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Old 01-23-2010, 01:22 PM   #1
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What is the purpose of the 2nd battery in a 5.4?

My 09 V8 5.4 has a battery under the body. What is this for and where does it sit in the general circuit? Does it get charged after the main battery? What is hooked up to it?

I am curious.

Thanks
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Old 01-23-2010, 01:41 PM   #2
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Re: What is the purpose of the 2nd battery in a 5.4?

The aux battery is listed in the 'Trailer/Camper section. When the key is 'ON' the batteries are tied together. At other times, the aux battery is isolated from the main battery by a relay. I guess you can hook it up to anything you want to hook it up to. SMB may (or may not) use it as part of the house battery system.

Mike
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Old 01-23-2010, 04:26 PM   #3
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Re: What is the purpose of the 2nd battery in a 5.4?

Mike,

Thanks for the reply. Is this "Trailer/Camper" section you refer to in the manual?

Paul
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Old 01-23-2010, 06:07 PM   #4
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Re: What is the purpose of the 2nd battery in a 5.4?

The van's accessory circuit will also run off the aux battery, so you can have key-on, engine off, and not drain the start battery (ditto the SMB aux switch that some vans get). It will also provide trailer power when the van is off.

Chances are, your dealer may not have actually connected the aux battery circuit (comes disconnected from factory). When looking under the hood, at the left side just below the windshield cowl there is a 4-way round connector. Typically, this connector has a yellow wire jumper plugged in. If that jumper is in place, you aux batter isn't doing anything, and is not getting charge. To connect he aux battery circuit, you remove the jumper, and plug in the other round connector hanging below, with matching wire colors.
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Old 01-23-2010, 06:23 PM   #5
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Re: What is the purpose of the 2nd battery in a 5.4?

So it's like a factory House battery that is included but disabled by default?
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Old 01-23-2010, 06:37 PM   #6
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Re: What is the purpose of the 2nd battery in a 5.4?

Quote:
Originally Posted by Nomadcat
Mike,

Thanks for the reply. Is this "Trailer/Camper" section you refer to in the manual?

Paul
Sorry, it is in the factory wiring diagrams under that section.

Mike
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Old 01-23-2010, 07:05 PM   #7
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Re: What is the purpose of the 2nd battery in a 5.4?

Thanks again. I love this forum.

One last thing, are the factory manuals available either online or for purchase somewhere?

Thanks.

Paul
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Old 01-23-2010, 10:12 PM   #8
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Re: What is the purpose of the 2nd battery in a 5.4?

Quote:
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So it's like a factory House battery that is included but disabled by default?
Yeah. Only took me 140,000 miles to figure it out too. I connected my inverter directly to the terminals.

I think it's disconnected by default so 2nd stage manufacturers won't leave it connected and short something out because they didn't realize there was still power.
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Old 01-24-2010, 11:01 AM   #9
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Re: What is the purpose of the 2nd battery in a 5.4?

I'm curious. What's the amperage on the van aux circuit battery? And can anyone explain how these relay switches work? I've seen some chatter on the forum about SMB's house battery circuitry connection via a relay to the radio, but I couldn't understand how it can be a liability to the starter battery if the radio is turned off and the relay is not switched back to the SMB house system.

I'm having a battery isolation switch installed to avoid the hassel of removing propane and CO detector fuses in order to prevent SMB house battery drain. Don't know if there's another way to get around this. Can the van's upfitter switches be used to my advantage here (not using them for anything otherwise)?
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Old 01-24-2010, 11:06 AM   #10
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Re: What is the purpose of the 2nd battery in a 5.4?

Quote:
Originally Posted by Nomadcat
Thanks again. I love this forum.

One last thing, are the factory manuals available either online or for purchase somewhere?

Thanks.

Paul
I have an 04 wiring manual for sale on this site. Other years are available on eBay.

Mike
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