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Thread: How to hook up two Batteries

  1. #1
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    How to hook up two Batteries

    I need to get an extra battery into my plow truck and I know alot of you have installed two batteries into your boats to cover your huge sound systems.

    How do you do it? Do you need a bigger alternator?

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    Connect them in parallel, negative post to negative post, positive post to positive post. This configuration will give you the same voltage and twice the crank capacity
    I do not know what size alternator you have installed but you should be fine
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    How do you intend to use the battery, this will determine how it gets wired.

    As noted above by lwndr, simply connecting them in parallel will give you 2x (in theory, if the 2 batteries are the same) the CCA's. If an ISO/Combiner is used, this will allow the both batteries to be charged by the alt, but isolate them from each other. This way, what even loads get wired to each one, will not draw off the other. Depending on what Iso/Combiner you use, you can also combine them for the starting, then isolate them again from each other's loads.

    Is this a true plow truck, or just an old beater with a plow mounted upfront? If I remember right, trucks like this: wrecker trucks, work trucks, ambulance chassis, etc, used to come with the highest amp alt available at the time, as it was expected to have the additional electrical loads placed on it.
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    if you wire pos to pos and neg to neg, your alternator will not charge both
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  5. #5
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    Quote Originally Posted by JUST-IN-TIME View Post
    if you wire pos to pos and neg to neg, your alternator will not charge both
    Yes it will. They will be seen as one large battery. Power simply flows through the cables. The question is how low will the batteries get, so will he need to upgrade the alternator to handle the extra charging capacity.
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  6. #6
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    no it wont trust me

    once one gets low it draws from the other one

    why do you think boats have 1/2 both switches then
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    Quote Originally Posted by JUST-IN-TIME View Post
    no it wont trust me

    once one gets low it draws from the other one

    why do you think boats have 1/2 both switches then
    Please explain your theory in detail, my boat must be the exception to the rule.
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  8. #8
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    Quote Originally Posted by JUST-IN-TIME View Post
    no it wont trust me

    once one gets low it draws from the other one

    why do you think boats have 1/2 both switches then
    If they are wired in parallel they will discharge at the same rate. If one battery is bad it would pull the other down but otherwise they will be just like one large battery. Most diesel trucks are wired like that from the factory.
    Last edited by ajholt7; 12-22-2010 at 04:55 AM.

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    Quote Originally Posted by JUST-IN-TIME View Post
    if you wire pos to pos and neg to neg, your alternator will not charge both
    Yes, please explain how 2 batteries wired together, in parallel, are charged and discharged separately

    If wired in parallel, battery 2 will not wait until battery 1 is half way down before it starts to drain it to it, which is called "equalizing". They are one large 12V battery and if they are of equal size, type and age, they will always equalize at an even level.

    Based on your statement:
    why do you think boats have 1/2 both switches then
    even these batteries will not get charged EVER by the battery, even though the switch combines then together in parallel when it gets turned to "Both". If that's the case, then what good is the battery and the switch because the battery will be discharged after a few uses, then dead because it's not getting charged.....
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    if they are the same batteries, same MCA and amp hrs, they might drain equally but i doubt it, due to cable and length of cables

    if you guys have them wired in parallel, get a volt meter on them both and look at the voltage draws and charge rates

    its going to be different

    some of your boats might have isolators on them, look at that also

    an ACR is what you want with a 2 way battery switch

    and you never want to run on both batteries, always save 1 to start he boat up

    you should have a starting battery and 1 deep cycle

    might want read up on the ABYC guide lines on electrical, its one step above the USCG regs
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  11. #11
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    Quote Originally Posted by JUST-IN-TIME View Post
    if they are the same batteries, same MCA and amp hrs, they might drain equally but i doubt it, due to cable and length of cables

    if you guys have them wired in parallel, get a volt meter on them both and look at the voltage draws and charge rates

    its going to be different

    some of your boats might have isolators on them, look at that also

    an ACR is what you want with a 2 way battery switch

    and you never want to run on both batteries, always save 1 to start he boat up

    you should have a starting battery and 1 deep cycle

    might want read up on the ABYC guide lines on electrical, its one step above the USCG regs


    This thread is about a plow truck

    2 batteries wired in parallel will equalize in voltage.

    If a system utilizes a diode type isolator, then the batteries are never combined, either in series or parallel, so it has no bearing in the discussion of weather or not 2 paralleled batteries charge the same or equalize, or not.

    "an ACR is what you want" Hmmmm, great idea! Umm, that's what I stated in the post where you told me I was wrong. So, an ACR connects 2 batteries, or battery banks, in PARALLEL, right? If so, then based on your original statement, the alternator will not charge the 2 batteries. If this is true, and its not, then why are you now recommending the very setup you claimed would not work

    If you never want to run the boat, or plow truck in this thread, on both batteries, then why recommend an ACR and a "2 way" switch, which is nothing more then a manual combining switch?

    "you should have a starting battery and 1 deep cycle
    " So, its not good to have 2 different types.....or is it

    Man, im even more confused by your statements now
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  12. #12
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    an ACR, is a switch, it switches to the battery that needs the voltage

    so you can run in parallel and still get the charge while not drawing down 1 battery

    painless wire kits have a kit like this

    i was referring to boats about batteries, so about the confusion, but if somebody searches 2 batts this will come up
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  13. #13
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    Quote Originally Posted by JUST-IN-TIME View Post
    an ACR, is a switch, it switches to the battery that needs the voltage

    so you can run in parallel and still get the charge while not drawing down 1 battery

    painless wire kits have a kit like this

    i was referring to boats about batteries, so about the confusion, but if somebody searches 2 batts this will come up
    Yep, I know what an Auto Combining Relay is.....I use them quite often.

    Sorry to disagree, but if the batteries are paralleled, then they can not be isolated at the same time. A mechanical device, such as an ACR, can not allow a battery to receive a charge, but isolate it from the others at the same time.

    What can do this, is a diode, and a diode type Isolator is not an ACR.

    Got a link or part number to what you use from Painless? A quick search yields no marine products, so i'm guessing they dont have anything that is ABYC or USCG certified, or anything "ACR
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    What is an ACR, and how does it work?

    * What is an ACR? An ACR parallels (combines) batteries during charging, and isolates them when charging has stopped and after battery voltage has fallen. An ACR is intended to keep a load from discharging both of the batteries.
    * How does an ACR work? An ACR senses when the voltage of either of the batteries rises to a level indicating that a charge source is active (13.0V for 2 minutes). The ACR′s contacts then connect and the ACR applies the charge to both batteries. If the voltage on both of the batteries subsequently drops to 12.75V for 30 seconds, the ACR will disconnect, isolating the batteries.
    * Why do I need an ACR? An ACR allows two battery banks to be connected so that they can share the output of a single charge source, allowing the user to charge more battery banks than the number of charging outputs. For example, an ACR can be used with a single-output charger, resulting in a simpler system at lower cost than a dual-output charger.
    * How many ACRs do I need? To combine two battery banks, one ACR is needed; to combine three battery banks, two ACRs are required.
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  15. #15
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    http://bluesea.com/viewresource/1366#What_is_it

    might want to read up on ACR's also
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