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My battery switch has melted 2 years in a row and I don't know why

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LouC has good advice.  I had an issue with my battery cable to the starter  where you couldn't see that the cable connection  was corroded under the insulation . When testing for voltage it was way low. 

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Remember when voltage drops, amperage increases, and that makes heat! 

For the same reason that a bad starter will kill a good battery and vice versa.

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On 3/17/2024 at 8:15 AM, The. Tin. Man said:

The A/B switch has melted from being hot, twice. they both still worked but I would like to change what ever I need to before the season.

It is a dual battery setup with a single outboard Yama 115.

The wires do not show signs of over heating.

Should I add battery isolator switch or charging relay?


Any ideas where I should start? I appreciate any advice

Please post the the root cause when you find it.

I love when we make way past the breakwater at first light and head out, there's nothing better, the whole rest of the world just melts away for me.

(*edited - member formerly known as 'windknot')

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I've been following this thread for a while without commenting, but seeing all the speculation and advice, feel the need to make a suggestion.


Spend a few bucks on a good marine electrician.


Your are dealing with a situation that, in a worst-case situation, could conceivably cause a fire.  When the flames break out, a boat will suddenly become a very small place to be (and yes, it will probably start as a smolder that you might readily suppress, but having once seen a 50-foot Viking go from a puff of smoke to full involvement in a very short time--less than a minure--I have a lot of respect for the flammability of fiberglass).


Should that occur, the money you saved by trying to fix it yourself will suddenly seem like false economy, particularly if you have a wife or kids on board.  Because when the flames start on a boat, you really have no safe place to run to.  There is only the water (and there's no guarantee that your floatation devices won't be on the other side of the flames).

"I have always believed that outdoor writers who come out against fish and wildlife conservation are in the wrong business. To me, it makes as much sense golf writers coming out against grass.."  --  Ted Williams

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There’s many scenarios that may or may not be causing the problem … what’s the switch rated for , volts  and amps ….any recalls on the electrics of the boat, hidden corrosion ?  But I would be extremely careful taking guests out and on the boat  … are their two batteries ? Are they connected in parrell or series? And what does the manufacture say is correct … is their chance that while  adding an option the power wire is crossed ? 

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Where is your battery switch and where are your batteries?


If you battery switch is in the console there is a good change there is a large terminal block someplace where the short leads from your engine are connected to longer leads coming from the console.  This is probably under the gunwhale or under deck, both spots with a high chance of corrosion.  I'd try and find that terminal block and clean it up.  High resistance at that point could cause your issue.



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