Trolling Motor advice 12v / 24v

[kudos1uk]

Can you guys offer some advice please,

I have a 2 man 3m Avon inflatable dinghy and would like to get a trolling motor for the river and maybe calm sea fishing but not so likely.

I currently have a powerful LiFePO4 48V 20AH 5A Battery for a DIY eecric bike I made, it would be good to use this on the boat too.

I want to get a reasonably powerful motor and was looking at either:

12v 65Lb
24v 86Lb

To use my battery on either I would need to step down the 48v to either 12v or 24v.

48v to 24v 22A 530W output
48v to12v 25A 300W output

I don't think either of these options will output enough wattage for either of the motors above a 55Lb is 636WAH, I'm not sure what the above motors rating are but will be higher.

Are there any other options to use the 48v battery pack, it would be nice to use it as it is so powerful and light.

I would also like to keep a back up battery to get me home in an emergency, for that I have a lead 24v battery, would that get me home on a 48v motor? would it run slower (limp me home)?

If 12v would not drive the 24v should I just stick to the 12v motor and keep a 12v back up, im guessing the 24v above would give me significatly better performance (and not much ore expensive).

If its not practical to use the 48v battery, should I just stick to a lead 12v - I have a brand new 85Ah 800A lead battery I bought for a van which I then got rid of so the brand new battery is doing nothing right now.

Sorry for all the questions but I'm just not sure what route to take for the best and hope someone can give me a push in the right direction.

 

[gm280]

kudos1uk, there are a few ways to drop 48 volts to 24 or even 12 volts for your needs. Some of them are better then others. The absolute worst way is to use a huge wattage dropping resistor in series with your trolling motor. While that will work, it is a real waste of battery ability. It basically converts the unused voltage/current to heat. Not a good idea in my opinion even though it is probably the easiest method.

The better method, but equally the harder to do is to convert the 48 DC volts into 48 volts AC or chopped DC. Then you can use a step down transformer to whatever voltage you want. But then you have to convert the AC output back to DC again. Those conversions waste efficiency but are way better then a dropping resistor setup.

The best but even more convoluted to do, is a PWM circuit. That would be the best option in my opinion and doesn't waste as much capability of the battery as all the others. But you have to either buy a good PWM circuit, or make your own. If you have no electronic experience, I would search EBay for a high current PWM circuit and use it. That will be the better option in my opinion.

How a PWM circuit works is it pulses the 48 volts on and off so fast that your trolling motor see only 12 (or 24) volts while being pulsed with 48 volts. It is the same technology they use in Digital Trolling Motors. Some TM manufactures call their Maximizer, while other call theirs Digital TMs. But they all use the same method to run their trolling motors while allowing the battery to last longer. That would be my option if I were doing it. JMHO

 

[Silvertip]

I question the data on the 65# motor. You say it is a 12 volt. Any motor brands I'm familiar with have the 12 volt limit at 55#. Anything more than that is 24 volts. A 55# troller draws about 46 amps under worst case conditions. Use that number to determine how far you can go in a given amount of time. Trollers are also propped for about 3.5 MPH, but probably a tad faster on a light hull.

 

[fhhuber]

You can get a custom regulator made to drop the voltage and have it handle the power.
I do not know of one "off the shelf" for dropping 48 v to 24 v or 12v that will handle the power.

I would first contact:

https://www.dimensionengineering.com/

And ask them for a quote or advice. (They may know who makes one)

Its actually a pretty simple circuit. These guys have the expertise in DC-DC voltage conversion to make it work right the first time

Of course a one-off product will not be the cheap solution.

********************************
I recommend:

Cheap and easy solution (appropriate for that size boat) is to just use a 12V "marine" battery and 12V trolling motor. 65 lb is more than you need. I'd go with 30 to 40 lb.

 

[kudos1uk]

Thanks guys thats really helpful.
the 12v is 660w and I think the 24v is about 900w

gm280
That sound a good option, is this the kind of thing I need:
http://www.ebay.co.uk/itm/282305224741

The power works:
12V: 720W(max)
24V: 1440W(max)

silvertip
Here is the spec of the 65:
http://www.ebay.co.uk/itm/282511015568

This listing is also for a 65 and also shows as 660w:
http://www.ebay.co.uk/itm/222516501883

Do you think they are just over egging the power and I would be better getting a 55 which would cost less?
http://www.ebay.co.uk/itm/301441912186

fhhuber / silvertip
As i'm only using it on a light inflatable and they are all propped for about 3.5 MPH, are you saying it is a waste of time and money (and probably battery power) going for a higher power trolling outboard?
Would I notice no benefit of a 55lb or 65lb over a 40lb on a light boat?


I notice the 40lb prop only has 2 blades while 55lb and above have a 3 blade prop, does this make a difference?
40lb: http://www.ebay.co.uk/itm/271710057214
55lb: http://www.ebay.co.uk/itm/301441912186

Sorry but I'm a complete newbie with trolling motors.

 

[fhhuber]

the 60 amp speed controller you linked is not a voltage regulator.
It is exactly what it says, a speed controller.
At full power setting it would let through full voltage 100% of the time and thus if using the 48 v battery you'd burn up a 12v or 24 v rated trolling motor.

The circuit involved is similar in some ways to the voltage regulator you would want for using the 48 v battery with a 12v or 24 v trolling motor, but not the same thing. Instead of regulating the voltage, it varies pulse width in order to get the speed control effect.

************

The larger diameter and/or more blades in a prop, the more power it takes to turn the prop the same rpm and the more power the prop can convert to thrust. More thrust means you can move a bigger boat.

The trolling motor used would draw close to its rated watts at full throttle, even if oversize, because a large part of the power goes to friction of moving the prop in the water. The larger prop will simply demand more power.

 

[kudos1uk]

the 60 amp speed controller you linked is not a voltage regulator.
It is exactly what it says, a speed controller.
At full power setting it would let through full voltage 100% of the time and thus if using the 48 v battery you'd burn up a 12v or 24 v rated trolling motor.

The circuit involved is similar in some ways to the voltage regulator you would want for using the 48 v battery with a 12v or 24 v trolling motor, but not the same thing. Instead of regulating the voltage, it varies pulse width in order to get the speed control effect.

Do you know what PWM stands for, I'm having no luck finding a PWM circuit on ebay, the speed controller is the only reference I get to PWM.

 

[kudos1uk]

PWM = Pulse Width Modulation

I'm confused, is that not what I linked to?

 

[fhhuber]

PWM
Pulse
Width
Modulation

Essentially, varying the % of time that you apply full power.

The concept is (for a 48 v to 24 v regulator) since 24V is half of 48 V you very rapidly turn on and off the power such that it is on 50% of the time and then use capacitors to level the resulting output, giving you the desired 24V.

It can also be used to encode a signal and this is how older AM and FM radio control systems work. Sequential pulses of varying "width" give the control positions.

When used as a speed controller, your pulse widths are increased and deceased (more % of time power on for higher throttle) to control motor speed.

The main difference between a PWM voltage regulator and PWM speed control is in what is controlling the pulse width. The voltage regulator will detect the output voltage (after the capacitors) and adjust to give the target voltage. The speed controller may not have any detection, just the circuit section that varies the pulse width timing.

A PWM based voltage regulator is more normally called a "Switching" regulator, because it constantly switches power on and off.
The switching circuits waste a very low % of the power as heat.

There are also "linear" voltage regulators which use variable resistance and waste a lot of power as heat. A linear regulator dropping from 48v to 24v will waste close to 50% of the power as heat.

 

[gm280]

To try and answer your question about the PWM circuit you linked to. Yes that can work. But you would need to possible modify that circuit so the maximum output would be 24 volts (if that it what your trolling motor uses that you buy) when it is at the max setting. Then you would vary the output from 0 speed to max from that mod. I know that sound really confusing so let me try to make it simpler.

The Pulse Width Modulation circuit pulses the output in percentages of "on time" to "off time". For instance, if you were to flip "on" and "off" a wall switch so fast that for 50% of the time, the light would be "on" and the other 50% of the time the light would be "off", the resulting voltage the light would see is half the house voltage. But the total current is not effected. So you have a PWM setup that would make the light look dim compared to being "on" all the time.

That is exactly how PWM circuit works. And since they don't effect the current, the trolling motor is getting only a drop in voltage but 100 percentage of the current it needs at and speed.

So to make a typical PWM circuit work in your situation, you would have to limit the variance so that only 50% on to off time could happen at max output setting. Otherwise you would toast the trolling motor. BUT, if you did limit the PWM circuit to a max of 50% duty cycle (on to off time is called the duty cycle) you would get your 24 volt trolling motor to run full speed and still be able to reduce that speed with lesser percentages of duty cycles as well.

And since you would still have 100 percent of the current, your trolling motor isn't harmed one little bit. And that is the only way I see you make your set up work without wasting half the battery capitally in heat.

And atypical mod would be something like this. If the pot that varies the speed is a 10k pot (just a hypothetical situation), then you would buy a 5k pot and a 5k resistor and run them in series in place of the 10k pot. Then when you dial the 5k pot to zero for max speed, you still have 5k resistor to keep the max output at 50%. Easy peasy. JMHO

Sorry for the long post! :facepalm:

 

[kudos1uk]

Thanks guys I think I have it sorry to be so dim

All the advice has been great, think I will go 12v 40lb for my needs and try and sort a way to bring the voltage of my 48v LiFePO4 down to 12v. The 40ib only draws 312w so hope it wont be so hard to correct the voltage.

Gm280
I have a friend who I am certain has the skills for this, I think I might have a chat with him as see if he can convert that speed controller as I simply can't find a PWM voltage regulator.

 

[Silvertip]

I think you are over estimating and over-complicating this project. Your 40# motor draws 30A under worst case conditions regardless how you get from 48 to 12 volts. So look at it this way, a group 27 deep cycle battery typically has a 185 minute reserve capacity at a discharge rate of either 23 or 25 amps. So operating your chosen motor could operate from that battery for roughly three hours at about 1/2 - 2/3 towards a worst case condition. Yes - the LiFe is smaller and lighter but it also has a very low capacity for this purpose.. But consider the complexity, potential for failure, cost, etc. I can buy a group 27 deep cycle battery on sale here for about $75 with a $10.00 recycle credit.

 

[kudos1uk]

I think you are over estimating and over-complicating this project. Your 40# motor draws 30A under worst case conditions regardless how you get from 48 to 12 volts. So look at it this way, a group 27 deep cycle battery typically has a 185 minute reserve capacity at a discharge rate of either 23 or 25 amps. So operating your chosen motor could operate from that battery for roughly three hours at about 1/2 - 2/3 towards a worst case condition. Yes - the LiFe is smaller and lighter but it also has a very low capacity for this purpose.. But consider the complexity, potential for failure, cost, etc. I can buy a group 27 deep cycle battery on sale here for about $75 with a $10.00 recycle credit.

That is very good advice, I did not expect it to be so complicated to make use of my existing battery, I thought I could get an of-the-shelf adaptor and off I could go with my existing battery.
Some times I think it's easy to get carried away and determined to solve a problem.

 

[gm280]

I think you are over estimating and over-complicating this project. Your 40# motor draws 30A under worst case conditions regardless how you get from 48 to 12 volts. So look at it this way, a group 27 deep cycle battery typically has a 185 minute reserve capacity at a discharge rate of either 23 or 25 amps. So operating your chosen motor could operate from that battery for roughly three hours at about 1/2 - 2/3 towards a worst case condition. Yes - the LiFe is smaller and lighter but it also has a very low capacity for this purpose.. But consider the complexity, potential for failure, cost, etc. I can buy a group 27 deep cycle battery on sale here for about $75 with a $10.00 recycle credit.

Silvertip. I would simply buy a marine type 12 volt battery myself instead of rigging up such a system to allow the use of another battery system like he is wanting to do. All I was doing was offering the different ways he could make his battery work with a trolling motor. Would I go that route he is wanting to do? NO! For probably less money, I would buy a battery and trolling motor, mount it all and happy boating. But once again, that is only my opinion. Is his battery and suggested system doable? Yes it is. But I like simplicity and work to that idea when I can. JMHO