switching DC trolling motor to generator

Question

Would like to drag the trolling motor prop while sailing to re-charge the battery. The trolling motor is 12 VDC and the max amps at full forward speed is 50. The sailing speed of the boat will turn the prop easily and at times will turn the prop 4 times faster than the full throttle speed of the motor.

My question is this, will the current crop of DC motor controllers with 4 quadrant control handle this amount of regen? and what circuitry do I need to alert me that the battery is fully charged and to pull up the prop?

Thanks in advance for any help with this. Answers from the Chesapeake Bay region can come aboard for a sail.

Answer 1

The sailing speed of the boat will turn the prop easily and at times will turn the prop 4 times faster than the full throttle speed of the motor.

If you can get the motor to act as a generator, that will create a braking load that will slow the speed. If the prop is inefficient when acting as a turbine as suggested by Robherc KV5ROB, it will not do well at overcoming the braking load.

Unless the motor has permanent magnets, it will not act as a generator unless it has enough residual magnetism to get generation started. If the battery can be properly connected to energize the field, the motor will act as a generator. If the field is connected in series with the armature, the most likely connection, that will present additional problems.

Added re PM motor comment:

With PM motor, this could be a good project. When operating as a generator, the voltage produced will be proportional to the motor speed with the normal supply voltage (12 V) produced at about the speed that the motor runs when supplied with 12 V. The maximum safe current for the motor acting as a generator will be full-load current for motoring.

You will need a charge controller that can convert the generated voltage to the voltage needed for charging, about 15 volts, and limit the current to whatever is safe for the battery. It may be possible that the regen controller can be adjusted to limit the voltage and current to the required level if the motor generates more than enough voltage. Otherwise, you need a boost converter / charge controller.

Before proceeding, it would be a good idea to connect some kind of resistive load to the motor terminals and see how much voltage the motor generates under actual operating conditions. That will tell you how well the prop works as a turbine when the generator is loaded. I expect that the prop will turn significantly slower when loaded. Any speed in excess to the normal maximum prop speed will proportionally increase the voltage above the normal supply voltage. I expect that the motor will tolerate some overvoltage, but I don't know how much. The controller may be less tolerant of overvoltage than the motor. You also need to be concerned about motor speed.

This forum is about design rather than guiding people to select off-the-self equipment. Questions of that nature are closed. Perhaps a site like Home Power would be helpful in that regard.

Answer 2

Most trolling motors I've seen have very inefficiently designed 'flat' blades on them. Combine that with the somewhat flimsy (under drag conditions) shaft, and the need to tear in & rebuild the main circuit...

Mightn't it be a better idea to sacrifice a bit of $$$ & weight in order to carry along a hydro-generator that was designed to work efficiently for the purpose?

With that said, since you're aware of the risks/complexity, here's what I've come up with so far:

• Since you said your current controller is shot, this controller seems to have the ratings you need (50A@12V), so long as you don't throw it into full reverse while clipping along at 18kts.

• Here's a quick and easily-built circuit you could use to power a row of LEDs to give you a "current state of charge/discharge" indicator display:
  

  1. The Green LED in the center stays lit whenever the circuit is powered.
  2. The left-side Yellow/Red LEDs come on around 10.7V/8.7V circuit voltage.
  3. The right-side Yellow/Red LEDs come on around 12.7V/13.7V circuit voltage.
     (All LEDs are assumed to safely handle 100-150mA DC current, and all transistors are assumed to have a ~0.7V 'bias/trigger' voltage.)

When using the above circuit, attached between motor & battery, you should be able to see instantaneous line voltage indication (under load) while the motor is 'on,' and be able to check the battery's charge state (based on residual voltage) any time the motor is 'off.'