battery isolator with an 85 115

[matthewp]

I've got an 85 115 Johnson and I want to add a battery isolator to the system to separate my two batteries so that one with a lower charge will not sap juice off the one with a higher charge. One starter battery - one for everything else.

My understanding is that it should go something like this: Wire from stator to input terminal on the isolator, output terminals on isolator to positive on each battery, positive from starter battery back to the positive lug on the coil.

My question is this: is it the stator wire I'm looking for, and where do I locate it?

Thanks!

 

[Chris1956]

Re: battery isolator with an 85 115

Matt, You do not want to do that on an OB. You will likely blow up your VR and you will need to run wires from the motor to the batteries.

Try this: Connect all your load to the house battery. Connect the motor to the other battery. Connect the two batteries negative poles together. Now insert the battery isolator or combiner between the postive poles of the batteries.

 

[tashasdaddy]

Re: battery isolator with an 85 115

this belongs in electrical.

 

[matthewp]

Re: battery isolator with an 85 115

Thanks for the heads up, Chris. With the arrangement you describe, the isolator would still prevent backflow of juice from starter to house battery. Forgive me if this is a naive question, but wouldn't the house battery need to be fully charged before the the juice gets to the starter battery?

My whole point in doing this is so I don't have to monkey around switching between batteries all the time. I want to design a system that will take care of itself.

Thanks for the help. I'd rather hear I'm going about it wrong 100 times than blow up anything that I'd need to replace, or worse yet, that would strand me.

 

[Chris1956]

Re: battery isolator with an 85 115

The setup described will charge the starting battery first, if you use a combiner. After the startng battery is topped off, the house battery will be combined and charged. With the isolator, both are charged at the same time, but you have a 1 volt drop across the isolator. If your charging system is 20Amps or so, it is not much of an issue.

 

[Don S]

Re: battery isolator with an 85 115

My whole point in doing this is so I don't have to monkey around switching between batteries all the time. I want to design a system that will take care of itself.

It's already been done. I have installed these for customers many times. The house battery and starting batties are completely isolated from each other.
You turn the battery switch on, and that's it.


http://bluesea.com/category/2/productline/overview/329

 

[matthewp]

Re: battery isolator with an 85 115

Ok, here is what I have:

http://www.marinco.com/product/120-amp-battery-isolator-0

Mine is the same make - only it's a 70 amp unit.

It sounds like Don S's suggestion is a package deal and would work with my boat. Great. I'm leaning toward using that solution if it's tried and true.

Next question: I WANT to put the batteries up in the center console to keep them high and dry. I've measured the wire path I'd use, and it amounts to about 20' of battery cable (each way). I called a marine wire distributor to ask about the proper gauge, and the guy made it sound like a crazy idea. soooo,
is this doable? Or should I keep the batteries in the stern?

 

[Boatist]

Re: battery isolator with an 85 115

I do not know your motor but if it has 20 to 30 amps of output your isolator will work well. If it only 8 to 10 amps of output then I would probably use a battery combiner instead and not expect much charge to get to the house battery or trolling motor battery.

On my boat I used a battery isolator for many years and never had a battery that did not last at least 6 years.

Remember that your charging system will put out a voltage in the 13.8 to14.8 range to the isolator. The isolator will likely drop .7 volts so will have 13.1 to 14.1 volts to charge the batteries. A house battery or trolling motor battery that is discharged 50 percent would take 10 hours or more to fully recharge.

Newer Large outboards can have 30 amp alternator but even then can still take 10 hours to fully charge a house or trolling motor battery.

Leave your starter conected as it is. Find the wire from you alternator or charging system and connect it with a wire as big or bigger to the common post on your isolator. Then with the same size wire or one size larger go from the isolator output 1 to battery 1 and output 2 to Battery 2.

When running the battery with the lowest charge will get more current then the battery with the higher charge. As batteries charge the current the batteries take will slowly decrease.

A large house battery or trolling motor battery will still take 10 to 12 hours to fully charge the same as it would on a 15 amp automatic battery charger.

The battery combiner charges only the starting battery until the starting battery reaches a certain lever then it will pick the relay to charge the house or trolling motor battery. Many times with outboards the load from the house or trolling motor will load the output of the charging system and pull the voltage down to the point that it dropps the charge realy for the house or trolling battery. Then the voltage go back up and it repicks the realy for the big battery. Many times will make the relay into a buser as it picks and drops. As the big battery charges then the relay will start to stay picked longer and longer. The combiner does not have the .7 volt drop so will charge a little faster near the end of the charge. Of course the big battery does not charge at all until the starting battery reaches a high voltage.

I also am not a fan of moving the batteries foward. It means the batteries get more pounding and you drop more voltage from long cabels.

Either way it is important for the house or trolling motor battery to be fully charged. So if your not running for 10 hours to charge you need a way to complete the charge at your dock or at home.