# burning up trolling motors- basic electricity



## linehand (Aug 25, 2011)

Been grazing arond and seeing a few trolling motor dead questions.
Here is some food for thought when using an electric motor.
Low batteries increase current flow. Hard on electrical connections and wiring.
So if using a troling motor in weedy water best to have a full battery.....
Ohm's law states that the current through a conductor between two points is directly proportional to the potential difference across the two points, and inversely proportional to the resistance between them.


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## parkerdog (Aug 26, 2011)

linehand said:


> Been grazing arond and seeing a few trolling motor dead questions.
> Here is some food for thought when using an electric motor.
> Low batteries increase current flow. Hard on electrical connections and wiring.
> So if using a troling motor in weedy water best to have a full battery.....
> Ohm's law states that the current through a conductor between two points is directly proportional to the potential difference across the two points, and inversely proportional to the resistance between them.




I don't know about food for thought.. Made my brain hurt thinking about that. Hm maybe too many beers last night. lol


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## Wallijig (Aug 28, 2011)

linehand said:


> Been grazing arond and seeing a few trolling motor dead questions.
> Here is some food for thought when using an electric motor.
> *Low batteries increase current flow*. Hard on electrical connections and wiring.
> So if using a trolling motor in weedy water best to have a full battery.....
> Ohm's law states that the current through a conductor between two points is directly proportional to the potential difference across the two points, and inversely proportional to the resistance between them.




Low batteries decrease current flow. 
Less voltage across a same resistance is less current flow. 
Ohms law I=E/R (I is current, E is voltage, and R is resistance)
So say your trolling motor has .5 ohms of resistance on high 
Example if is at: 
12v with 5ohms = 24 amps of current flow 
11v with 5ohms = 22 amps of current flow
10v with 5ohms = 20 amps of current flow

Ohms law calculators:
https://www.the12volt.com/ohm/ohmslawcalculators.asp

I think by having extra load causing motor to work harder would be cause for higher failure rates like having to small of trolling motor trying to pull a big boat or weeds wrapping around prop. 
Kinda like trying to pull a big trailer with a 6 cylinder engine. One may be able to do it but not for as long.


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## linehand (Aug 29, 2011)

Wallijig said:


> linehand said:
> 
> 
> > Been grazing arond and seeing a few trolling motor dead questions.
> ...


-------------------------------------------------------------------------------------------------------------------
You are absolutely correct on Ohm's law. Good catch Wallijig. Let me rephrase what I was trying to say here. I stated "current flow increase" actually I meant to say "current demand" will increase. Think about it like this: as voltage drops and CURRENT DEMAND increases from a dc battery which has a non linear capacity, you'll get "thermal transfer". Basically the battery will try to displace heat once there is a temperature difference between itself and the electrical components on the circuit.i.e. motor, connections, switches, etc. they will in effect become heat sinks for teh battery.
Which increase the resistance on the circuit proportionally.
What I am saying is you will ALWAYS be better off with a fully charged battery(ies) 
Conclusion: low batteries = damage to connections and wiring so especially in weedy lakes keep a full battery.


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## wihil (Sep 1, 2011)

Well, a motor isn't a resistor - a linear resistor anyway, though it does have resistive elements, which makes Ohm's Law a bit of broad guide when used like this. There's a lot going on, especially with a DC motor/capacitive phase shifting/etc.

The other thing to think about is batteries do go bad, even if charged religiously. Once the load handling of the batteries is gone, it'll fall flat on it's face when asked to do real work. 

C


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## linehand (Sep 2, 2011)

wihil said:


> Well, a motor isn't a resistor - a linear resistor anyway, though it does have resistive elements, which makes Ohm's Law a bit of broad guide when used like this. There's a lot going on, especially with a DC motor/capacitive phase shifting/etc.
> 
> The other thing to think about is batteries do go bad, even if charged religiously. Once the load handling of the batteries is gone, it'll fall flat on it's face when asked to do real work.
> 
> C



Who said a motor is a linear resistor? I stated that a battery has non linear capacity. That means it will not loose charge in a straight line. It will fall off faster as the charge decreases. Also the resistance will be at any connection, contacts, etc. 
Ohms law does apply. I could have left that out. I just wanted to clear up that the resistance increases proportionally as heat is transferred from the source.... Pm me if you want to get into the nitty gritty of electrical theory. This is what I do for a living.


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## Zum (Sep 3, 2011)

I think I missed the question
We all know we should have fully charged batteries and they normally start off that way.
When they get used they loose some charge and we have to turn them up,to get the same results.
Let's say, you draw 20 amps at half open(full charge) and now you are at 3/4's(used abit) to get the same speed/power.I'm thinking it would still only be at 20amps,not say 30amps like it might be if fully charged.
If you use the right size wire and breaker and don`t see any problem.
I thought you were only hurting the battery,if running it to long,towards a dead state.
Theres no "juice" left in the battery to hurt anything else.


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## linehand (Sep 4, 2011)

Zum said:


> I think I missed the question
> We all know we should have fully charged batteries and they normally start off that way.
> When they get used they loose some charge and we have to turn them up,to get the same results.
> Let's say, you draw 20 amps at half open(full charge) and now you are at 3/4's(used abit) to get the same speed/power.I'm thinking it would still only be at 20amps,not say 30amps like it might be if fully charged.
> ...



Hey ZUM. There wasn't really a question I was just trying to help out the people burning up trolling motors.The motor will be asking for the "juice" but if there isn't enough voltage there to push the current, components won't be "happy". Where ever the weakest link is on the circuit will fail. Hopefully that is the fuse or breaker. Make sense? Did you ever have a poor connection and feel or see it get hot? If you can filter through what I stated above this is what is going on. 
Hope this clears it up for you.


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## Zum (Sep 4, 2011)

Thanks for trying to clear it up.
Makes alittle sense to me and I am trying to see what your saying.
It's just that when a battery gets low,I've never seen it blow any fuse/breaker.
The TM just stops working or goes slower;then you swear and say I'm getting another battery,because this is when the fish are really biting
I'm having a hard time with a battery with low voltage being able to put out too much current.
Even if your TM is on high asking for 55amps,if your batteries low,your not going to get it.
Kinda like me...


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## linehand (Sep 4, 2011)

Lol. Well to make it simple. If they make a higher "thrust" motor usually they will want it connected to a higher voltage like 24 or 36v. This is because at a higher voltage there is less amp draw. So if your motor is made for 12v and the battery goes down to say 10.5 or 11v there will be more amp draw. This is usually when something fails. Make sense?


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## Zum (Sep 4, 2011)

Nope.
Although I should...
I understand the 24/36V draw less amps then a big 12V TM.
Heres my conundrum:
My TM is 50lbs thrust,so lets say it draws 50amps at wide open.(for max,thats is all it should ever want,if working good)
It's never going to draw more then that unless theres something wrong(bad connection,loose bolt,etc.) 
Why would the amps ever go above what the TM wants?

I thought if a battery was pretty much dead at 11.9 volts.
If it starts out at say 12.6 or 12.8V,how many more amps would it draw anyways?

Do those amp meters read dc amps?
If so I have to hook one around one of my wires and see whats going on.
Thanks for your time.


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## linehand (Sep 5, 2011)

Yeah I'm getting confused...lol. It's like asking my wife if I can keep my whole paycheck after she already spent some of it. It is not there to give. And this causes heat and resistance. The motor asks for something it can't get. It wants say 30 amps but there isn't enough voltage to push it there, ohms law. This is now not a happy circuit and the battery gets hotter and tries to displace the heat. Something will eventually have to give. Its not going to give it 35 amps it's going to try and still give it 30 but can't.


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## linehand (Sep 5, 2011)

Thought of a better analogy. Like sucking a milkshake through a straw. Current is the milkshake and voltage is the straw diameter. Motor is the person trying to drink it. The problem as he sucks the straw gets smaller. Eventually the vein in his forehead bursts..lol


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## wasilvers (Sep 7, 2011)

Soooo, At full charge, I am good running my troller on high. At say 50% of full charge, I shouldnt' run on high anymore cause it will cause heat at the battery and then terminals, and then the troller too? 

So I should run at slower speeds once the battery is drawn down to avoid heat and possible trolling motor damage? This is counter productive to my fishing style of get there and move fast! :?:


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