NEED HELP WIRING! Adding a 2nd battery

burnsadam45

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I recently purchased a new 12v motorguide x3 trolling motor and want to add a second battery to my boat. Right now I have the 40hp evinrude connected to a cranking battery, then the radio and trolling motor and fishfinder to a seconf battery. I connected both batteries with a lead wire to both negatives like the manual from the trolling motor showed. Any help is much needed as I do not know much about wiring.
 
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JoshOnt

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I would leave them desperate since you don't want to kill them and have no way of starting your boat.
 

burnsadam45

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JoshOnt That's what I was thinking. The manual from the trolling motors is showing both batteries wired together in the 12v system. I just keep second guessing myself as on what to do.
 

thumpar

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Keep them separate. An outboard can't charge the battery just maintain it. If you want to go with 2 batteries for the accessories get another one but leave the crank battery for just the engine.
 
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burnsadam45

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Thanks guys. I'm recently new into boating so any help is great. My mindset was that while running the motor would charge both batteries. I realize it wouldn't be much of a charge though.
 

thumpar

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Thanks guys. I'm recently new into boating so any help is great. My mindset was that while running the motor would charge both batteries. I realize it wouldn't be much of a charge though.
There are kits to add an alternator to an outboard but I don't which ones they work on. Stock it is just a few amps. It would take all day or more to charge a battery from being dead.
 

burnsadam45

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Roger that thumper. So if I leave the connections like they are, both batteries will die at the same time? I'm getting the idea that the best option is to leave both batteries separate with all the forums I have been reading.
 

thumpar

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Roger that thumper. So if I leave the connections like they are, both batteries will die at the same time? I'm getting the idea that the best option is to leave both batteries separate with all the forums I have been reading.
If it is just ground connected then you don't need to worry about it. It is the positive side that you need to keep separate.
 

burnsadam45

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So far I have trolling motor, fishfinder, and radio one battery and engine on the other. The negative terminals are connected by a higher gage wire. All the positive leads connected either have a breaker or a fuse. How does that setup sound thumper?
 

Silvertip

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Let's understand this a bit better. Are you actually adding a second trolling motor battery (making a total three - two for the troller and one starting battery) or are you thinking about using the starting battery for the troller as well? Understand that if your current wiring is actually charging both batteries, that constitutes a parallel situation so they are not separate at all. But lets deal with your current situation (one troller & one starting battery). Those two batteries do not and should not have the negative terminals connected together, nor should the positives be connected whether they have fuses or breakers. I suspect you already have a 12 volt parallel system which means the weaker of the two batteries will suck the life out of the better battery whether or not you are using the troller. What you "say" you have is fine as long as the two batteries are not interconnected and only the starting battery will charge. You need to have a smart charger that you plug in at home or the dock to keep the troller battery charged as the engine simply cannot do that unless you run wide open for hours.. If there is a current path between both batteries you definitely have a 12 volt parallel system and no -- that is not a good system. It will work, but you run the risk of having a dead starting battery.

If you indeed plan to add a third battery (second troller battery) I would suggest a simple switch to connect and disconnect the two and again, keep those two 100% separate from the starting battery. The switch could be ON when using the troller, and OFF when not in use. You get very slightly longer run time that way.
 

burnsadam45

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You cleared everything up for me right there Silvertip. Pretty much what I'm taking for that is, I should leave the cranking battery alone and just run the trolling motor and accessories off of a complete separate battery. If I want to add a 3rd battery (2 for trolling motor, 1 for cranking) how should I wire the two batteries up for the trolling motor in a 12v system? Once again thanks for the great tip!!
 

thumpar

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You cleared everything up for me right there Silvertip. Pretty much what I'm taking for that is, I should leave the cranking battery alone and just run the trolling motor and accessories off of a complete separate battery. If I want to add a 3rd battery (2 for trolling motor, 1 for cranking) how should I wire the two batteries up for the trolling motor in a 12v system? Once again thanks for the great tip!!
Hook the positives and the negatives together. This will still give you 12v but 2x the capacity. You may want to use a switch in between for charging purposes.
 

burnsadam45

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But if the negatives and positives are connected wont the weaker battery still drain the stronger battery? thumper Plus doesn't that make a 24v system?
 

thumpar

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But if the negatives and positives are connected wont the weaker battery still drain the stronger battery? thumper Plus doesn't that make a 24v system?
If one is weaker then yes but just get the same batteries. It would only be 24v if the positive of one battery went to the negative of the other. That would make them in series. Connecting the positives together and negatives together puts them in parallel. The voltage stays the same.
 

burnsadam45

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So when in series they bump up to 24v but when parallel they act as one big 12v? I have two of the same brand new batteries so it should be fine as long as I keep them properly charged. My only question now is why did the motorguide manual tell me to connect just the negative terminals? They said nothing about connecting the positive terminals as well. thumpar
 

Silvertip

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Batteries in parallel have the negative posts connected together and the positives connected together (hence - parallel). This creates one "big" 12 volt system. Batteries in series are like batteries in a flashlight. Positive on the outermost battery to negative of the inner. Positive goes to the load and the remaining negative is ground. Hence - series. With two 12 volt batteries this makes a 24 volt system. Three batteries in series is 36 volts, and four in series makes 48 volts etc. The OP has a 12 volt system.
 

thumpar

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I am not sure about why they would have that in there. Maybe for grounding purposes. Just remember if you connect the positives together it becomes 1 battery and they will both drain at the same time.
 

burnsadam45

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Thanks for both of your guys help! Seriously answered all my questions and now just to decide if I want to run them separate or in parallel.
 

thumpar

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If your motor has a pull start and you can get it going that way it wouldn't worry me too much. Make sure you test that out if it does have one. If it is electric only I would not connect them.
 
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