NEED HELP WIRING! Adding a 2nd battery

burnsadam45

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Mar 15, 2016
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I recently purchased a new 12v motorguide x3 trolling motor and want to add a second battery to my boat for it. Right now I Have a 40hp evinrude connected to my cranking battery then the trolling motor, radio, and fishfinder connected to the deep cycle battery. The manual from the trolling motor showed that I need to connect the two batteries with a lead wire connected to the negative terminals of the batteries. I do not know much about batteries and wiring so any help is much needed. Thanks!!
 

fhhuber

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Jun 19, 2014
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If paralleling,
you can just connect with a heavy wire from negative to negative. Use equal to the wire from the batteries to the trolling motor or heavier.
The positives need a fuse or breaker between EACH battery and where their leads join.

The batteries should both be within 0.5 v of each other when you connect them. Then they can just be treated as one big battery until one or both fails from age.

Note that paralleled batteries will generally both fail about the same time because you don't notice one going bad until it is already damaging the other. Thus its best to buy them as a pair, so an old battery doesn't damage a newer one.
(This is why you don't mix fresh batteries with old ones in flashlights or other devices)
 

burnsadam45

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Mar 15, 2016
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So far that is what I have done. The lead that connects both batteries is of heavier gage. Both batteries are brand new and I installed breakers on both positive connections. So now the only thing I have to watch is when the batteries get weak both batteries will be weak at the same time? Also will any juice go back into the batteries when the main engine is running (40hp evinrude)? I'm just trying to figure out the correct way of hooking my electrics up. Thanks fhhuber
 

fhhuber

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Depends on other wiring of the boat... Often the trolling motor is not connected to the engine in any way and depends on use of a plug in charger. There are systems to connect the trolling motor to the engine for charging without it being able to be used to start the engine. And there's a system that lets you hold an over-ride switch to use the trolling motor battery to start the engine if the engine battery goes down for some reason (like running a radio or lights off of it too long)
 

burnsadam45

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Mar 15, 2016
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Ok the helps out. The trolling motor uses a plug in at the bow then the wires run straight to my second battery along with the radio and accessories. the engine is the only thing connected to the first battery minus the negative lead wire running to the second battery. As you can tell I keep second guessing my wiring of the boat so just looking for others opinions. As of right now I use two battery tenders to charge the batteries but look at a two bank on board charger. fhhuber
 
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