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I must have miss stated what I am going to do. I am not wireing anything.The spare battery will just be sitting in one area the jumper cables will be sitting in another area both under the hood. NO connection. If the battery hooked up to the car goes dead I just grab the jumper cable under the hood and jump the spare battery to the dead battery.
Marine grade battery switch? The ones I saw just switch from one battery to the other and the heavy duty switch's (that can handle up to 350 Amps)cost around $25-$30.00. What does handle up to 350 Amps mean? If the battery is 900 Amps will it blow the switch? I am confused. How will it charge the spare battery?
I guess I didn't read thoroughly like I should have... my bad...
A battery that is 900A means it has 900 Cold Cranking Amps available, but it doesn't mean it ever puts out the full current, (unless you drop a wrench across the terminals). The starter only draws a couple hundred... if the switch is rated for marine use, most marine inboards by Mercruiser are just Chevy engines, and the starters are conventional automotove starters, so it shouldn't be an issue. A regular starter draws all of a couple hundred amps at most.
My point was that if you're going to have two batteries (under the hood or wherever) why not use the vehicle's charging system periodically top off the charge? Plus, if you ever need it, the aux is just the flip of a switch away... no need for fancy cables or sparks (and no chance of inadvertently crossing the cables either). A switch would allow you to leave it set to the primary battery 99.99% of the time. Occasionally (maybe once a month, or every other month) you'd want to flip the switch to the secondary battery and use it... start with it and let it charge with the engine running. Take it for a drive, and when you park it, flip the switch back to the primary battery, leaving the backup battery stored and fully charged in case you need it.
A cheap one like this will run about $45 and is good for 300A of current continuous, 1500A intermittent.
A more expensive one rated for 450 continuous, 1200 intermittent will run about $150
I used this one in my boat for years, but it was a 150 Hp outboard... the starter was a bit smaller than an automotive starter, but the switch was rated for 250A continuous, 360A intermittent. and only ran about $35
If you've got a winch on your truck, you'll want to watch the current draw on the winch and size it accordingly if you want the winch to draw through the switch. However, in that case, I'd probably wire the winch up to draw right from the battery anyways.
My other suggestion is that if you're going to pay the money for a heavy-duty switch, you're not far from the cost of a battery isolator.
For $210, your alternator will ALWAYS charge both batteries, but your starting system will only draw off the one for starting purposes. You'd still need a set of jumper cables or a switch to put the batteries in parallel in the event the primary battery dies, but that's not such a big deal.
I know Cole-Hersee makes a good isolator too, but IIRC, it's somewhere around $300....
-Joe
