I have a machine project, which is going to work with a Lithium-ion battery. As you know, as a battery is used, its voltage drops with time, and I’m looking for a way to maintain constant power to the machine, and I'm asking for a method that will really work.
This machine uses a fan having a minimum required air flow speed of 3,000 RPM for cooling. But the dropping battery-voltage presents a big problem in maintaining the fan's minimum RPM. Do you think the method which I am going to describe now will meet this requirement?
To begin with, I found a DC motor having a wide working voltage range. For example, I found a DC motor having a 40 V maximum working voltage, which produces 3,000 RPM at 24 V, so I built an 8S Lithium-ion battery. When the battery is fully charged, it produces 8 * 4.2 V = 33.6 V, and the motor will be driven using 72 % duty-cycle PWM, which will produce an average voltage of 24 V, which will achieve the necessary 3,000 RPM. After a while, the battery voltage will drop, which will be sensed using a voltage divider circuit, yet we're going to produce the PWM modulation that will maintain the required 3,000 RPM (meaning that, with each voltage drop, the PWM will be slightly increased). Finally, when the battery voltage drops to 3 V, the microcontroller will produce a 100 % duty cycle PWM modulation. Using this method, a constant motor RPM will be maintained in spite of changing battery voltage.
Do you think this method will work? What is your recommendation?