I have a 30hp mariner with electric start and a Minolta edge 45lb trolling motor.  I recently bought a new agm group 31 battery. And was planning on hooking everything to one battery. (I used to have a cranking battery just for cranking and everything else hooked up to my deep cycle wich consists of bilge aeration and a small lawrance)When the trolling motor is isolated and hooked up to the battery its fine. Nothing else hooked to the battery at this point.

 

As soon as i touch the positive lead from the big motor to the battery positive terminal. I get spark and the fuse (25amp glass tube) on the trolling motor positive lead breaks. 

 

What's going on here. Both the big motor and the trolling motor hook up and work fine on there own....but they do not play nice together.

 

At first i went to the front of the boat and undid the trolling motor plug.  Because I had the starter for the big motor hooked up and tried hooking up the trolling motor. As soon as i touch positive. POOF.  Everything was off and isolated. There is no bad wires or connection.  

 

Just these two systems a red and black lead from the big motor and a red and black lead from trolling motor. As soon as i try to hook them both to the same battery fuses blow.

 

Everything works fine on separate batteries.  I had everything hooked up before for  4 years with zero issues.  Why can't i hook my trolling motor and my big motor to the same battery?

Voltage across battery is 12.76 volts. The fuse for the trolling motor is 25amps ....if i hook up the leads from the big motor starter everything's fine. As soon as i touch the positive lead from the trolling motor i get an arc and the fuse goes. The negative isn't hooked up or touching anything. 

 

Same thing happens the other way around. If the trolling motor is hooked to the battery by itself its fine. As soon as i touch the positive lead from the big motors starter to the positive on the battery...I get an arc and the fuse on the positive trolling motor lead blows. 

 

Both the big motors starter and the trolling motor work fine connected to the battery by themselves.

 

I touched off on the starter to the big motor with a multimeter and got a slight reading for a few seconds (.04)  then i got nothing the second time. I am baffled as to.  why i can't hook both these things up to one battery.

Using 1 battery to run your TM, Elecrtonics and crank you engine isn't a good plan because of the voltage drop as the TM uses up amp storage and voltage drop.

Keep the 2 batteries separate with 40-50 amp circuit breakers. You have something connected to ground that the positive wire to the TM is shorting to causing the arc.

Tom

There is nothing else on the circuit. Its a lead strait from battery  to the trolling motor plug (wich i cut out just to eliminate fault). And the factory  lead that's three feat long that goes to the big motors starter. All wiring and connectors are in good shape.  I never run the bilge unless emergency....the boat stays dry. Or the aeration. I just pull the plug and let in fresh water or bail water into THE LIVEWELL.   I use  the lowrance only when running the big motor. There is no excessive draw. Its a minimalistic boat set up for bass fishing. Smaller lakes and river backwater. I have never seen the end of my trolling motor battery. In a full day fishing not even close....its a group 31 pulling a lightweight 16 foot alum boat. Its a five speed. Trolling motor and i rarely run it on speed three. Mostly the dial is on one or two. It only draws maybe 6 or 8 amps under that load. Its a 105 amp hour battery. And the motor is a 30hp with a pull start if i need it. Idk....im baffled.   

 

 

I guess i don't really need the electric start. It is nice on cold mornings....with a cold blooded carburated two stroke. After the she's got that first heat in her she starts as soon as you hit the key. Same with the pull start. Its easy. But that first start of the day she's kind of picky

Not Having two batterys...saves space and weight. That's the only reason for trying to go down to one battery. 

Can't help much with your issue, I did do a little research on the trolling motor you are using and Minkota recommends a 50 amp CB.  The 45lb thrust edge can draw up to a max of 42amps.  One thing did cross my mind during your description of issues.   For current to flow through the positive lead of the trolling motor and blow the fuse, the circuit would have to be grounding in some fashion.  Try turning the speed dial all the way down to zero and placing the momentary/off/continuous switch to off when you attempt to connect the positive lead.   Another thought I had was about a possible short in the windings on the trolling motor armature or commutator segments.  

I cut out the trolling motor plug. Its not even wired in the circuit. So its not anything to do up front. I will have to pull the rear deck out and have a look.at wiring. It all appears clean from what i can see. And i would think nothing is wrong... i bought the boat used so who knows. Ill just run without the starter and use the pull start for now....its really strange issue that i didn t expect. I can also just run two batteries like I was. But I'd like to find the issue

DC is very simple; positive ( power)  and negative (ground) there isn't common current. The current can only flow if your TM switch is on or it's defective. Positive wire aching then shorting a fuse indicates a direct ground short. You need a circuit breaker to prevent over loading the circuit and burning wires. My gues is with all the cutting and splicing 1 wire is crossed cussing your short. I have no idea how the boat is wired, what colors were used to indicate positive (power) and negative ( ground). 

He says that he can hook each thing (TM, and Outboard) up separately, and they both work fine, and no fuse blowing. 

 

If that is a true statement, then your issue is very strange.  Take the positive and negative lead (TM) off the battery, and test for continuity to from positive to ground, and from negative to ground.  There should be no continuity.  If there is, your TM wiring is shorted to the boat somewhere. Do the same for the outboard, any continuity to the boat means a wire is grounded.

 

I do agree that you "shouldn't" put an outboard on a deep cycle battery, they don't like the burst of amp draw from a starter, they are made for steady current draw over long periods.  It should work, it just won't last as long as if you had the proper battery for the application.

 

And you do need a bigger fuse/breaker on your trolling motor for sure.

 

Put the old battery in and see if it works like it did before you changed the battery, if it works fine, the battery is causing the issue for some reason.  If the fuse continues to blow, something happened with the wiring during the battery swap.

 

Thanks guys...I think the problem was the little spring loaded carrier that the bus fuse locks into. I cut that out stripped fresh wire and rewired new in and insulated everything with marine shrink tube and put a 40amp fuse in instead of the 25 that was there before.

....seems fine so far.

  Im not worried about using the trolling bat as a cranking battery. The starter only requires 350 MCA.   The battery is rated at a thousand MCA and 105 amp hours it starts the instant the key is hit. Strain on battery is minimal. I don't need two batterys its un necessary. Its not a tournament boat. Its a light  16x42 that drafts about nine inch of water. I could get away with a battery half the size. Its an oversized battery for the application for sure. And its designed for cranking and deepcycle