Adding aux battery in the back - with a twist - Hot Rod Forum : Hotrodders Bulletin Board
Hotrodders.com -- Hot Rod Forum



Register FAQ Search Today's Posts Unanswered Posts Auto Escrow Insurance Auto Loans
Hot Rod Forum : Hotrodders Bulletin Board > Tech Help> Electrical
User Name
Password
lost password?   |   register now

Reply
 
LinkBack Thread Tools
  #1 (permalink)  
Old 12-05-2010, 09:50 PM
curtis73's Avatar
Hates: Liver. Loves: Diesel
 
Last wiki edit: How to find cheap parts
Last journal entry: 1999-2001: Getting it on the road
Last photo:
Join Date: Dec 2005
Location: Carlisle, PA
Age: 41
Posts: 5,153
Wiki Edits: 16

Thanks: 2
Thanked 5 Times in 5 Posts
Adding aux battery in the back - with a twist

So I was thinking the other day. I know, terrible idea. Most vehicles/trucks with a 7-pole trailer wiring harness include a 10ga terminal for 12v+. In my world of towing mostly RVs, that terminal was used by the RV for charging coach batteries, courtesy lights, and other 12v requirements in the trailer.

So, I'm planning on adding a small-ish battery in the cap of my truck since I use it for camping occasionally, but something concerns me. If a battery is capable of dumping 500+ amps all at once for starting, and an alternator can provide up to 100 amps like in my truck, doesn't that mean there is the potential for 100A to be available for "flow" to the secondary battery once you start the engine? How can that happen without frying such relatively small wire?

I want to tie in my secondary battery with the factory rear end harness since the truck has an isolation solenoid as part of the factory setup, but really just a 10ga wire is ok for that?

    Advertisement
Reply With Quote Quick reply to this message
Sponsored Links
Advertisement
 
  #2 (permalink)  
Old 12-06-2010, 05:25 AM
Registered User
 
Join Date: Sep 2010
Location: illinois
Posts: 50
Wiki Edits: 0

Thanks: 0
Thanked 4 Times in 4 Posts
hi,
you say the truck has an isolation solenoid. how is that solenoid triggered? it may not be connected to an ignition circuit, possibly an accessory circuit? an accessory circuit won't have power during cranking. plus it will probably have circuit protection of some type to protect the wire. what is year, make and model of the truck?
mark
Reply With Quote Quick reply to this message
  #3 (permalink)  
Old 12-06-2010, 06:42 AM
cobalt327's Avatar
WFO
 
Last wiki edit: Intake manifold
Last journal entry: 1980 Malibu Wagon
Last photo:
Join Date: Mar 2009
Location: Atlanta
Age: 59
Posts: 5,037
Wiki Edits: 1616

Thanks: 128
Thanked 597 Times in 546 Posts
Quote:
Originally Posted by curtis73
If a battery is capable of dumping 500+ amps all at once for starting, and an alternator can provide up to 100 amps like in my truck, doesn't that mean there is the potential for 100A to be available for "flow" to the secondary battery once you start the engine? How can that happen without frying such relatively small wire?
While there is the "potential" for a large current flow to occur, the battery would need to be basically dead to cause the alternator to hit it w/that much current. If the auxiliary battery was being used in conjunction w/the other ('main') battery, I believe it would "buffer" the alternator output to it.

In any event, if there's not already a fuse to protect the circuit, I'd wire one in myself.
Reply With Quote Quick reply to this message
  #4 (permalink)  
Old 12-06-2010, 07:05 AM
bubbahotep's Avatar
Registered User
 
Last photo:
Join Date: Aug 2008
Location: NYC
Age: 42
Posts: 487
Wiki Edits: 0

Thanks: 1
Thanked 1 Time in 1 Post
In my work car I have an extra batt for accessories when the car is off (fan, chargers, DVD player etc). It's connected to the alt with thinner than 12ga (unrated cheap extension cord wire). I dead that battery 3 times a month entirely and the wire still hasn't fried. It'll pull only as many amps as the batt needs to charge and unless I overload it with accessories I'm not sucking more than a few amps

charging amps is low (30amp??) to the battery which is why it's a small wire from the alt, cranking amp is high, as evident from the fatty wire from battery to starter.
Reply With Quote Quick reply to this message
  #5 (permalink)  
Old 12-06-2010, 07:49 AM
curtis73's Avatar
Hates: Liver. Loves: Diesel
 
Last wiki edit: How to find cheap parts
Last journal entry: 1999-2001: Getting it on the road
Last photo:
Join Date: Dec 2005
Location: Carlisle, PA
Age: 41
Posts: 5,153
Wiki Edits: 16

Thanks: 2
Thanked 5 Times in 5 Posts
Quote:
Originally Posted by ween
hi,
you say the truck has an isolation solenoid. how is that solenoid triggered? it may not be connected to an ignition circuit, possibly an accessory circuit? an accessory circuit won't have power during cranking. plus it will probably have circuit protection of some type to protect the wire. what is year, make and model of the truck?
mark
98 F150 4.6L
I haven't actually figured it out yet but I'm going to print out some wiring diagrams from Alldata today. What prompted me to check was this: I was towing a little travel trailer with it a few weeks ago and ran the camper's heat all night. When I got up in the morning, the coach batteries were dead. I worried thinking the truck would be dead too but it cranked normally. That prompted me to check, and the B+ pole on the connector only gets juice when the key is on (not sure if its ignition or accessory but I'll check)
Reply With Quote Quick reply to this message
  #6 (permalink)  
Old 12-06-2010, 03:15 PM
curtis73's Avatar
Hates: Liver. Loves: Diesel
 
Last wiki edit: How to find cheap parts
Last journal entry: 1999-2001: Getting it on the road
Last photo:
Join Date: Dec 2005
Location: Carlisle, PA
Age: 41
Posts: 5,153
Wiki Edits: 16

Thanks: 2
Thanked 5 Times in 5 Posts
Ok, I looked at the wiring diagram (the appropriate page is attached below)

It is a relay that is hot during run. I also have the locations of all the necessary components so I'm going to see what the connections look like, the wire gauge to and from, etc. It looks like it might be a plain-jane 20A relay and 10-ga wire according to the research I've done. What I might do is upgrade the wire to 8-ga and stab in a 30A relay.

... not sure why... other than the fact that I like overkill.
Attached Thumbnails
Click image for larger version

Name:	f1502-3.gif
Views:	94
Size:	78.5 KB
ID:	50636  
Reply With Quote Quick reply to this message
  #7 (permalink)  
Old 12-07-2010, 01:07 PM
ogre's Avatar
the 'Duracell Project'
 
Last photo:
Join Date: Nov 2009
Location: Motown
Posts: 1,432
Wiki Edits: 0

Thanks: 0
Thanked 117 Times in 102 Posts
your thought proccess is correct, the math does not work, but...

your charge wire off the alternator is 10 ga.
all after market harnesses use a 10 ga wire to the battery with no protection (fuse)
and you are correct; that wire is use for charging a big battery in the tow along
just use it as is.

if you killed the main battery and had a fully charged aux battery you would blow the fuse if you tried to start it.
if you keyed on for a bit, you might be able to flow charge the main battery back

that same wire in my 09 sierra is always hot. no isolation relay.
the factory fuse is (a very expensive) 30 amp slow blow fuse
killing an rv battery would also kill my main battery
i use that wire to power a scooter lift and do not need the key on

where is doc vet when you need him... rip doc
Reply With Quote Quick reply to this message
  #8 (permalink)  
Old 12-07-2010, 05:41 PM
SSedan64's Avatar
Registered User
 
Last photo:
Join Date: Dec 2007
Location: Macon, GA.
Age: 50
Posts: 6,016
Wiki Edits: 0

Thanks: 0
Thanked 46 Times in 46 Posts
I installed 8 gauge wire with a 40amp self resetting Circuit Breaker on the Aux. on mine.
The Brake + called for 30amp breaker w/10 gauge.
Reply With Quote Quick reply to this message
  #9 (permalink)  
Old 12-08-2010, 11:11 PM
curtis73's Avatar
Hates: Liver. Loves: Diesel
 
Last wiki edit: How to find cheap parts
Last journal entry: 1999-2001: Getting it on the road
Last photo:
Join Date: Dec 2005
Location: Carlisle, PA
Age: 41
Posts: 5,153
Wiki Edits: 16

Thanks: 2
Thanked 5 Times in 5 Posts
I'm going to pose a theory from my limited grasp of the subject....

Could it be that batteries with their potential for large amperage out are because the load has no voltage?

I'm thinking of it this way. A battery has 13v of "pressure" out to the load, so between the battery and starter you need a cable of sufficient cross-section to carry the amperage. But, going the other way, the alternator is only providing slightly more voltage than the battery, therefore reducing the load.

The voltage differential between the battery and the starter is 13v to 0v. Recharging even a dead battery, the potential between the alternator and the battery is from 14v to 10v thereby reducing the amount of potential amperage that actually charges the battery.

In this way (by my amateur theory), its like draining a large water tank from the bottom... its capable of venting large volume and pressure, but then if you try to feed water back through the drain, it doesn't refill nearly as fast as it drained because you are fighting the weight of the water (the voltage of the battery.)

So, while a 100A alternator can provide 100 amps at peak, charging a battery would only require a small portion of the electrical load since its fighting the voltage "pressure" of what's left in the battery.

Does that sound plausible, or am I just pulling stuff out of my anus?
Reply With Quote Quick reply to this message

Recent Electrical posts with photos

Quick Reply
Message:
Options

Register Now

In order to be able to post messages on the Hot Rod Forum : Hotrodders Bulletin Board forums, you must first register.
Please enter your desired user name (usually not your first and last name), your email address and other required details in the form below.
User Name:
If you do not want to register, fill this field only and the name will be used as user name for your post.
Password
Please enter a password for your user account. Note that passwords are case-sensitive.
Password:
Confirm Password:
Email Address
Please enter a valid email address for yourself.
Email Address:

Log-in

Human Verification

In order to verify that you are a human and not a spam bot, please enter the answer into the following box below based on the instructions contained in the graphic.




Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)
 
Thread Tools

Posting Rules
You may post new threads
You may post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
Need help please with an electrical headake bowtieorbust Electrical 15 01-26-2006 03:01 PM
Starter keeps starting and battery exploded Huug Electrical 14 10-06-2005 05:48 PM
Removing fenders? Jag Daddy Body - Exterior 3 11-26-2003 04:42 AM
body panel alignment gmonkey Body - Exterior 5 09-01-2003 06:50 PM


All times are GMT -6. The time now is 04:43 AM.


Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2014, vBulletin Solutions, Inc.
Search Engine Optimization by vBSEO 3.6.0 PL2
Copyright Hotrodders.com 1999 - 2012. All Rights Reserved.