Steps on How to Run 2 Batteries Off of 1 Alternator?
Originally Posted by KandyZ
I was hoping that someone could walk me in the steps of hooking up 2 batteries off of 1 Alternator. Thanks! 
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Joined: Oct 2006
Posts: 1,003
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From: Champaign/Chicagoland
First off...why do you want another battery?
DEFINITELY use a battery isolator instead of wiring the batteries in parallel - if you really want them in parallel as one load though for whatever reason, buy 2 new batteries that are identical to one another in every way possible.
--Either way though, if you want plan on killing the battery a lot with the car off and that's your reasoning, go with a good deep cycle battery and skip the second battery (or even if you do get the second battery, the reason you are doing so I would assume is to run it down so you could still benefit from a deep cycle batt.).
DEFINITELY use a battery isolator instead of wiring the batteries in parallel - if you really want them in parallel as one load though for whatever reason, buy 2 new batteries that are identical to one another in every way possible.
--Either way though, if you want plan on killing the battery a lot with the car off and that's your reasoning, go with a good deep cycle battery and skip the second battery (or even if you do get the second battery, the reason you are doing so I would assume is to run it down so you could still benefit from a deep cycle batt.).
I run 2 batteries without an isolater like ww mentioned, but I also got 2 brand new identical batteries. Also, depends on your climate and how often you drive the car. You want to avoid having the 2 batteries at different temperature extremes for too long, such as one under the hood in hot southern climate, while the 2nd one in the hatch with the AC on. Basically, the internal resistance of the batteries will change over time as the battery ages, and you want these values to be as close as possible between the 2 batteries. So if one battery is always hot, and the other always cool, the hot one will degrade faster. Since my car is primarily a show car and driven only one long trips a few times a year, it's no big deal.
The guy who I bought my truck from added another battery, and they weren't the same brand either. He hooked the positive of the first battery to the positive of the second battery. Don't know why he did that, but the second battery will be coming out soon.
Last edited by cooltc2004; Apr 2, 2007 at 06:17 PM.
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The isolator helps by keeping the car battery from being drained when the car is off. So you can start your car. The battery in the back should also be ina safe storage container with it vented to the outside.
The reason for using 2 of the same battery's is to keep the resistance differences to a minimum to help with equal charging.
http://www.bcae1.com/battiso.htm
The reason for using 2 of the same battery's is to keep the resistance differences to a minimum to help with equal charging.
http://www.bcae1.com/battiso.htm
Last edited by Richiec77; Apr 3, 2007 at 05:17 AM.
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Brett Foote
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Joined: Dec 2004
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From: Zanesfield, OH
Actually if the two batteries are of different type you run the risk of the batteries discharging each other over time, due to each battery having different properties.
The isolator doesn't allow each battery to "see" each other so the risk of discharging wont happen.
I use to compete on the highest level of spl competion and at one time had 7 batteries all ran parrallel. This was the best for what I needed to accomplish.
I think you could get better information if we knew a little more about what you are trying to accomplish.
The isolator doesn't allow each battery to "see" each other so the risk of discharging wont happen.
I use to compete on the highest level of spl competion and at one time had 7 batteries all ran parrallel. This was the best for what I needed to accomplish.
I think you could get better information if we knew a little more about what you are trying to accomplish.
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From: Zanesfield, OH
Originally Posted by fredmr39
The worst thing that can happen if you run different batteries in parallel without an isolator is....fire 
Regardless of how often this happens, I wouldn't take the chance.
Regardless of how often this happens, I wouldn't take the chance.
For sure!!! Just not worth the chance.
Originally Posted by fredmr39
The worst thing that can happen if you run different batteries in parallel without an isolator is....fire
Regardless of how often this happens, I wouldn't take the chance.
Regardless of how often this happens, I wouldn't take the chance.
15 years, I've never seen one catch on fire using two new identical batteries and a fuse between them. TECH Fanatic
Joined: Oct 2006
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From: Champaign/Chicagoland
Originally Posted by wickedwarlock
false, that's what the fuse between the batteries is for. 15 years, I've never seen one catch on fire using two new identical batteries and a fuse between them.
15 years, I've never seen one catch on fire using two new identical batteries and a fuse between them.
Originally Posted by fredmr39
maybe...but that doesn't stop any current from flowing back into the cars electrical system when the motor is shut off and both batteries can still discharge one another. I'm not saying you wont be fine your way.....I'm just trying to defend myself at this point.
The only issue you will have is running two different batteries in this case or one new battery and an old one. That's the only way they will ever try to cancel each other out.
The fuse is to prevent a spike or surge.
The only reason for an isolater is to keep the electrical system seperate.
I have seen it for years and never has ever been an issue with a proper setup. BTW, an isolater wil drop you up to .5 volts. So those looking for a dual setup for a car audio should consider not using one.
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From: Zanesfield, OH
Originally Posted by wickedwarlock
defend yourself from what?
The only issue you will have is running two different batteries in this case or one new battery and an old one. That's the only way they will ever try to cancel each other out.
The fuse is to prevent a spike or surge.
The only reason for an isolater is to keep the electrical system seperate.
I have seen it for years and never has ever been an issue with a proper setup. BTW, an isolater wil drop you up to .5 volts. So those looking for a dual setup for a car audio should consider not using one.
The only issue you will have is running two different batteries in this case or one new battery and an old one. That's the only way they will ever try to cancel each other out.
The fuse is to prevent a spike or surge.
The only reason for an isolater is to keep the electrical system seperate.
I have seen it for years and never has ever been an issue with a proper setup. BTW, an isolater wil drop you up to .5 volts. So those looking for a dual setup for a car audio should consider not using one.
In my competition car I didn't even run fuses, so I didn't kill a run due to a fuse that went bad. This isn't recommended but if you plan your system right some things you just dont need. Of course it helps when your sponsored and you dont care if you burn up an amp or 16 amps
Teching In
Joined: Sep 2003
Posts: 40
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From: Utah
Orion MBR 70 multiple battery isolator I use an Orion MBR 70 multiple battery isolator with a hot wire (4 gauge) running from the front battery with a fuse directly after the battery connection to the back of the car. I connect this to another fuse (to protect the isolator in case of wire chaffing) then run the hot wire directly to the Orion MBR 70. I added another 4 gauge wire (for the grond) and grounded it to the rear of the car (needs to be a good clean ground area). The MBR 70 is electronic and completely separates the front and rear batteries and monitors the votlages of both batteries. The MBR makes sure the front battery (for the car) is charged first then charges the rear battery but can do simultaneously depending on how low the batteries are...
I use two batteries as I do not want to drain the car battery when I am out on the beach, in a park or wherever as the battery in the rear is used and only drains when the ignition is off. Thus the main battery is always ready to start the car at any time... I also use a Jacobs electronics 1500 accuvolt that ensures my amps are getting a continuous 13 to 16 volts (depending where I set it at) as my amps draw a great deal of power... If you compete or have a very good ear and want clear crisp sound, the amps require coninuous power without power drops during high bass sections of a song (keeps you SPL and sound wave where they need to be)...
I use two batteries as I do not want to drain the car battery when I am out on the beach, in a park or wherever as the battery in the rear is used and only drains when the ignition is off. Thus the main battery is always ready to start the car at any time... I also use a Jacobs electronics 1500 accuvolt that ensures my amps are getting a continuous 13 to 16 volts (depending where I set it at) as my amps draw a great deal of power... If you compete or have a very good ear and want clear crisp sound, the amps require coninuous power without power drops during high bass sections of a song (keeps you SPL and sound wave where they need to be)...
Just another view from experience. . my 97 tahoe that used to be a demo vehicle for rockford fosgate had two yellow tops, 3 fosgate batteries, a 250 amp alternator, and a piggy back 300 amp alternator, went through, more isolators than I care to remember. Took it off, no problems after that. that was back in 98, and she's still going strong. The fosgate batteries started crapping out a few years ago, but they did pretty good considering the abuse they were under. I replaced them with two optimas. personally I have a slight disdain for isolators, although I do believe in fusing each battery. My 94 chevy truck has always had two optimas, and my current 01 firebird has two optimas as well, all with no problems, and no isolator.
