The Official Dual Battery Thread

Not to compicate this thread, but how about adding a third battery? For 2 years I've had two batteries with just a simple isolator switch between the two. They worked great overall and no problems with the alternator. But last year, I added a 5th amp to the mix and I'm almost positive I'm overtaxing the Optima that is supposed to run the stereo. If I add another Optima, would it be okay to just wire those two together, or should I isolate them as well?

[Diesel]

Originally Posted by skitilldark

Not to compicate this thread, but how about adding a third battery? For 2 years I've had two batteries with just a simple isolator switch between the two. They worked great overall and no problems with the alternator. But last year, I added a 5th amp to the mix and I'm almost positive I'm overtaxing the Optima that is supposed to run the stereo. If I add another Optima, would it be okay to just wire those two together, or should I isolate them as well?

No need to isolate the third battery or the forth, fifth, etc.............. The isolation only protects the starting battery from draining while running the stereo. The isolation ensures you will always have a fresh battery for the engine.

Just wire the third battery in parallel to the auxiliary battery. This way the isolator still only sees one big 12 volt battery. I would recommend that you buy two new identical batteries to prevent current disparencies between the hardwired batteries. In addition if I know I am not going to be using my boat for a month or longer I will disconnect one of the terminals connecting the two auxiliary batteries to prevent current flow between the batteries.

Also make sure you do not overload the capacity of your isolation device. Double check the rating and measure your current draw from the 5 amps to ensure the isolator is not overloaded.

Originally Posted by Diesel

No need to isolate the third battery or the forth, fifth, etc.............. The isolation only protects the starting battery from draining while running the stereo. The isolation ensures you will always have a fresh battery for the engine.

Just wire the third battery in parallel to the auxiliary battery. This way the isolator still only sees one big 12 volt battery. I would recommend that you buy two new identical batteries to prevent current disparencies between the hardwired batteries. In addition if I know I am not going to be using my boat for a month or longer I will disconnect one of the terminals connecting the two auxiliary batteries to prevent current flow between the batteries.

Also make sure you do not overload the capacity of your isolation device. Double check the rating and measure your current draw from the 5 amps to ensure the isolator is not overloaded.

Thanks a ton for the info!

[rmbrinner]

I am very inexperienced with this type of thing. Can someone please explain in detail exactly how to connect the relay to the ignition circuit? I would really appreciate it!

Also, is it going to be a problem if I keep the battery that I have now for starting, and add a blue top D34M for accessories? All I know about the battery I have now is a one year old deep cycle marine battery, but is not a blue top.

[RobertT]

Not wanting to complicate things further, but is there a easy way to diagnose current drain? I bought a new battery a few months ago, and have not used it since other than just a few tests on my sound system.

My battery is dead again.

I have not disconnected it and trickle charged it though.

Any ideas gurus??

Robert; YOu can put a VOM that reads current in series with the positive lead off a charged battery. With everything off you should see some current draw if your battery is being killed. Then you will have to find that problem!

[BrianS]

There is a similar discussion going on here: http://www.tmcowners.com/teamtalk/showthread.php?t=6569

I'm getting ready to install dual batteries in my Prostar 190 and I'm planning on using a different isolator/combiner. Specifically this unit: http://www.yandina.com/c100Info.htm

I believe there is one drawback to using a "dumb" relay. When the key is switched to ON the batteries are immediately placed in parallel. If one of the batteries is significantly drained it will pull the other battery down and equalize at a voltage level somewhere in the middle. This could all happen between the key hitting ON and START. Potentially the voltage could drop low enough to prevent the boat from starting. I have never had this happen but I think it is theoretically possible.

The Yandina combiner only places the batteries in parallel when one of the batteries reaches 13 volts, ie it is being charged. This only happens when the alternator is turning, ie the boat is running. So starting the boat will drop the voltage of the starting battery down, once the alternator has it fully recharged, the batteries are placed in parallel to allow the aux battery to charge.

I'm not trying to knock the method above, obviously it works very well. Just throwing out another alternative. I'll draw up a diagram for the setup I'm planning and post it.

[Byrdman]

GREAT INFO DIESEL, THANKS!!!! I just added another amp and sub to my system and decided to do this as well. With this info, I'll be able to get this done in a fraction of the time. FYI, If anyone is in an agricultural area, you can get tractor batt. cables that already have the double post clamps as well.

[BrianS]

Here is the diagram:

[prostar205]

This is great info. I have a thread that I started about this same subject. I will piggyback onto this one and ask a few questions.

It appears to me that people are either using the relay method or battery combiner. I don't see alot of folks using a pure isolator between the AUX and Starting battery. Why is that?

I have had a dual battery setup in my boats since 1989 and have always used an isolator without any problems. I am now installing a three battery system in my X30. Two Optima Blue Top D34Ms wired in parallel for the stereo and a single battery to start the engine, run the gauges, ballast pumps, bilge pumps, etc...

Here is my block diagram.

Can someone tell me if this looks correct?