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"What you NEED is a good understanding of your boats basic electrical system. "
Hear hear!

Keja, I'd suggest a trip to any chandlery, or online, to find a basic book on 12-volt systems, batteries and chargers. You can approach the batteries on a boat two or three ways:

1-Curse at them constantly while perpetuating or ignoring problems
2-Curse at them constantly while perpetuating or ignoring problems and throwing gobs of money at them, i.e. replacing batteries every two or three years
3-Learn how to inspect the entire system, charger(s), cables, batteries, power loads, and set up the entire system so it is running properly.

Number three really is the least exciting way to do it, but also, the least expensive in the long run. $20 for a basic book, $20 for a basic multimeter, and you're on the way.
 

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John-
If you've got a common 3-wire harness on the alternator (like almost all cars do) then all someone has to do is disconnect the voltage sense lead (one of those wires) for about 30 seconds and it will fry the diodes and ruin the alternator. This is a common scam for rip-off garages to pull on customers, they disconnect the wire in front of your eyes and then hook up a meter to show you "See, it's dead" and it sure is now.
Then they'll blame it on the battery being bad, so you'll buy both a new battery and a new alternator.

But it simply is a weak point in the common alternator design. Disconnect the voltage sense lead, the alternator says "Oh the battery is so very low, I have to put out full power!" and in doing so, it can burn out in 30 seconds. And damage all the electronics and light bulbs, if they are on.

So...not necessarily a problem caused by the battery connection, although that lead goes from the alternator to a battery, same as the charging lead does.

Boats can be a hard place for electrics and unfortunately it can be a physical PITA to get close enough to make sure all the connections are clean and tight.

FWIW, If you have a "one wire" system, it doesn't have that problem, but there are always other tradeoffs.
 
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