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Unread 06-26-2008, 07:11 AM   #1
4mudder
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Question Dual battery alternative

I was wondering what the pros and cons are of wiring a second battery in parallel (pos to pos and neg to neg) with the main battery. My electrical knowledge is limited but it seems that I would get double the amperage, reserve capacity, etc... with the same voltage. It's a much simpler (and cheaper) setup than a true switchable dual batt system. Any thoughts?
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Unread 06-26-2008, 08:09 AM   #2
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Well, it kinda defeats the purpose when you do it that way, because if one battery goes dead they both go dead.
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Unread 06-26-2008, 09:59 AM   #3
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Most dual battery systems does connect the 2 batteries in parallel, they just have a seperator to isolate one battery from the rest. Typically the isolater is automated to connect when the engine is running, so both batteries gets charged, and then disconnect when the engine is off to isolate.

This way one battery can be used for accesories without fear of the accesories drainign the starting battery. Thsu the concept of main adn second battery. WHere the main runs the main system electrics and starter, and then the secondary abttery is sued to dirve accesories like fridges, etc.

Most of these isolaters have a override switch to double capity or help out with starting if needs be.

In the above setup the main battery is a normal starting battery, but the secodn one is a deep cycle unit to prevent it from dieing if it gets drained coimpletely.


In very cold countries it seems Toyota ships the vehicles with a 2 batteries connected in parallel for extra capacity (no seperator or isolater).
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Unread 06-26-2008, 10:34 AM   #4
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Quote:
Originally Posted by Mikestang View Post
Well, it kinda defeats the purpose when you do it that way, because if one battery goes dead they both go dead.
I would really like a more technical explanation of why if one goes dead the other will too. Let's say one goes bad, gets sulfated whatever, how would it specifically affect the second battery?

BTW, thanks bulldog for a very clear and concise explanation of how the dual battery / isolator circuit works together with my main battery. Now... about that spelling...
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I had that Jeep thing... But I went to my local Toyota dealer, bought a 4Runner and it cleared that crap right up.
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Unread 06-26-2008, 10:47 AM   #5
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Quote:
Originally Posted by 4mudder View Post
Now... about that spelling...
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Unread 06-26-2008, 02:13 PM   #6
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Quote:
Originally Posted by 4mudder View Post
I would really like a more technical explanation of why if one goes dead the other will too. Let's say one goes bad, gets sulfated whatever, how would it specifically affect the second battery?
Without an isolator and the batteries connected directly terminal to terminal, if one battery died it would simply drain the other battery since it's connected directly to it.
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Unread 06-26-2008, 04:04 PM   #7
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Quote:
Originally Posted by Mikestang View Post
Without an isolator and the batteries connected directly terminal to terminal, if one battery died it would simply drain the other battery since it's connected directly to it.
Chris,

In this case, we are talking about "dieing" due to being drained by accessories, not chemical or mechanical failure of the battery.

Accessories or system shorts would just drain two batteries, instead of only one. Murphy's law says this would happen, when you were the furthest from civilization.
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Last edited by Photog; 06-26-2008 at 04:08 PM.
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Unread 06-26-2008, 04:16 PM   #8
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Quote:
Originally Posted by Photog View Post
Chris,

Murphy's law says this would happen, when you were the furthest from civilization.
Alas, I'm very familiar with that law... Point taken, I'll do it the right way the first time.
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I had that Jeep thing... But I went to my local Toyota dealer, bought a 4Runner and it cleared that crap right up.
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Unread 06-26-2008, 06:19 PM   #9
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If two batteries are connected together when the car is not running (car not running) the weak battery will drain the strong battery.

A very simple anology is two buckets of water with a hose connecting them at the bottom. If one of the buckets has less water in it, water will flow from the other until both buckets have the same amount of water.

Connecting two batteries together without an isolator/separator is generally not reccomended for this reason. Your weak, damaged, or dead, battery will kill your good battery. Also if the batteries are not the same as each other this condition can be aggravated.
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Unread 06-27-2008, 12:06 AM   #10
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As stated, the weaker battery will drain the stronger one.

Basically, unless you buy 2 new batteries (preferably from the same batch, close serial numbers are nice) then your new battery + your stock battery will always be different and will drain each other.
Even then, all it does is give you more reserve capacity, an isolator is much better.

In my old car I had 2 new matching blue tops run parallel with each other, with isolators running to the stock battery. Top notch, but pricey
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Unread 06-28-2008, 08:37 AM   #11
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Ok all,
Good analogies and explanations. I appreciate the feedback.
A few hundred bucks for peace of mind in the middle of nowhere is actually priceless...
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'05 4R SR5 V8, SAW 2.5 front, OW/Bilstein 5100 rear, SW Sliders, 4Crawler 1.5" BL, extended rear swaybar links, front swaybar delete, AR wheels, GY Duratracs 33", Yaesu FT-8100r, Rola roof rack, Aux backup lights, Nuvi 255W, Speedohealer.
I had that Jeep thing... But I went to my local Toyota dealer, bought a 4Runner and it cleared that crap right up.
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