Dual battery set up?

I am thinking about having two batteries. One dedicated to running the lights, starter, etc, and the other for stereo and other accessories. Is there a way to wire it up so that if one or the other battery dies I can flip a switch over to the 2nd one? Like if I had a dead main battery, and it wouldn't start, I could use the back-up. Or switch the secondary over to main duty in order to have the alternator charge it?

Yes you can use an isolator like this one: http://cgi.ebay.com/Stinger-SGP32-200-AMP-Current-Relay-Battery-Isolator-F_W0QQitemZ220485366320QQcmdZViewItemQQptZLH_DefaultDomain_0?hash=item3355f3b230

Mike, you are the man!

Can you help me please understand a little better how this works? I clicked on the link, and even did a web search for this item, hoping to get a little better understanding of the concept. I found another Stinger model that referred to hooking up two batteries, and that it came on with the ignition, assuring you that the primary battery wouldn't be drained when the ignition was off. I guess that means that the secondary would be charged while the vehicle was running, which is great, but would the secondary be available for juice if the primary was dead, by turning on the ignition? Do you just wire them both to the top posts (that's what confuses me - seems like there should be 4 posts, or at least 3 with a common ground).

OK, my ignorance of electricity is probably showing now. Thanks again. The price is OK so I'll probably order one.

I don't know much about electricity either but when you have this isolator hooked up both batteries charge when the motor is running. Your sound system may run down the battery it is hooked to when the motor is off but the other battery will have a full charge to start your van. I always use Optima batteries. With two batteries you will need a better alternator, one with at least a 135 amp rating. I have a 200 amp one on my Econo I bought on ebay for around $80.00. Don't get a chrome one since chrome does not dissipate heat. If you are going to have a sound system with a lot of amps you may need to get a capacitor that stores electricity to give you that needed boost when necessary such as when you have your headlights on and the stereo turned on. Remember the louder you have your stereo the more juice you use from your battery.

Southern man,
Maybe I can help you on this. I have put dual batteries in just about everything I own and have always been glad I did.

Connecting multiple batteries can intimidate the do it your self installer, however the process is fairly simple when broken down. No matter how many batteries your adding each one of them needs to be isolated. What exactly does isolated mean? I will use the isolator that vanishingbreed linked for you to make it easier.

At this point I was going to insert pic's but clearly the pic server service is not working again....

The first thing you will notice when looking at relays is an amperage rating. This refers to how much current (amps) that the relay is able to handle internally. This means if looking at the top of the relay it refers to how much current can transfer from one post to the opposite post. This number is important since it will tell you how much energy can flow between the two batteries.

Second, most relay isolators will specify how many posts it has. This will determine how many batteries can be connected the isolator. For instance if a relay isolator features 3 posts then it is possible to connect up to 4 separate batteries to the single relay isolator.

So why do you need a relay isolator to run two or more batteries? Isolators serve two purposes. First they allow a set amount of current to travel between the positive terminals of two or more batteries. So for instance if a system was set up with a relay isolator that is rated to handle 200 amps this means up to 200 amps of current is able to flow from one battery to another. Second, all batteries have a different potential voltage so this means if two batteries are connected together without using a relay isolator the batteries would actually drain each other until they are both completely discharged (two completely dead batteries). Since each battery would have a different potential voltage they would push and pull on each until there is no energy left. This is where the relay comes into play; the relay will ONLY connect the two batteries together when triggered to the “ON” position (while the car is running). So when the relay is switched “OFF” the batteries are disconnected and no energy will be flowing between them (when the car is off).

As far as the connections that will be found on a relay they all share similar inputs regardless of the specific model. The first connections are the actual posts, depending on the relay there will be two or more of these. These connection points are used to hook up each positive battery terminal to the relay. Second is the ignition wire connection. This will either turn the relay “ON” or “OFF” depending if 12 volts are being run though the wire or not. This wire needs to be connected to a 12 volt source that shows 12 volts both while the car is cranking and when in the run position (a true 12 volt ignition wire can be found in the main power harness, under the steering column). The last connection is a ground. This allows for the relay to make a complete circuit. Below is a diagram that shows the process.

Keep in mind this is for the Isolator Mike linked: http://cgi.ebay.com/Stinger-SGP32-200-AMP-Current-Relay-Battery-Isolator-F_W0QQitemZ220485366320QQcmdZViewItemQQptZLH_DefaultDomain_0?hash=item3355f3b230

You can also read a basic breakdown on isolator wiring at: http://www.bcae1.com/battiso.htm

Wow, thanks Dan and Mike! I think I have a much better understanding, and can probably figure it out with the link Dan provided (and likely the directions with the iisolator Mike linked to).

Two more questions. I'm assuming the Stinger is a diodide type isolator? Looks like it from Dan's link (didn't see it specified on the Stinger sale site).

Second, how can I tell what the output is on my current Alternator? The motor was put in by the PO, and it's a 200 six from a Mustang (if that helps).

Thanks again to both of you guys!

I think you can take it to Auto Zone or Pep Boys and they can test it for you. If it is stock it is probably no more than 80 amps.

I would follow Mikes Auto Zone suggestion on the testing but if your planning on going to a dual batt sys. I highly recommend an upgrade to at least a 160 or 180 amp. I have a 220 amp. In my jeep for all the assy’s. And plan to put a 180 amp in the van for it’s dual setup.

I have a 200 amp alternator in my 65. It powers all three TVs (one is a 32"), a 1000 watt Arc audio amplifier. a 350 watt amplifier, a microwave oven and a 3500 watt power inverter as well as the DVD player. I do have two Optimas and a 25 farad capacitor too. Will be adding a train horn to it next year as well as a bunch of interior LEDs and under body LEDs and a Wii.

great thread guys!!!

Two questions.

How do I determine how many amps the isolator should be rated at? 200amps with a 300 amp surge vs a 200 with a 600 surge vs a 100 with a 400 surge. what does all that mumbo jumbo mean to me?

Optima is obviously my battery of choice, but should I run a red top as my primary for staring and operation and a yellow top as my secondary battery or do I need to run the same type i.e. two yellow tops?

eventually I plan on running a small compressor and some power tools out of the rear doors, maybe even a small fridge. I just want to keep my options open.

P.S. I'm building a new wiring harness so I might as well do this all now while its torn apart.

You can run two batteries parallel without any isolation almost every diesel pickup built in the last twenty plus years is wired that way
If you do want to isolate the batteries but still use both to start the engine I would use a boat type perko switch and as a bonus you can switch both batteries off if you are going to store your van for an extended length of time