How To Figure Battery Run Time From Amp Hours ?

[Jim Mac]

I'm looking into buying a 40 lb. thrust Endura Max motor, and I'm trying to choose a battery. It's for my kayak(Frontier 12') so I'd like to keep the weight down if possible.

 

My first choice is a sealed battery.

 

I'd also like a battery smaller than a Group 24. I'm normally not out longer than 4 hours.

 

What do you guys suggest?

 

Thanks.

[Wayne P.]

Generally , the amp draw of a trolling motor is somewhat close to the max thrust rating.

40A used for one hour = 40AH.

 

Divide that into the AH battery rating.

[Jim Mac]

Thanks Wayne. So a 40 amp hour battery would only give me about 1 hour run time on high?

[Way2slow]

No, about half that.

A small battery with that heavy a load would only be about 50% efficient.

Amp hours on the label DO NOT directly equate to run time on the TM. Most batteries are rated using the 20 hour rating method, which means they are only drawing 4-5 amps during the test. As the amp load increases the batteries efficiency decreases. The smaller the battery, the less efficient it is with a heavy load. Group 24's are only about 75 Ah @ a 20hr rating. With a 40 amp load, it probably won't last an hour.

Understand also, that's not maintaing the same speed as when you first start. As the batteries charge state decreases, so does your speed. When you get to it's 11.8 volts recommended minimum charge level, you are going to be barely moving, so you had better hope the wind is not blowing.

[Jim Mac]

Thanks

[Team_Dougherty]

If the motor you are buying is a 5 speed motor. Speed 4 will draw about half of the current of speed 5. But, you will not go twice as fast on speed 5 as speed 4. So, running on speed 4 will give you twice the run time with out affecting speed that much.

 

This should give you a good idea what to expect

 

http://tufox.com/hobie/TrollingPerformance.html

[Jim Mac]

I ordered the motor today, and will have it Monday. The MK variable speed(Endura Max) 40 lb. thrust.

I'm normally out fishing 4-5 hours max. I'm thinking of a group 24 battery, but would like to go with a lighter weight battery, if possible.

I do want to get a sealed battery also. Any suggestions on battery, and where I can get a good deal?

Thanks.

[Loop_Dad]

If the motor you are buying is a 5 speed motor. Speed 4 will draw about half of the current of speed 5. But, you will not go twice as fast on speed 5 as speed 4. So, running on speed 4 will give you twice the run time with out affecting speed that much.

 

This should give you a good idea what to expect

 

http://tufox.com/hobie/TrollingPerformance.html

 

T_D, I've read many times that 5 speed Minn-Kota's use same amount of current no matter what the speed is because it control the speed by applying more resistance series to the motor. That's why I bought variable speed. This study totally contradict to those claim. (But I believe this study.)

[Team_Dougherty]

T_D, I've read many times that 5 speed Minn-Kota's use same amount of current no matter what the speed is because it control the speed by applying more resistance series to the motor. That's why I bought variable speed. This study totally contradict to those claim. (But I believe this study.)

 

I do not know where people got the idea that the 5 speed motors draw the same current on all speeds. When you increase the resistance of a circuit the current draw drops. I=E/R. Ohms law.

 

Me and another fellow go into a 15 page forum discussion with another guy, who is a minnkota repair center owner, about the same thing. The Minnkota guy kept claiming the 5 speed motors drew the same current no matter what speed.

 

I finally had to make a video to prove otherwise to him.

 

[F14A-B]

That's some Exacting work there T_D , I like that..

[Team_Dougherty]

That's some Exacting work there T_D , I like that..

Thank You.

[Way2slow]

First off, they don't use resistors to drop the voltage. I think They do it by selecting different turn counts on the windings, basically making it five different size motors, not just one motor a changing resistors. Even being water cooled, you would be amazed just how large that motor would have to be if they tried to use restores to drop the voltage.

For max run time, you want to go with a variable speed motor instead of a select speed type. On max, both draw the same current for the same size motor, but at lower settings, the variable speed uses PWM or similar circuit and develops the same thrust using much less current.

[Loop_Dad]

I do not know where people got the idea that the 5 speed motors draw the same current on all speeds. When you increase the resistance of a circuit the current draw drops. I=E/R. Ohms law.

 

Me and another fellow go into a 15 page forum discussion with another guy, who is a minnkota repair center owner, about the same thing. The Minnkota guy kept claiming the 5 speed motors drew the same current no matter what speed.

 

I finally had to make a video to prove otherwise to him.

Nice work T_D!

 

It is almost funny you argue with Minn-Kota repair guy and ended up making video to prove your point.

 

But you're right, if the circuit is very simply motor and variable resister connected serial, it is just that simple ohms raw. I don't know why I didn't think of that.  Do you have the wiring diagram? (I have feeling you might have )  I am curious now.

 

About 'constant' thing...if I had to guess, someone must have mixed up with constant current and constant voltage somewhere, maybe lost in the translation between engineer and sales person.

[Loop_Dad]

First off, they don't use resistors to drop the voltage. I think They do it by selecting different turn counts on the windings, basically making it five different size motors, not just one motor a changing resistors. Even being water cooled, you would be amazed just how large that motor would have to be if they tried to use restores to drop the voltage.

 

Ok, that's interesting. So the motor itself is made for 5 speed. That sounds too complicated. I just assumed the motor itself is the same thing between 5 speed and variable speed, just plain dc motor.

[Way2slow]

Both are DC motors, the difference is how the voltage is applied. Big difference between straight DC and PWM. With PWM, it's being switched on and off, not a steady on voltage.

[Team_Dougherty]

I do not have a schematic per se.

 

But, they're resistors, two in fact, in the motor housing. They are coils of wire each a different resistance. The wires are the sized to the inside diameter of the motor housing,  They are wired in before the brushes. Lets call them one and two,  The 5 speed switch will either put one, then two, then one and two in series with each other, then one and two in parallel with each other, for 4 speeds. The 5th speed bypasses the resistors to give full speed.

 

A quick resistor lesson two resistors in series with each other will have a total resistance of both resistors. two resistors in parallel with each other will have a total resistance less than the lowest resistor in the circuit. A one ohm resistor and a two ohm resistor in parallel with each other will have a total resistance of .667 ohms

 

here is what a speed coil, resistor, looks like.

[Loop_Dad]

Both are DC motors, the difference is how the voltage is applied. Big difference between straight DC and PWM. With PWM, it's being switched on and off, not a steady on voltage.

 

I am actually familiar with PWM. It is actually on the control side, not the motor side. That's why I assumed the five speed change is also done in the control side, totally separated from the motor itself.

 

I do not have a schematic per se.

 

But, they're resistors, two in fact, in the motor housing. They are coils of wire each a different resistance. The wires are the sized to the inside diameter of the motor housing,  They are wired in before the brushes. Lets call them one and two,  The 5 speed switch will either put one, then two, then one and two in series with each other, then one and two in parallel with each other, for 4 speeds. The 5th speed bypasses the resistors to give full speed.

 

A quick resistor lesson two resistors in series with each other will have a total resistance of both resistors. two resistors in parallel with each other will have a total resistance less than the lowest resistor in the circuit. A one ohm resistor and a two ohm resistor in parallel with each other will have a total resistance of .667 ohms

 

here is what a speed coil, resistor, looks like.

 

Ok, so those resistors (coils of wires) are meant to be just resistors to adjust current, not meant to be some sort of additional coils to increase/decrease magnetic filed, correct? They just happened to be coiled up right?

[Team_Dougherty]

I am actually familiar with PWM. It is actually on the control side, not the motor side. That's why I assumed the five speed change is also done in the control side, totally separated from the motor itself.

 

 

Ok, so those resistors (coils of wires) are meant to be just resistors to adjust current, not meant to be some sort of additional coils to increase/decrease magnetic filed, correct? They just happened to be coiled up right?

 

Yes they are just act as resistors. They are coiled so they touch the motor housing all around. this way they are cooled by being submerged in water. You should limit the amout of time the motor is run out of water. Those resistors see about 60 watts of power and can burn up pretty quick if over heated.

 

Resistor do not adjust current. They drop voltage. for example; If you have a motor that has 1 ohm of resistance. you apply 12 volts to it. it will spin at full rpm. Now, if you put a coil of wire, resistor, that also has 1 ohm of resistance in series with the motor. Both the resistor and motor will see 6 volts. the motor will spin at half it 12 volt rpm and the resistor will just get hot. The current flow will be 6 amps. Now, change the 1 ohm resistor to a 2 ohm resistor and the motor will see 4 volts and spin at a third of its rated speed. The resistor will see 8 volts and, The current flow will be 4 amps.

 

[WRB]

Keep in mind you are powering a very light water resistance craft with very little drag or wind resistance in a kayak. You will more than likely use most of your amp draw running back and forth to where you fish, very little amp draw while fishing. I would calculate by using 50% amp hour draw or more than 5 hours.

Tom

[Team_Dougherty]

Item #12 in the speed coil, resistor.