Going from 12V to 24V

[BigTerp]

I have an older Motorguide Pro Series 41 trolling motor that has a switch to run on 12V or 24V. Currently it runs on 12V on it's own battery. I have a seperate battery that is used for cranking, sonar (standard black and white), lights (interior, nav, all led), stereo and bilge pump. I've been told that if I run my trolling motor on 24V I'll get double the thrust lbs. then when it's running on 12V. Is that the case? If so, how can I get 24V power to my trolling motor yet still have 12V power for cranking and all my accessories. Is it just as simple as running a jumper cable from the negative of my cranking battery to the positive of my trolling motor battery, like in this thread? https://www.tinboats.net/forum/viewtopic.php?f=5&t=35003 I guess I'd have to rewire the trolling motor also to be hooked to the positive of my cranking battery? Next question is how much of a drain would this have on my cranking battery? Wouldn't want to run it dead, obviously. I've run it for almost a full day on it's own battery and it's done fine. I just could really use some more power when in the swifter parts of the river.

 

[onthewater102]

It isn't quite that simple - my thread (the one with the diagram you referenced) was looking at using a dedicated 24v trolling motor and verifying my understanding of Kirchoff's Circuit Theory. You need to be able to switch your whole circuit between having the batteries in series and having them in parallel. A jumper fixes the two in series. Somehow your switch has to act as your jumper.

Not sure how the dual voltage motors operate - in theory if they have the same motor resistance in either configuration then there would be a noticeable increase in power running on 24v relative to 12v. In theory quadrupling the power output, but the inefficiencies of the power transfer through the propeller will skew this as they're non-linear (spinning a propeller twice as fast doesn't induce twice as much flow.) I'm certain this isn't the case (4x power output) as the reason you'd want to increase the voltage is to decrease the current draw on the battery and still get similar or better power output while having your batteries last longer too. There must be a different resistance in the 24v setting than the 12v, but by how much I can't tell without your unit.

If you measure the resistance of the motor in the 12v configuration and the 24v configuration you would be able to figure how much more power you have.

Power (watts) = Current (amps) x Voltage

Current (amps) = Voltage / Resistance (ohms)

Using those formulas you could calculate the power in each configuration if you knew the resistance of the motor in each - it would be a rough approximation of how much increase in thrust you would generate, though the actual results would be less due to the propeller inefficiency.

 

[BigTerp]

Thanks!!

I followed a bit of what you said, but must admit electricty isn't my strong suit!! Currently the motor runs only on 12V. I had my buddy rewire it so the switch that works for 24V/12V now works for constant power or intermintent power. Meaning switched one way the motor only runs if I have the pedal depressed, switched the other way and the pedal acts like an on/off switch. If that makes sense? If I were to wire my batteries to run the motor on 24V, I would rewire it so it only was on 24V.

 

[onthewater102]

your motor didn't have that feature (constant on vs. intermittent with the pedal button) already?

odd

So if you restore the 12/24 switch to it's original configuration you'd loose that functionality right?

If so that's an easy fix - you could buy a simple toggle switch (don't get one with a built-in light as it will be for 12v only) and mount to the side of the unit somewhere. The switch would simply bypass the button on the pedal - you'd leave the pedal button wired as-is and just splice into the feed and the return from it and send the splices to either side of the toggle switch. With the switch in the off position there isn't a connection so the pedal button would control the power. With it in the on position you would have power regardless of whether the pedal button was pressed or not.