continuous duty 100 amp relay to join battery banks 1 and 2

Discussion in 'OnBoard Electronics & Controls' started by sdowney717, Aug 4, 2019.

  1. sdowney717
    Joined: Nov 2010
    Posts: 1,175
    Likes: 85, Points: 58, Legacy Rep: 274
    Location: Newport News VA

    sdowney717 Senior Member

    Some time ago, installed a continuous duty 100 or 150 amp relay to join battery banks 1 and 2
    With 2 engines and 2 alternators, its been working good, the load on alt 1 drops in half and alt 2 picks up the difference when I throw switch on. This is clearly visible on the ammeters connected to alternators 1 and 2. Alternators are 12SI Delco.

    I was told by some it would not work! That they would fight each other, and one would loaf.
    Without the relay, depleted house battery bank 2 works alternator 2 really hard while alternator 1 just charges the starter bank 1 and loafs almost unloaded. The relay is extending the life of alternator 2. One time before I had this relay, alternator 2 belt burned up due to overloading and alternator got pretty hot.

    The other advantage (if battery bank 2 is not depleted), the relay boosts the starting power for the engine starters, as they then have both battery banks 1 and 2 to draw from. I pretty much leave the helm switch on all the time, and you know its working because it makes an audible click when turning on.

    The combining relay's on switch at helm was wired to the fuel pump of engine #1. I have since realized, I could use 2 diodes in the incoming power to the combining relay helm switch, so that either engine 1 or engine 2 could be on and the other engine off and the relay can come on. Right now engine 1 has to be running for combining relay to work, so I will probably make that mod soon. The diodes have to be big enough ones to handle the current for the relay coil. End of each diode would connect to the fuel pump or ignition on of each engine and other ends joined together into power in at helm switch for the relay. You have to get the diode facing the right way, think of it as a one way valve for electricity. Power flows to switch and blocked if was coming from switch, that way turning on one engine ignition wont activate (backfeed) power to other engines ignition circuits.

    When ignition is off, the relay is off, and the banks are not combined.

    And I cant say if it would work with different kinds of alternators.
    Last edited: Aug 4, 2019
  2. sdowney717
    Joined: Nov 2010
    Posts: 1,175
    Likes: 85, Points: 58, Legacy Rep: 274
    Location: Newport News VA

    sdowney717 Senior Member

    Got it done today so that either engine coming on will power the relay.
    Created album of how I did it.
    Power from each engine ignition wires goes into power diodes with white bar mark facing towards item you desire to power up.
    Other ends of both diodes joined as one to power the coil of the relay.
    No power can backfeed, the diodes prevent this. Diodes act like one way valves for DC current.
    Each diode drops the forward voltage by 0.4 vdc, which is meaningless for the relay coil.

    I do think people who have trouble soldering, the metal has to be super clean, if solder wont flow, its either not hot enough, or dirty. I use electrical solder with rosen flux, but if the metal wont take solder, I mechanically scrape it, and will use Oatey water based plumbing flux. Then clean it off afterwards with rubbing alcohol or water. Never failed me yet.

    These diodes are good sized, likely 10 amp capable at least.
    And that wire is over 50 years old, salvaged from another 60's model Eggharbor that was crushed, and like new. they used this wire for the overhead lighting in those boats and a few other things. The only old wires that are failing are the old SO rubber coated, the insulation beginning to crack, and I have one wire run left to replace, goes to an aft bilge pump.
    Diodes I salvaged from a busted 3000 watt inverter, but they are cheap to buy.
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