Idea, use relay to only energize starter wires when you start the engines

Discussion in 'OnBoard Electronics & Controls' started by sdowney717, Jul 29, 2021.

  1. sdowney717
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    sdowney717 Senior Member

    Idea being the very large positive starting wires would then only have power when needed. Would lessen the risk of bad shorts occurring when wires are not needed. Why power the wires all the time.
    Course, my boat is 50 yrs old and has never had a short like that, the relay is on the starter, so would have had no benefit for my boat.
    Someone forgetting to turn off the rotary battery switch and working on and around the starter, might help them. Or might help if something happens and you're not around to stop it.

    Most starters already have a starter solenoid that acts as a switch, so relays exist on the shelf, mostly Ford style. Most of those relays ground through their steel base, but you can get ones that dont. I got a bunch here doing nothing.

    One relay could turn off the positive starter buss and its wires. Having multiple engines is not an issue, like backfeeding power when you turn the key to the other engine. If needed, you can put a 10 amp diode inline with each starter solenoid on the solenoid wire for each engine, they act like 1 way valves. Power can only flow one direction, so turning one engine starter key, wont also crank the other engine. It does add some complexity to the system.

    Basically, turn on all the starter buss wires, only when someone turns the engine start key. If there is a short, letting go of the starter key, the short goes away.
     
    Last edited: Jul 29, 2021
  2. gonzo
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    gonzo Senior Member

    All starters already have a solenoid. Do you propose having two solenoids for each starter?
     
  3. philSweet
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    philSweet Senior Member

    They probably need power all the time for the alternator and other stuff. If you reroute that other stuff, you'll just be adding back about as much hot wire as you are disabling. But maybe not - if you already have the alternator wired to something other than the starter lug, like a diode combiner or BSR, then the starter circuit doesn't have to be hot when the engine is running. But at any rate, don't use the key switch. Use a separate toggle switch mounted next to the blower switch.
     
  4. philSweet
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    philSweet Senior Member

    Yes, or maybe, but they function differently. There is a "suck in" coil in the starter that engages the cog and then makes the battery to the starter motor (in the US, you may call this a relay, but not a solenoid). On biggish engines, this coil draws enough amps that it is controlled by a slave solenoid (which you must call a solenoid and never a relay). This is remotely mounted on most installations. But not all engines have one - mine doesn't, the key carries the suck-in coil amps (and I still have an ammeter next to the key switch - real old school setup that is frowned upon these days). So these are about reducing the amps running through the engine control harness. The slave solenoid typically is tied to the key switch run position, not the start position. And it often provides power to necessary accessories like lift pumps. So yes, for what he wants, it would be best to leave the existing system as is (mutatis mutandis) and to add another one that only acts on an isolated run to the start motor. Keep as much of the engine wiring as normal as possible.

    (But I don't know why you would go to such lengths to do this <shrug>)
     
  5. missinginaction
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    missinginaction Senior Member

    sdowney,

    I'd suggest (and I'm doing this with the utmost of respect) that you spend this upcoming winter with a book that really helped me come to a good understanding of marine electrical systems and how they should be designed and installed.

    https://www.amazon.com/Boatowners-M...t=&hvlocphy=9004606&hvtargid=pla-333901114316

    I spent a winter with it years ago. Read it repeatedly. Studied it. Thought about what I read.

    It helped me, immeasurably.

    MIA
     
  6. sdowney717
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    sdowney717 Senior Member

    No, just a single solenoid (relay) right at the starter bank positive battery post. Keep all downstream switches and starter wires dead, no power. No power no chance of causing a short.
     
  7. sdowney717
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    sdowney717 Senior Member

    Not if it is a pure starter only bank. The alternator that charges the starter bank could be fused with an MRBF terminal fuse on the battery positive post. All associated big starter wires remain unfused.
     
    Last edited: Jul 29, 2021
  8. sdowney717
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    sdowney717 Senior Member

    yes, may not be practical. Idea here is the very large starter wires would not need any fusing, but short protection would be much greater seeing they will only be powered when you're actually cranking over the engine, all other times, those wires are dead, having no power. No power, no potential to short.
    And since they are only powered when you turn the key, your right there to notice a shorted starter wire.
    And when you let go of key in crank position, if it was shorting, the short would instantly stop, (unless the solenoid-relay stuck closed). But that is not too likely, i have never seen a starter solenoid fail closed.


    a shorted starter wire or starter itself, when you turned the key, of course the engine wont crank. You will instantly understand something is wrong, a problem with starter, wires, connections, or the battery. And you will naturally investigate the cause.

    a short term dead short on a battery starting wire is not going to instantly melt the large wire cable, but it will get warm to hot. Whereas a short that does not stop may cause a fire. And could happen when no one is around.
     
  9. sdowney717
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    sdowney717 Senior Member

    while a standard switch would work, you have to remember to turn it off to deenergize all the starter wires. Whereas using the ignition key on at crank, it releases the power every time you let go of the key switch, so more fail safe and simple and natural, nothing extra to remember to do.
     
  10. sdowney717
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    sdowney717 Senior Member

    here is the basic schematic idea. The 2 diodes prevent the ignition key(s) from cranking both engines when either key is turned to crank. Otherwise either key would crank both engines at same time. Red lines are the large starter cables. Diodes act like one way valves, power can not back feed to the other side.
    this uses one starter bank for 2 engines with 2 ignition switches.
    The diodes are cheap and reliable , only size for amps drawn by the relay coil, plus some more for safety margin. The relay coil will draw little power to energize its coil.

    The relay takes the place of a disconnect battery switch for the starter bank.
    For emergency starting, run a line from the house bank rotary switch over to the starter wire lug on the relay. In my own boat, I use position 2, an unused position, so if I put house rotary switch on 'both, 1+2', power is sent to the start bus.
    starter-idea.PNG
     
    Last edited: Jul 29, 2021
  11. philSweet
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    philSweet Senior Member

    Well I sure have. I shut down six lanes of traffic at rush hour towing a 6 ton RV. Stalled the truck on a hill. Starter relay bolted when I tried to restart the engine. By the time I got the hood up and ripped the battery lead off (six lanes of rushhour traffic remember), I had a nice little fire going under the hood. Cops showed up and blocked traffic at bottom of hill. With a dead engine, I rolled backwards two blocks - no engine, no brakes, no power steering, no trailer brakes - backed onto a side street and then into a parking lot.
     
  12. sdowney717
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    sdowney717 Senior Member

    Interesting. Stuff happens. Engine just madly cranking by itself.
    Very tough to back up that far.

    Even though if the added in relay failed closed on the schematic, engines wont crank as their is not a key on event to turn on the starter solenoids at the starters, so adding in the center relay at the battery hurts nothing. With the schematic wiring, you would have to have 2 relays simultaneously failing closed. Means the schematic adds another level of safety against a starter solenoid failing like that, about impossible for what happened to your truck then to happen on your boat. Another plus for this idea.

    This idea will work on any truck as well as a boat for the starter wires.

    I have had a Bendix drive gear get stuck in the flywheel on a Chevy 350. Spinning the starter at 20,000 rpm, the armature threw off all its copper segments on the commutator.

    I was thinking if just the solenoid flat copper washer disc inside failed closed, stuck, with the schematic wiring of this extra relay, the starter will still function normally.
    The bendix drive gear is still pushed back the big spring.
    The relay at the battery will function then as the solenoid relay.
     

  13. sdowney717
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    sdowney717 Senior Member

    Just thinking about this again.
    If a dead short existed in the starter wire, with a starting relay in series. I think the voltage drop from the short would open the starting relay over and over, like when you try to start a car with a dead battery and you hear that click, click, click sound. At least for a typical flooded lead acid battery, not a lithium battery bank which can dump over 20,000 amps thru wires for a very short time.
    Put that kind of electrical pressure on a battery and the volts will drop a lot during the key start turn event short. The relay is sensitive to low voltage, likely will drop out, go open circuit. That should be easy to test with a battery, key switch, some wires and a start solenoid relay. Even test with an old car that uses a remote start solenoid (Fords), short the starter wire and turn key see what happens. This would be another good reason to use an inline series starter solenoid relay at the start battery.
     
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