Dual Battery Setup - What size relay/isolator?

[jstrahn]

Hi guys,

I'm installing a second battery in my boat to run the stereo and want to add a relay/isolator between the two batteries so that the amp only draws from the secondary battery when the engine is not running.

That said, I'm not sure what size isolator I need. Here is the setup:

Primary Battery runs the following (Most will not be run simultaneously):

Running Lights

Bilge Pump

Blower

Head Unit

Horn

Hydraulic Trim Tabs

Hydraulic Outdrive Trim

Secondary Battery runs:

Alpine PDX-5 - 100 rms x 4, 360 rms x 1 at 4 ohms (Two 30 amp fuses)

So with that said, Stinger makes an 80 amp isolator and a 200 amp isolator. I'd rather have some head room for peace of mind but I don't want to spend money that I don't need to.

Any advice is appreciated.

Click to read more...

 

[sacsking916 5,000+ posts]

what suze us your alt?

 

[Bettr n' Revrse 5,000+ posts]

Also Id like to know where these are wired into the setup... Sorry for the thread jack OP but I thought you may wanna know too?

 

[jstrahn]

It is a 55 amp alternator I believe. Is that what would determine the relay size needed?

As for where they are wired in, they would both be in the engine bay in the stern of the boat. I'm not sure if that's what you are asking or not but they'll be right next to each other.

Edit: Oh are you asking where the relay/isolator would be wired in? It goes between the two batteries on the positive side.

So there would be a positive connection from motor to main battery, positive connection from main battery to isolator and positive connection from isolator to second battery. I would then have the amp wired to the second battery.

 

[ultimate157 5,000+ posts]

If your alt is 55amp, and your amp can draw ~60A on full tilt (highly unlikely to reach 60 though), then a 60A or higher isolator will be needed.

ETA: I was ready to jump in here and tell another person not to buy an isolator... But I was pleasantly surprised to find someone who needed one for the correct reason.

 

[Targa]

Very simple solution that I used is an AOPEC Smart Battery Isolator. Simplest install available and I bought it online just google it or you can find it on the DIY website dfna.info . They were a little higher priced so I shopped around and found it on craigslist. I ended up paying $56 with shipping. It is completely automatic and isolates the battery when a charging source isn't available. The nice thing is that I didn't have to connect it to the alternator or the ignition. Simply mounted in next to my starter battery ran a cable from the start battery positive terminal to the isolator terminal "A" then another cable from isolator terminal "B" to the second battery. Also connected the isolator and both batteries to a common ground and it was finished. When I start my outboard the isolator allows starter battery to reach full charge 13.3 volts before it connect second battery for charging. When I shut outboard off as soon as the voltage drops to 12.8 volts the isolator disconnects the two batteries. It is very small unit and marine approved so it can handle moisture and vibration. It works great and I don't have to remember to turn a switch and it doesn't allow my alternator to be over worked. Outboard alternators are very expensive!!!