Connecting multiple DC motors together to charge a 12 V battery

Question

I have five small DC motors all rated for 12 V. I am trying to get them all connected together so it will act as multiple wind turbines and charge a battery. However, I am very new to electricity and circuits and such and am struggling to figure out how to do it. I don't want to ruin the battery or the motors. I do believe from what I've found that I will need to use a series circuit, but I need the motors to be running power to the battery, instead of the battery running power to the motors. Any help is appreciated!

Answer 1

You can determine the voltage produced by each motor by spinning the shaft with an electric drill of known speed. This provides the motor's Kv, or volts per 1000 RPM, and is also known as back EMF. This will be approximately linear over a reasonable range of RPMs.

Then you can find how much current they can supply by adding a resistor of perhaps 1 kOhm across the terminals. The internal resistance will be approximately the change in voltage divided by the current, which will be equal to the voltage under load divided by load resistance. You might also just read the motor resistance, but that will not be as accurate.

The motors can be connected in series, and the total voltage will be the sum of their individual RPMs times kV. Their internal resistances will be added together as well.

You can make sure the motors only charge the battery by adding a series diode. The total voltage will likely be too high to connect directly to the 12 V battery, so you will need to add something to control the voltage and current. Simplest, but least efficient, would be a series resistor that limits the current to a maximum value that is safe for the motors and the battery. A linear current regulator would be better. And ideally you could use a switching buck converter. Here is an example:

^{simulate this circuit – Schematic created using CircuitLab}

Note: This shows behavior for generator voltages from 12V to 60V. I show 10*V(batt) to see slight differences due to internal resistance of batteries set at 10 ohms, but most batteries will have less than 1 ohm.