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  1. #1
    Join Date
    May 2001
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    433

    Default Two way battery switches

    I have a fairly standard 2 way switch - connects battery 1, both or battery 2. I heed the general advice that if you link two batteries together, the lower-charged one draws from the other.
    My question is how long does this take? I sort of assume that it's instantaneous. However that would mean that every time I switched between battery 1 and 2, the discharge would occur when the switch was - momentarily - on "both". I can understand that the reason for the the switch always connecting to a battery is to prevent damage to the alternator, but is the price I pay the fact that I part-
    discharge one of the batteries.
    Or does the whole process of equalisation take much longer than I believe?

    Brendan

    Brendan

  2. #2
    Guest

    Default Re: Two way battery switches

    You are correct that the lower battery takes power from the other one but it is not instantaneous. I have the same type of switch and have no problem with it. This type of switch allows you to manually select which battery you are charging, instead of a blocking diode.
    What I do is, number 1 is my engine battery and 2 for services. I start my engine on 1 and then I switch to BOTH for charging. If I'm only running my engine to charge I often let it run on both for half an hour or so, then I go to 2 so my domestic batteries get all the charge. When I'm finnished with the engine I switch to 2 for domestic use. Then I don't use any of the power in 1 and can start my engine.
    Hope this helps answer your question


  3. #3
    pvb's Avatar
    pvb is offline Registered User
    Location : UK East Coast
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    Default Throw it away!

    These switches have no place on a boat. They are confusing and offer lots of ways for owner or crew to set them wrongly - potentially allowing the engine starting battery to be discharged (ever tried hand-starting a diesel?). So throw it out and fit a diode blocker instead (with a battery-sensing lead to the alternator, or a smart regulator).

    New boat arrived! Now being commissioned!

  4. #4
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    Sep 2001
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    Default Re: Two way battery switches

    I ahree thats the right way to do it! If you have one of those switches. I dont agree with the other answer, how do you start your engine form the domestic battery in an emergency? I am probably missing something!

    If I agreed with you we'd both be wrong.

  5. #5
    pvb's Avatar
    pvb is offline Registered User
    Location : UK East Coast
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    Default Yes, you\'re missing something...

    If your batteries (or battery banks) are properly isolated, there's no reason for your engine starting battery to fail. If it does fail, and this is highly unlikely, surely it's not beyond the ability of most boat owners to link the wires up to another battery?

    New boat arrived! Now being commissioned!

  6. #6
    Join Date
    Sep 2001
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    18,495

    Default Re: Yes, you\'re missing something...

    In that case I did not miss anything, tried dis-connecting, connecting batteries in a seaway? and why should you, batteries go flat for all kinds of reasons and it will always happen when you least expect/need it, Sods third law I beleive! Switch and splitter pherhaps?

    If I agreed with you we'd both be wrong.

  7. #7
    pvb's Avatar
    pvb is offline Registered User
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    Default ? Pherhaps?





    New boat arrived! Now being commissioned!

  8. #8
    Join Date
    Sep 2001
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    Default Re: ? Pherhaps?

    Perhaps, ferhaps, pherhaps, happen, furcaps, who gives a ****, the message was the same!

    If I agreed with you we'd both be wrong.

  9. #9
    Join Date
    May 2001
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    359

    Default Better Still Keep Switch and Blocking Diodes

    Keep the double battery switch and fit blocking diodes in parallel as well. That way, even if the battery switch is in the off position, by accident while the motor is running you will not internally haemorage and will still be charging both batteries at a slighlty lower voltage as the diodes cause a voltage drop[ 0.6V if my memory is right]. The selection switch then will allow selection of whichever battery [or both] for whathever purpose with the motor stopped and while the motor is running it will allow full charge, over riding the diode voltage drops, to the selected battery [or both], due to the voltage drop function of the diodes. Further the diode will prevent discharge of one battery into the other. A case of the two systems producing more than the sum of the parts.


  10. #10

    Default Re: Two way battery switches

    ROYG gives a very sensible answer. One problem you get when asking a question like this is that there are numerous correct answers/solutions. My system works like royg`s and I have never had a problem. The more complicated you make the system the more there is to go wrong. I am fortunate in having a Yanmar 18hp engine which has decompression levers and is very easy to hand start should the batteries fail (they never have failed)


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