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Old 07-12-2015, 08:22 PM   #1
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Battery Isolators

I installed an old-school battery isolator in the jetboat. I had it out today for the first time this year, first time with the house battery and isolator. Will be replacing the isolator with a Battery Doctor/Wirthco isolator later this week when it is delivered.
The isolator had ~13.6v coming in, but only ~12.4v going out to each battery. At that rate they'd never reach a full charge.

I'm thinking about scrapping the old-school isolator in the Sun Stream and replacing it with a Battery Doctor/Wirthco unit.
Battery Doc® 12 Volt 150 Amp Battery Isolator - Wirthco Engineering, Inc.

Not a huge deal for the MoHo since most of the time the house battery is charged by the power center with either shore power or gen, but with the old style isolator the starting battery isn't going to reach a full charge.

Anyone replaced their old style isolator with a newer, lower-loss isolator? Good/bad/otherwise?

1985 Gulf Stream 34' Sun Stream
1997 Newmar Mountain Aire 3757
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Old 07-13-2015, 06:51 AM   #2
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Location: Snowbird - Waterford Mi and Citrus Springs Fl.
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You've identified the reason they are no longer used in any OEM install that I'm aware of. Running power through a diode will drop the voltage. The amount varies, but 3/4 - 1 volt is a good rule of thumb. There is no upside to using one of these to offset that loss.

A constant duty relay will accomplish the same purpose, are reasonably inexpensive, and easy to wire up. They're also much easier for John Q. Public to diagnose and replace as necessary....

I used to ask for something to use in a snow plow application at the auto supplies. Those are generally rated for constant duty (vs. one used for starting purposes), yet they're still reasonably inexpensive.

Placed in a boat bilge though? I don't think so.....

1997 37' HR Endeavor, 275hp Cat, Freightliner
03 CR-V Blue Ox, Ready Brake
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Old 07-13-2015, 09:30 AM   #3
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On a boat, solenoids are are a poor choice for battery isolation. Buddy uses a Perfect Switch isolator which is not solenoid based, and has no voltage drop like old school isolators. He told me its not cheap, but nothing in the boating world that's decent quality is cheap, fwiw.
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Old 07-14-2015, 05:17 PM   #4
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Check to see if alternator voltage is adjustable.

Many are...not standard automotive but larger ones maybe.

Ours is adjustable and we have diode isolator and simple adjustment to compensate for isolator loss.

Another possibility is the control source for alternator may provide reference voltage for output and if on low side of isolator it will self adjust but your measurements indicate otherwise.

Confirm exactly what you have before making any changes.
Tony & Lori
1989 Country Coach Savannah SE
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