Main Engines Starting Draw

I have a twin gas engines configuration and have recently noticed that while attempting to crank the second engine, the first engine cuts out. This is while having all 4 batteries on. Is the draw of cranking the second engine enough to kill the first engine stealing too much voltage or could it be a short in the electrical system?

When it cranks the second engine, is it laboring hard to turn over?
When I set up my twin engines, I have a separate battery just for starting.
So I have 3 batteries, two for house, one for starting both engines.
I mean why not? One battery can start an engine, then start the other engine.

And exists 2 battery switches, so I can combine or remove batteries for starting or house use. An obvious advantage of a distinct starting battery, is you can not run it down by accident. If somehow I drained the house batteries, then I can still start the engines.

There is no apparent laboring while cranking the second engine. It spins just fine but if it takes more than a few seconds cranking, the other engines dies, I assume from low voltage. I have a cranking battery for each engine and two house batteries wired in series & 3 battery switches. I sometimes open up all switches so that all 4 batteries are cranking the engines.

Is this happening regardless of which engine is started first?

Is your cranking battery also running the engines?
I do believe my house battery is running the engines.

If you have 2 cranking batteries, perhaps each one should start and run it's own engine, or rewire so the running engine takes power for spark from the house battery.
I found it was quite simple to just use one battery for starting and the house batteries to do everything else.

I also have a generator and those switches, I can combine or separate all the batteries.
When an engine cranks, voltage can drop low. Some starter solenoids bypass the coil resistor to give more energy to the coil on the engine being cranked. There would be an extra wire on the starter to do that.

Needing separate batteries or electrical circuits as well as alternators for each engine? Believe every engine deserves its own alternator.

Yes, this appears to happen whether the port or starboard engine is started first. And yes, the cranking batteries are running the engines, although if all 3 switches are in "all" position, the engines are running and/or cranking from the starting & house batteries. I must say I have never noticed this phenom during the 5 years I have run the boat, I just started to notice this over this past weekend.

Are your engines fuel injected, Jim?

Your probably dropping the voltage below what is called for on the fuel solenoid of the running engine. Just a guess. Voltage drops and fuel is shut off.

I have carbs...I'm old fashioned! Simple

the drop in voltage to the running fuel pump sounds realistic, I just don't know why it would have just now become apparent????

Ok, someone correct me if I'm wrong. If both engines have their own alternators, then starting one shouldn't affect the other. With a good alternator, you can actually disconnect the battery once it's running, the alt will put out enough juice to keep it running. Perhaps the second starting is actually stealing fuel from the first. Might be as simple as clogged fuel filters.

Fuel is definitely not the issue. Each engine is running off its' own tank and filters. Alternator on running engine is definitely producing output as volt meter registers around 14-15 volts when not cranking the other engine. I can't say what the volt meter reads while cranking the other engine, I didn't check at the time.

Man, that's quite a puzzle.



I'd throw those switches to isolate the running engine's ignition power battery from anything connected to the second engine then try it.

?
I hope after you find it you post what did it. Weird.

I had an idea- I would inspect behind the ign sw panel and use my voltmeter to check switch functions/connections/wiring and supply voltage and grounds, whatever else that's involved, like relays or whatnot. See if their locking rings are loose or if anything else looks odd.
Just to eliminate it. I'm sure boats use different switch set ups, but in cars a faulty ignition switch can have you pulling out your feathers.

Starter drops volts in the batt bank shared by the engines. If a new symptom, batts may be getting old and weak, that causes volts to drop further. Always dicey having two engines share a start bank, but can be done with decent engineering.

If you have a start bank on each engine and crosstie switch is closed, try opening the switch. That would allow each to start without dropping volts on the other.

"With a good alternator, you can actually disconnect the battery once it's running, the alt will put out enough juice to keep it running. "

This is correct in theory , however the alt will usually destroy one or more diodes if disconnected from a running engine.

The better battery switches will have a FIELD disconnect circuit , (break before break )to keep the alt. alive should some one actually turn off the battery connection.

If the engines will not start and run independently, you need to replace both start batteries and clean terminals pronto.
A few questions:
Do the batts live on a charger full time?
What happens if you try to start with the charger off for a day?
Electrolyte levels good?
Battery age?

Jim Cooper said:

... Alternator on running engine is definitely producing output as volt meter registers around 14-15 volts when not cranking the other engine. I can't say what the volt meter reads while cranking the other engine, I didn't check at the time.

Click to expand...

Monitoring V at Batt'y (w/ help if req'd) while starting would be worthwhile.

Also might try leaving first eng run for longer time before starting 2nd to see if it makes any difference.

since this is new I suspect that the batteries are getting old. How old are they?

MY Guess is there is resistance in the ground to the engines, and the voltage drop when starting #2 is enough to kill #1.

No cost, take out the bolt on the engines ground , wire brush them clean and reassemble with out steel or SS washers , copper if you can find it.

thanks for the responses, folks. Two of my four batteries are within 6 months old and a third is probably about one year or more. They were all fully charged when I experienced this issue. I'll be checking negative ground blocks this weekend!

One or more batteries could still be defective. Check for a voltage drop when trying to start the engine. Or have them load tested.