Adding a secondary battery - Relay/Isolator size?

[jstrahn]

I'm adding a secondary battery to my boat to run the stereo and some other accessories. I think I have a fairly good handle on things. I want to put a relay/isolator between the batteries so that the secondary runs things when the engine is off and both charge when running.

Stinger makes an 80 amp model and a 200 amp model. I don't mind spending the extra money but is it necessary?

Any other tips you can give me about the installation are appreciated.

[mrfixxall]

I'm adding a secondary battery to my boat to run the stereo and some other accessories. I think I have a fairly good handle on things. I want to put a relay/isolator between the batteries so that the secondary runs things when the engine is off and both charge when running.

Stinger makes an 80 amp model and a 200 amp model. I don't mind spending the extra money but is it necessary?

Any other tips you can give me about the installation are appreciated.


just install a battery switch,i run mine on 2 then switch to all when under way to charge them..10 yrs and counting ;)

[jstrahn]

just install a battery switch,i run mine on 2 then switch to all when under way to charge them..10 yrs and counting ;)

I considered that but I am comically forgetful. I'm the type of person that walks from one room to another to do something and when I get there think "Now what was I going to do again?" :)

The 80 amp isolator is only $25 so I'm wondering if that would be sufficient.

[gcarter]

If I remember correctly, the rating is it's ability to charge off the alternator.
An 80 Amp charge is pretty healthy I think.
You can add a Ford remote starter solenoid that can be activated to momentarily connect the two batteries for emergency starting if you want to.
One thing about isolators, they'll drop the charging voltage from the alternator to the battery through the isolator about one volt. Normally this isn't an issue, and it's only when you're charging through the isolator.
When you've selected a battery through the switch, the charging bypasses the isolator, or maybe it's in parrallel w/the isolator.
I'm all for isolators.
When you're talking about multi-engine/multi battery bank boats, you wouldn't think about NOT having isolators. It's just done that way.
It should be on our boats too.

[MOP]

I have been running an Arco 70 amp isolator for quite some time, it works flawlessly and I never ever sweat either battery not being fully charged.

[Walt. H.]

Yep!
80 amp model isolator is all you need since your alternator output is most likely 54 or 60 amp max anyway, so you're well within working limits.

[zelatore]

If I remember correctly, the rating is it's ability to charge off the alternator.
An 80 Amp charge is pretty healthy I think.
You can add a Ford remote starter solenoid that can be activated to momentarily connect the two batteries for emergency starting if you want to.
One thing about isolators, they'll drop the charging voltage from the alternator to the battery through the isolator about one volt. Normally this isn't an issue, and it's only when you're charging through the isolator.
When you've selected a battery through the switch, the charging bypasses the isolator, or maybe it's in parallel w/the isolator.
I'm all for isolators.
When you're talking about multi-engine/multi battery bank boats, you wouldn't think about NOT having isolators. It's just done that way.
It should be on our boats too.


That's it exactly. On our larger boats we don't mess with battery switches. Kinda hard to do that when you're dealing with 3, 5, or 10 batteries. The isolator is ideal if you want dedicated starting and house banks.

The one place you might want to use a simple 'on/off' battery switch is for paralleling the two bats. 99% of the time you'd simply leave it off, but if your starting battery should fail, just throw the switch and you combine both of the to get yourself started and back home. This would do the same thing as the Ford solinoid George mentioned, it's just a little simpler and stays 'hot' after you turn the switch vs only being momentary on with the solinoid.

About as fail-safe as you can get.

Oh, and yeah, I think 80 amps should do it - just depends on what your alternator output is, and I doubt you'd find a high-current alternator on a small Donzi unless somebody got crazy with it in the past.

FWIW, a silicon diode as used in the isolator drops .7 volts. (I remember a little bit from my days in school!) If you have a one-wire alternator it won't matter, but if yours has a sense wire, you have to be carefull to connect it to the battery side of the isolator instead of the input. Otherwise you'll always charge the batteries .7v low.

[jstrahn]

Thanks for all the information guys. I went ahead and grabbed an 80 amp isolator.