HP500 alternator over charging
 

I've got one of my alt randomly over charging to 16+ volts which in turn sends my audio amp in to protect mode. I unplugged the two wires (trigger for regulator??) with no difference. I run both batts all the time but did figure which one it was.. I was going to upgrade it to a 105 amp one wire alt if it wasn't a simple fix. Any ideas??

Regulator, which is internal Priceyif you can find one . For most it’s worth getting new alternator ?

In the meantime, don’t run your boat anymore this way. Can damage batteries and electronics

its often the 90 amp fuse down on the starter

How does that cause the issue?? Ive never heard of that before...

What??????

If the 90 amp was fuse was blown, the engine would not run.

I’ve found this to be true on some Merc outboards. The fusible link blew and the alternator was putting out 19-21 volts.

if the fuse is bad the power does not go to the batt instead goes into the harness feed just happened here on a 525 to test run a good sized wire from alt output to the pos on the battery

I found out a number of years back not to run the switches on "BOTH" with a twin engine setup. After blowing up 3 alternators in one season i started digging around for a reason why it was happening. Finally got an answer from a company that makes huge audio alternators.
Basically batteries go through 3 phases of charging, (They actually had it divided into something like 12 stages but overall its like 3 just to keep it simple) Bulk Charging, Acceptance, and Float charging.
First phase is bulk charging where the regulators are given a target voltage and the alternators are giving max current output. So let say you have 100amp alts and you start the engines and the batteries are accepting 80amps charging current. The alt on the engine you started first will be cranking out 80a while the other is spinning at idle doing 0amps. So now its building heat which is hard on bearings and electronics...IE the regulator goes out....
Next phase is Acceptance, in which the voltage is kept constant at 14.2v but the current keeps going down on which the batteries can take, down to where its like 1-2% of their capacity, then the batteries are considered charged and the system goes into Float mode at which point the regulators are looking for their set points to turn back on and give some love to the batteries.....this is where they can yo-you back and forth between the two and the constant on/off fight between the two is not good.
And if you have never had a problem with your setup running them in "BOTH" more then likely you have one alt that has settings higher then the other and is doing all the work while the other just sits there waiting its turn...you will never even know one has gone bad until both are toast. :(
Its better for the charging system to run the engines on their respective batteries and let the alternators be separate systems unless you are absolutely dead and need the other side for starting briefly.

Thanks Wally. Been saying to tin septette for years and keep hearing but they are fine all connected. Like talking to crickets sonetines, lol. No biggie, not my boat(s).

Anyway, great info and well written that will hopefully get people to make the switch. Pun intended.