque at lower rpm, lowering the voltage should also reduce the current. The simplest way to reduce motor voltage is to just put a resistor in series, which drops a voltage proportional to the current flowing through it (Ohm's Law).

So if your motor draws 1A at 10V and you insert a 5Ω resistor in series then the motor will only get 5V, which would slow it down to half speed. If the load requires less torque to spin at this lower speed then current consumption will also drop, causing the voltage and current to reach equilibrium somewhere between 5~10V and below 1A.

However if the load demands more torque then current will increase and the voltage will drop even lower. Changes in loading will cause the motor to speed up and slow down dramatically. If too much load is applied the motor may stall and not be able to start up again.

Another problem with using a resistor is that it will waste power and get hot (Power = Voltage x Current, and the resistor converts that power into heat).

The speed variation problem can be solved by using a transistor with a feedback control loop that varies its 'resistance' to keep the motor voltage constant while allowing it to draw the current it needs. However this 'linear voltage regulator' will still waste power and get hot.

Luckily there is another solution that doesn't waste power - PWM (Pulse Width Modulation). This makes use of the motor's mechanical inertia and inductance to control the speed without having to drop voltage and pass current at the same time.

Switching the motor on and off rapidly causes it to run at a speed proportional to the average voltage, so at 50% PWM it will spin at half speed just like it would with 5V DC. However unlike a resistor or linear regulator, the MOSFET is turned fully on and off so it either drops (almost) no voltage or passes no current, with the result being almost zero power loss.

The diode in this circuit perform two functions. Firstly it prevents the MOSFET from being damaged by the voltage spike (caused by winding inductance) which occurs every time the motor is switched off. Secondly it diverts the spike current back into the motor, improving efficiency and keeping the current going so the motor runs smoother and responds better to PWM control.