forgive the hijack but whats the advantages of having a second battery other than a back up?
Actually it's not a hijack but dead on topic!
Depends on how you have them hooked up. Lots of the diesel trucks, for instance, have two batteries from the factory (I'm intentionally ignoring the CUCV's here -- don't go there.)
They're just hooked up in parallel, plus to plus and minus to minus, and give you more cranking power when it's cold. Both get charged, and used, at the same time and same rate.
In the 'wheeling and stereo worlds, the second battery is generally desired as an *auxiliary* battery, i.e. separate from the main one. Some sort of circuit separates it from the primary battery, so that your big honkin' stereo, or your winch, lights, etc, wear down your auxiliary battery, but leave your primary one charged. The ideas is that you can sit and listen to the stereo for hours, or run the winch, etc, until the auxiliary battery is down. Then, when you turn the key, the primary battery still has juice in it, you crank, and off you go.
There are complicated and expensive solutions, which have been mentioned.
The next cheapest/simple solution is a solid-state isolator, which is basically two big diodes in a heatsink (generally blue, for whatever reason.) The parts stores usually sell these; they're rated by the maximum current and you'd want one rated at least for the maximum output of your alternator. The alternator goes at the center, and a battery goes on each side. You leave the starter, headlights, and other factory stuff on the main battery, and hook your new stuff (winch, offroad lights, whatever) to the secondary battery. This way when the alternator's charging (i.e. the engine is running) both batteries get charged, but the load on each battery can only discharge that battery and doesn't touch the other one.
Hey, look, $35.
http://www.partsamerica.com/ProductDetail.aspx?MfrCode=SRI&MfrPartNumber=9523AD&CategoryCode=3018
I don't think the fourth post is necessary for your alternator, but they come with instructions.
The *really* simple (and inexpensive solution), as mentioned, is a big honkin' continuous duty solenoid. Mine, IIRC, was a replacement part for a John Deere =)) It connects the auxiliary battery to the primary when the ignition is on ... that is, when the engine is running, which means the alternator had best be charging. It disconnects when the key is off, i.e. there's no charge, and so I can run down the auxiliary battery to my heart's content without fear of discharging the primary battery.
These actually came from the factory in the early 70's as an option, I gather, but my parts house only had the JD replacement and not the Chevy that day. Something like this would work:
http://www.napaonline.com/MasterPag...54140&Description=Solenoid+Switch+-+Universal
$28, and notice it says "Continuous Duty" in big letters. While a Ford starter solenoid might work, they are not rated for continuous use.
If you intend to kill your auxiliary battery often, as in discharge it all the way, a so-called "deep-cycle" (aka "marine") battery is preferable. Ryoken will likely step in as this gets complicated, but in general it's hard on lead-acid cells to discharge them all the way. The stereo people do this; I don't know that in our case it's so necessary ... once your battery gets below 10V or so your lights and winch won't work for crap anyway.
Now, speaking of winches, during a good pull you're prolly gonna want to leave your engine running anyway, to get just that more juice into the winch (i.e. by using the alternator), meaning in that instance an isolator isn't really important.
It's the times that your engine won't start 'cuz you're sideways on the trail, or the like, that the isolator would come in handy for winching.
The one thing the big fancy isolators CAN protect you against is running both batteries down WITH the engine running. (That is, presuming they have an "off" position where the alternator doesn't charge the aux battery. You could also do this really simply with the solenoid solution, just by putting a switch inline with its circuit to the ignition.) That is, if you're draining more power than your alternator puts out, you'd kill both batteries even with the engine running. This is most likely the case for the stereo people, though if you have dozens of huge KC lights in use and/or are running your winch at full pull while driving (neat trick!) this could be an issue
Jeebus, that was NOT supposed to be so long-winded ... but again, 1978k5, I think you're thinking too much. For your use, the big solenoid or the solid-state solution would do the trick. With a big alternator, and the big solenoid, some nice #6 or so wiring (should be good for 100A or so) and you oughta be able to run lights, winch, or a small-third world country off your aux battery and call it good.
-- A
) used this. I like the manual over-ride switch part. What ya guys think think? 
