Dual Battery System Setup
 

I'm using Dual AGM batteries but now decided I want to use an Isolator or DC-DC converter. I want to be able to run my AV, etc. at shows without killing my starting battery. I'd also like the ability to switch to the second battery in case my starting battery was low and would not start the car.

Anyone have any experience with these?

In the past I have used diode isolators with two batteries for a stereos. These work well for doing what you want to do.

For switching from one battery to the other.....

You could switch the cables from one terminal on the isolator to the other for the times when you want use the AV battery for starting. However, I wouldn't suggest this to often. The terminals on the isolators aren't real rugged.

Another way to achieve what you want to do is to connect a relay isolator between the two outside terminals on the diode isolator and put a switch on the switching terminal (small terminal). This switch would normally be open and only activated when you needed to use the AV battery for starting the car. Then deactivated once the car is started.

Here is a link that has some good info on both diode isolators and relay isolators. https://www.youtube.com/watch?v=CGD8HAeg5UA

Bass Pro Shops
 

Don't the bass boat guys do this for their fish finders & trolling motors?

You'd think Bass Pro Shops would have something for a dual battery set-up like you're wanting...

A lot of racers use this as well. You can get dual battery switches but they only let an AGM battery charge to about 80%. The DC-DC Isolators let both batteries charge to 100% but don't seem all that adept at switching big current in the event a jump was needed ot the primary. At least not the ones I have run across but I'm just learning about them.

I'll give it a look. I was thinking a battery made to switch between or combine both batteries and also using the Isolator to keep both batteries charged. I'd mount the switch between the two batteries as they are now in parallel and only turn it on in a hard start issue.

At least that is what I have come up with so far based on very limited research. I'm also going to add an alternator idiot light to the voltage gauge so I can see if it actually quits charging before running all the juice out the battery operating the car.

I put together a couple of multi battery wakeboard boat stereo systems. Keeping it simple I just used a starter solenoid.
Key on engine running solenoid closed all batteries connected and charging.
Key off or acc. Solenoid open, batteries isolated.

Simple and works, for years.

I would but my research led me to a few articles that point out that the AGM batteries require a DC-DC isolator to charge to 100%. Apparently they otherwise only charge to 80% with an basic switch. Same thing with 120v chargers. You should use one designed for AGM batteries verses wet batteries.

So the agm battery in my car doesn’t charge ?

That is your statement not mine. The point being is that you cannot hook them up exactly as you did in your boat because they only get an 80% charge that way. Look it up.

The bottom line is you need better equipment to keep your AGM battery fully charged and to maximize it's life. The voltage regulation needs to be more precise than a wet battery.

I also imagine your cars starts fine with a semi fully charged AGM battery. It will just not live nearly as long as it should.

This stuff is not as simple as your are making it. Using the same old style charger a Lead acid wet batteries and also an AGM is like trying to run the same fuel pump for both carb to fuel injection. Can you get it to work? Yes. Will it last? No

Here's some info to help show that 12v isn't just 12v.

https://deepcyclebatterystore.com/ho...ain-batteries/

I think a car alternator charges the agm battery just fine, they wouldn’t use an agm in a automobiles all these years if it was an issue.

The battery doesn’t know if you pulled the cable off the post or simply opened and closed a disconnect switch.