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  1. #1

    Default 2 Batteries Wiring Diagram

    Where can I find wiring diagram to connect secondary battery but ensure first
    battery charges first for starting engine?
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  2. #2
    VicS is offline Registered User
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    Look at the details of the VSRs and the BEP VSR isolator switch clusters on the Merlin Powerstore website.

    Almost any VSR supplier should have some wiring diagrams on the website

    You could of course do it by manual switching. Just remembering to switch in both when changing batteries when the engine is running.

  3. #3
    shmoo's Avatar
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    Quote Originally Posted by C Salt View Post
    ... ensure first
    battery charges first for starting engine?
    If the batteries are simply connected in parallel under charge the battery that needs the most charging will take the most current, which is what you want I think. We have split charge diodes that were on the boat when we got it, but I have never really been persuaded of the point of them.

    Actually, I don't think it is the cranking battery you want to worry about. Starting the engine takes very little power except on cold mornings. Look after the domestic battery: it works harder and if you have a 1 - both - 2 switch you can always use it to help the cranking battery.

    150A for 30s to start the engine is 1.25Ah from the cranking battery. And 30s is a long time cranking!

    4A for 8h on an overnight sail with modest lights, radio, instruments, occasional pump etc is 32Ah from the domestic battery.
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    VicS is offline Registered User
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    We have split charge diodes that were on the boat when we got it, but I have never really been persuaded of the point of them
    .Well they do enable the two batteries to be automatically charged from the same source without them actually being connected together, and will prevent a duff battery draining a good one. The down side of diodes is the volts drop across them, which means in order to properly charge the batteries you need the alternator to be "battery sensed" rather than "machine sensed". If it were not for the volts drop a diode splitter would probably be close to the ideal system for two batteries.

    You don't really want a 1/2/both/off switch with any sort of split charging system. Better to fit separate isolators to the two batteries with an emergency linking switch between the engine and domestic circuits.

    The American way of thinking, and I believe what Calder suggests in his book, is not to have dedicated starter and domestic batteries but rather to have an all purpose battery, or battery bank, and a reserve battery.

  5. #5
    shmoo's Avatar
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    Quote Originally Posted by VicS View Post
    in order to properly charge the batteries you need the alternator to be "battery sensed" rather than "machine sensed".
    Yup, that's what we have. The charge regulator senses voltage and temperature at the battery terminal. At the domestic (or "all purpose" battery, see below) terminal, that it.

    Quote Originally Posted by VicS View Post
    The American way of thinking, and I believe what Calder suggests in his book, is not to have dedicated starter and domestic batteries but rather to have an all purpose battery, or battery bank, and a reserve battery.

    I think I agree with this, although I have never seen reference to it before. It's how we actually use the batteries, whatever we call them. We almost inevitably start the engine on the 'domestic' battery, apart from very cold mornings, when we use "both" or occasionally to make sure the other one is still healthy.

    Of course, we never charge on "both" since the diodes do their job for us.
    Last edited by shmoo; 07-04-10 at 15:28.
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  6. #6
    VicS is offline Registered User
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    The American way of thinking, and I believe what Calder suggests in his book, is not to have dedicated starter and domestic batteries but rather to have an all purpose battery, or battery bank, and a reserve battery.

    I think I agree with this, although I have never seen reference to it before.
    It has been debated on here.

    The "British" way is to have a dedicated starter battery that can be a compact high CCA battery, not necessarily with a huge Ah capacity, and a high capacity domestic battery that is designed for deep cycling but is not necessarily suitable for regular engine starting loads. More of a "horses for courses" approach.

    The other thing Calder and his followers believe in is not having an isolating switch between the alternator and the battery(ies). That avoids all possibility of frying the alternator diodes by disconnecting the batteries from the alternator while the engine is running.
    The British way of thinking is more along the lines that when the isolators are OFF everything (with the exception of things like auto bilge pumps) should be OFF, and you make darn certain nobody other than yourself is allowed to touch the battery switches.
    Last edited by VicS; 07-04-10 at 15:49.

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    VicS is offline Registered User
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    Here's mine
    If you click on edit above the thumb nail in your P'bucket album you can rotate the image through 90



    Whats the thing bottom right? VSR?

    .

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    Quote Originally Posted by VicS View Post
    Whats the thing bottom right? VSR?
    .
    Unfortunately my eyesight ain't good enough to read the print on this but that is where I think VSR should be in a circuit like this.

    I am about to install a VSR (Smartbank) split charging system also although will be retaining my 1/both/2/off switch initially (let's call it a staged re-wire!). My intention is to eventually move the alternator onto the domestic battery, have a Smartbank to manage all charging sources and change the Big Red Switch to two separate isolator switches. I would have a cranking battery and a service battery. My Smartgauge will allow emergency paralleling if necessary.

    I agree with Shmoo that the cranking battery should seldom present a charging challenge. The domestic will almost always require more filling.

    The American system of one battery (bank) for everything has the disadvantage for me in that there may be problems with instrument power during cranking resulting in a need for rebooting (recent thread describing exactly this). Although I'm sure I've read of clever ways around this.

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    Quote Originally Posted by VicS View Post

    Whats the thing bottom right? VSR?

    .
    Correct Vic it is a VSR. sorry about the quality it's a jpeg from an autocad drg.

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