Onan MCCK charging diode

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Onan MCCK charging diode

Post by BeaconMarineBob »

Important information for those of you who have a Onan MCCK gas generator.

If you hook up a battery charger to your Onan when it is running you will burn out the charging diode.

Also there is a good change you will burn out the resister in the electric fuel pump.

The house battery charger should never be connected to the Onan battery.

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Re: Onan MCCK charging diode

Post by mrrudely »

been there, done that, to both
I am wondering if there is a way to have the genset starter battery hooked up
to the charger but disconnect automatically as soon as the genny starts
my charger has 3 wires for 3 banks
rr
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Re: Onan MCCK charging diode

Post by prowlersfish »

Quick answer is NO. Install a good battery and run the generator once in a while no need to have a charger. If you must charge it, I would just use a portable charger once in a while. Just because you have a 3 banks charger does not mean you have to have it charging 3 banks. All can be used to charge 2 banks. But they have to be hooked up properly. That way you do it varies on the make and model of charger.
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Re: Onan MCCK charging diode

Post by mrrudely »

Prowlersfish
You are the second no answer I got
But I am not convinced
I have very limited knowledge when it comes to electricity
There are a lot of Trojan owners here on the forum and I suspect a lot have Onan mcck generators
I can't be the only one with this problem
Also, (this may be rhetorical) why would Onan design a system that cannot be left connected to a charger? Most boats do not get used that much.
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Re: Onan MCCK charging diode

Post by prowlersfish »

mrrudely wrote: Wed Dec 21, 2022 12:01 pm Prowlersfish
You are the second no answer I got
But I am not convinced
I have very limited knowledge when it comes to electricity
There are a lot of Trojan owners here on the forum and I suspect a lot have Onan mcck generators
I can't be the only one with this problem
Also, (this may be rhetorical) why would Onan design a system that cannot be left connected to a charger? Most boats do not get used that much.
Yes there are thousands and thousands with these generators and a lot right here.

What have I missed? is there something you have not told me? What exactly is the problem your having other than changing normal opp? Maybe I am missing something.

Why would they design a system that way? Because there is no draw on the battery until you try and start the generator. So the battery would normally stay charged for a reasonable length of time. The generator was deigned to be run, not sit for months at a time. Durning long storage like winter lay up you can disconnect a cable and hook up trickle charger.

Unless you are having other issues You really don't have a problem other than the battery. As you posted you have a bad battery You need to replace it. that and recheck everything.
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Re: Onan MCCK charging diode

Post by prowlersfish »

I may add Trojan wanted the Generator isolated from the other batteries so a fault elsewhere (like the charger or power to it) would not drain the genset battery. This is a common school of thought on other brands too. The MCCK is old tech (from the 60s) but a rock-solid simple design. I would bet there were more these built than any other.

Let me know what exact charger you have, and I would be happy to explain how to hook it up for 2 banks
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Re: Onan MCCK charging diode

Post by mrrudely »

I don't think you missed anything
I have had the exact problems Beacon Bob is warning about
Just wondering why I have not heard more about it

I will install the new battery and keep it isolated

But I am still wondering why I can't hook up one leg of my charger to the generator start battery through a relay
that would disconnect once the generator started

Your answer was no and someone else told me it would burn up the fuel pump
I'm not convinced it can't be done

Protech 3 bank charger
I don't see why I can't just use 2 of the three banks
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Re: Onan MCCK charging diode

Post by prowlersfish »

I guess you could but then you have to worry about the voltage spike front the relay coil. And the limited amps from generator DC output to power the relay. Thousands and thousands of these out there maybe millions in every brand of boat you can think of and its non-issue. It is a primitive and simple design and that is its charm and why it's so reliable. They came out about 60 years ago The only thing that kills them is lack of use or bad fuel (from lack of use). Is it perfect? No. IMO your trying to fix a non-issue. There are More up to date and better units out there with much more robust charging systems (real alternators) Perhaps you may want to upgrade?


Pro tech? can't find any info of them. On some brands you need to hook up the 3rd bank wire to one of the other banks to work on others you don't recheck the brand please.
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Re: Onan MCCK charging diode

Post by mrrudely »

can't thank you enough
you are right, no sense trying to fix a non-issue

charger is pro mariner protech i
will work with only two banks connected
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Re: Onan MCCK charging diode

Post by BeaconMarineBob »

Prowlerfish is correct

From the factory Trojan always used two of the three banks of the battery charger. They never used the third bank for the generator. The reason was to keep the generator isolated. If anything went south with the charger or something else the captain could always start the generator.

As for Onan the same was true the generator should be isolated from the rest of the DC and AC system till the generator starts.

In the master Onan repair manual it stats that you should not hook up a battery charger to the Onan when she is running.

We have found over the years two common issues with the Onan MCCK when a battery charger is hooked up to the Onan.

One and most common is the factory electric fuel pump fails or the factory replacement fuel pump fails.

Second the charging diode fails

Best to run your Onan regularly and make sure its group 24 battery is healthy.
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