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hertfordnc 04-18-2008 09:07 AM

alternator wiring mystery
30 year old boat with 20 HP yanmar- some time ago the alternator was converted to a Delco - it currently has a 10SI 60 amp Delco alt used on most chevy cars in the 70's.

The field voltage had been wired directly to the starting battery- makes no sense- drains battery even with all switched off.

I wired it to a spare breaker - works great- except.....

If I select the starting battery , which happens to be a bit weak at the moment- the breaker for the alt pops a few seconds after the engine is running. The alternator still makes charging voltage even after it loses the exciter voltage.

If I start the engine from the larger bank- which happens to be fully chargerd, the breaker does not pop.

I don't know the rating of the breaker.

So, my guess is that the lower voltage from the weaker battery draws more amps for the exciter voltage and the higher draw pops the breaker.

Is that logical?


Lancer28 04-21-2008 11:04 AM


I think you should re-wire your exciter voltage to a hard line to the starting battery (or to the battery switch would be an even shorter run).

The alternator draws just a fraction of a milliamp and doesn't really kill the battery like you'd think. Non-use maintenance is to visit the boat once a month or every other month and run the generator or charger for a couple of hours. If the battery is dying sooner than that, it is probably not good.

The cycle for the alternator above is something like this Start-wait about 5 seconds-RPM up-go into exciting mode (creates a LOT of draw and power loss for a minute)- then it charges max amps at about 14.75 volts until the battery starts to top off, then it goes to about 5 amps and lowers output voltage.

You're probably popping the breaker from drawing too much through the wire to recharge the dead or bad battery. I do know that a lot of delco alternators didn't push full amperage unless the line voltage was below a cetain point. you may have one of those.

Unwire the breaker -- if the battery is bad replace it, don't crutch. check the wiring to the battery and terminals for cleanliness and resistance too. If this is the one with the voltage regulator built into the back of it, you should run the lead all the way to the battery terminal itself, then the charge leads are ok going to your battery switch. You should also have a line that goes to ground coming off the back with a lamp in series to show when the alternator is not charging. Check to see if that lamp is operating or not too, if you can't get a good ground then the charging system greatly suffers or stops completely.

Hope that helps! Charging systems are a total pain in the ass unless you've owned the boat since it was born!

Lancer28 04-21-2008 11:09 AM

another thought - 60 amps is REALLY high for just a 20hp motor. You may be nearly killing the engine trying to recharge your batteries. If you were caught with your pants down in an emergency and needed full throttle, that alternator would likely make the engine bog down bad.

I would suggest going to a mitsubishi 20-25 amp with built in regulator to reduce the parasitic horsepower loss when your bateries are low. Anything should bolt and wire in as long as the hinge bolt and pattern matches your old delco. (replacement is only about $40 at autozone).

camaraderie 04-21-2008 11:27 AM

Lancer...60 amps is only a couple of horsepower. He is not killing anything.

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