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wasabi_VA

Voltage drop while trolling motor is running

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My deep cycle battery is less than a year old and I ALWAYS put it on the charger as soon as i get back from a fishing trip. I use it to run a 12V 55 lb thrust bow mount trolling motor. The battery is at the rear of the boat and I have a 20' run of #8 wire to it.

Today, just a couple minutes after leaving the dock, I noticed on my depth finder the voltage was 11.9. I was currently using full power to get to my fishing location. The depth finder uses the same battery but is wired off a fuse panel fed by a different wire - I think I have about 8 feet of #10 feeding my electronics. Way overkill but I sometimes run a spotlight. So the voltage being displayed is the voltage available at the depth finder, not necessarily the result of the voltage drop across that 40' run of cable up to the front of the boat.

I guess I'm wondering if perhaps this suggests my battery is weak. It seems to charge fine. When I stopped the trolling motor the voltage went back up to something like 12.5, it just goes down to 11.9 or even 11.5 while the motor is actually running (after running it for awhile). I'm thinking that is getting kind of low and not giving me the power I should expect at the motor.  :-/

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Nope! Its working fine. The motor is putting a huge draw on the battery relative to everything else you're using, and the decreased voltage is how the battery reacts to load. If it makes you feel any better I have a completely identical setup, 55lb motor, 20' cable and everything else on a seprate panel. Mine does the same thing.

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I expect the motor to drop the voltage while it is running, just not that low. I think it is a sign the battery may have weakened. It is still under warranty (Walmart - but lets not have a brand depbate  ;) ) and I know I can return it with no argument, I just don't want to do it if it is a normal condition.

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It's perfectly normal and explained by ohms law.

As current increases, voltage decreases.  

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Bad (loose, dirty) connections on the positive or negative battery terminals will give the same symptoms.  It wouldn't hurt to remove them, clean thoroughly with a wire brush and tighten.

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Bad (loose, dirty) connections on the positive or negative battery terminals will give the same symptoms. It wouldn't hurt to remove them, clean thoroughly with a wire brush and tighten.

Ann-Marie is right, it wouldn't hurt.  After they are clean coat with a battery terminal grease to keep them from corroding so fast again.

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Also what gauge wires are you running, Maybe the wires aren't thick enough (rated) to handle the amount of amps. touch them while in use, maybe there is a break somewhere in the line.

or simply check the connections like mentioned before.

memo

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is it a maintenance free battery, if not check the fluid levels in the cells of the battery, if low refill with distilled water, unless extremely low then go to the auto store and get some battery acid.  Otherwise the drop isn't that drastic, check and see if the voltage comes up with a slower speed, if it does it is just the load of the trolling motor would be my guess.  You can also do a load test on the battery if it would make you feel better, but it all sound fairly benign to me.  You should put a ohms meter on your car battery and have someone crank it, you will see the same phenomena.

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