Another electrical engineering question

Dune Posted By Dune, Jul 14, 2012 at 10:22 PM

  1. Dune

    Dune
    Minister of Fire 2.
    NULL
    

    Jan 14, 2008
    3,128
    273
    Loc:
    Commonwealth Of Massachussetts
    Back story;
    I just bought this boat. It is in Nova Scotia and I am getting it ready to sail it back to Cape Cod.
    It had an obsolete electrical system and I am re-wiring the entire vessel before I cross the Bay Of Fundy.

    Most of the electrical needs can be met with 12 volts, except the engine starter motor. The 855 cubic inch diesel requires a 24 volt starter. I had the 12 volt alternator rebuilt and bought a new 24 volt alternater to charge the starting batteries.

    My question is, instead of 2 battery banks, one 12 volt bank consisting of (2) 8 d batteries in parallel. and one 24 volt bank consisting of 2-8ds in series, should I return the 24 volt alternator, replace it with a 12, run 2 12 volt banks and then combine them to 24 volt, just for starting? This would give me a spare battey bank for electronics, instead of 24 useless volts just sitting there most of the time, plus alternator redundancy.

    My only issue is my lack of understanding of battery banks. When I tie the two packs together to achieve 24 volts for the starter, what prevents all the circuits from suddenly becoming 24 volts?

    Would I need a contacter or soliniod to make this work? Diodes?
    I am using swithes like this, http://www.jmsonline.net/hd-selector-switch-350a-cont.htm?gclid=CLuflpzPmrECFUlN4Aod5XpYdA and should be able to use this switch to join the two banks
    for 24 volts.

    I know the first way I designed it will work, but with a lot less options.
    Any input is greatly apreciated. Thanks
     
  2. basod

    basod
    Minister of Fire 2.
    NULL
    

    Sep 11, 2009
    937
    351
    Loc:
    Mount Cheaha Alabama
    You can do this. the only down side is one battery will be loaded and the other will be getting "over-charged" sligthly during the cycle.
    It can and proabably will over time shorten the battery life of the one with no load or if you plan. I would connect the load to the second battery in the charging circuit( the one going to ground)
    The same will be true if you have unbalanced 12V load on the 2 batteries.

    The question about what prevents everything from becoming 24v is basically the circuit potential, any load between 2 posts on the battery will be 12V, the alternator has 24v between its +/-.(first battery + and second battery -)
    You can't ground the 12V load on any frame it will have to be grounded directly to the battery(-) otherwise you will have 24V going from the load to ground(24V potential) vs 12V to the negative post.
     
  3. greg13

    greg13
    Minister of Fire 2.
    NULL
    

    Jan 5, 2012
    587
    124
    Loc:
    CNY
    Older trucks used a series/parallel switch. 12v to the running circuit & 24v to the start. I would check with some of the Alternator/starter rebuild shops and see if they can find a 12v gear reduction starter.
     
  4. Dune

    Dune
    Minister of Fire 2.
    NULL
    

    Jan 14, 2008
    3,128
    273
    Loc:
    Commonwealth Of Massachussetts
    Thanks. Good, helpful answer.
     
  5. Dune

    Dune
    Minister of Fire 2.
    NULL
    

    Jan 14, 2008
    3,128
    273
    Loc:
    Commonwealth Of Massachussetts
    Thanks, I would have liked to use a parallel swith. I am going with the 24 and 12 volt, and will run some lights on 24. I figure I will hook up all the 8ds to each other in parallel with switches, In case I loose either alternator at sea.
     

Share This Page