I was planning on putting some extra batteries (2or3), under the truck to power some accesories (tv, vcr, fan &other stuff) for when I go camping, I want them to be charged by the alternator, with a switch to disconect them when im parked. do I need to install another alternator? Im already running dual batteries up front, any info needed.

-truck is an 88 460efi, my mini earthroamer.

 

Perhaps your could run a power relay from one of the batteries under the hood to one of the spare batteries you want in the back to charge it before you use it.

 

You can use a constant duty solenoid to bridge the battery's (ignition triggered)

As far as a second alternator, I guess it would depend on:

1. How big is your alternator now?
2. How many and what type of battery's will you use?
3. How much do you plan of discharging the battery's?

If you plan on running the battery's down a lot, you might consider a smart solenoid. This has a voltage monitor on it and will only bridge the battery's when there is sufficient voltage available.

 

Use a battery isolator. They are available from the marine parts industry, but at a premium.

They are basically a BIG diode (rectifier), that allows current to flow from the alternator to the secondary (tertiary) batteries, but you can't draw current from the batteries to the truck. Wire anything you want directly to the batteries. With fuses

 

I have seen way to many isolators go bad, That's why I like the solenoid design, if it goes bad you will have no problems with the factory charge system.

 

Ever think about a solar panel to recharge the batteries instead of running your truck, or running it as much anyway? I use a 60 watt panel to recharge a camper battery. Puts out 17.5v/3.5 amps on a clear day. You just hook it up and it keeps the battery topped off. You do have to re-point it towards the sun several times a day but it beats having to run your vehicle or a generator.

 

Isolators should (at least I would) be spec'd to at least twice what the alternator puts out to be safe. And protected from over-current with a circuit breaker.

If done right, they last forever.

 

Isolators are good to keep a dead battery from draining down a good battery. You can also put a solenoid in to comlete isolate the battery when the truck is off. To control the solenoid, the best way is an oil pressure sensor. That way regardless of what position the key is in, the batteries are tied in parallel if the engine is actually running. There is tons of data on this in 4x4 jeep forums, since dual batt's are handy for winches,
Ed

 

You could charge them off the trailer battery charging circuit. I have 4 trojan 6 volt batteries wired for 12v and they get discharged a fair amount when camping, but are topped off by the time I get home. The trailer wiring circuit will protect your alternator by design. Just tap into the wire at the trailer plug.

Eric

 

The trailer wiring circuit is controlled by a plain-jane relay with a 30AMP fuse.

You'll blow that fuse if the batts are really low.

My plans for two extra batts:

1) Mount them under the bed, either on the frame, or under the bed itself.
2) Run a dual isolator rated for 250 AMPs PER batt.
3) Put a 50AMP self-resetting circuit breaker between the isolator and alternator
4) Wire everything up with BIG stuff.
5) Setup connectors to plug stuff in, rated 50 AMP or so, plus run inverter off one of the batts.

This way, the 130AMP (optimistic) alternator can't be pulled to 100% charge by the secondary batts, they will be limited to 50AMPs. Better yet, I have BIG resistors that can current limit to less than 50 AMPs, so I may put one in between the isolator and alternator.

 

I've had my trailer batteries way down, haven't blown it yet. I think the long run of relatively small wire limits the current naturally. I know I can be like 70% discharged (based on battery voltage), and it'll charge it if you drive far enough. The battery pack is 520 amp hours.

I've been planning on doing dual optima's up front. Whenever the stubborn factory battery let's go. 5 years and going strong.

Eric

 

Which is a fire waiting to happen Just kidding

A fully downed marine deep-cycle battery is lucky to hit maybe 10-15 amps draw from my battery charger, so you are probably just under 30 amps

 

Ok, I have a X-Power 1200 watt inverter and an 105AH deep cycle, 4 gauge wire, batt box and frame to attach to rail under bed.
Do you have any specifics on the isolator?
Can ya give more detail on #5 and the part in the last paragraph about the resistors limiting to 50 amps? Kind'a lost on that....
Thx!

 

The isolator I've got looks like this:

http://fords.kilonet.org/krewat/arcoisolator.jpg

It's sold by Arco, who sells rebuilt/new marine alternators/starters/etc. THe one in the pic is only a 70 amp, the one I have is huge, and is rated 250 amps. Since the alternator is only 130 amps, that's a lot of overhead and unlikely to blow the isolator. But I'd still put a 50amp circuit breaker inline before the isolator just for circuit protection.

Hmm... I'd like to mount one of those big 50amp connectors they use on computer UPC batteries, like this:

http://fords.kilonet.org/krewat/powerplug.jpg

right on the back bumper, like the trailer plug.

But that's probably a dream, and I'll just use a big setup of alligator clips to hookup anything.

The batts will be hard wired to the isolator, etc.

As to the resistor, that's just a stupid comment I made, shouldn't have said anything about it.

 

Here's a pic of what I'm going to use. It's a BIG pic.

http://fords.kilonet.org/krewat/arco...tor-240amp.jpg