My onan MCCK generator is overcharging the start battery

[Boat Design Net Moderator]

Guys, let's please try to keep the discussion friendly and not go too far with the jabs. Thanks.

 

[CDK]

We ar here to help this guy not argue but are you telling me its 10 deg C under a car bonnet ( hood) ..

10 C. under the bonnet is highly unlikely. But only one of my 3 cars has the battery there! Sadist car manufacturers put the battery in places where the owner can't find them, like a recess in the car bottom, under a rubber mat and upholstery.

Anyhow, imho the correct way to tackle the overcharging Onan (from the book of Genesis: the masturbating son of Judah, weird name for a generator...) is to find the circuit diagram, then check and replace faulty components.
Or discard the circuit and make something 21th century worthy.

 

[pistnbroke]

Quote ... find the circuit diagram, then check and replace faulty components.
Or discard the circuit and make something 21th century worthy.

Quite right CDK dont go and fit a power hungry series regualtor to an already faulty generator....

 

[sdowney717]

picture shows the center tap power resistor part number is 8.

The center lug is offset?
I think it is, I will thorougly investigate as I dont understand why this is putting out such high volts.
Perhaps the high and low charge side wire hookup on the resistor have been reversed? I would think the low charge side should have higher number of windings. I will check. This is a 6 ohm 225 watt wire wound resistor with 2 outer adjusting lugs.

From series field of gen, power comes into center lug of resistor. I think center lug is fixed, not moveable, only outer lugs move.
when the batt volts are low, both sides of the circuit are in.
When the batt volts rise, the 2 step charge reg K5 opens the points leaving only the low charge circuit in.
Voltage output is entirely dependent on RPM of gen, and resistance setting. Seeing RPM is steady, then resistance must be adjusted. excess output is given off as heat. HOW ABOUT hooking up a light bulb in series with low charge wire? To reduce voltage, then would also shed light instead of just heat.

If I cant figure what it is doing wrong, then I will likely disconnect low charge wire. GEN powers the AC charger which does charge the gen battery.

even though prior to spec C, it is the same part. My gen is spec C. Manual shows all the parts for spec A, B, C
I show this picture cause it better at showing the off set center lug.

The schematic and wire diagram are on page 43 here
you can full screen and zoom around to see the charge circuit, it is very simple design.

http://www.docstoc.com/docs/34443851/Onan-Operators-Manual-and-Parts-Catalog?
K5 is the two step charge relay
R1 is the center tap lug resistor

 

[Frosty]

All my customers used to say ---yeah but ---it was working yesterday. You blew the tyres up and now it wont charge. It was alright yesterday.

In my infinite wisdom I would say that there is something wrong with it.

It is not regulating --replace the regulator.

 

[pistnbroke]

Anything man can make man can repair

Its just getting a print of the diagram thats the problem

 

[sdowney717]

I pulled the reistor out of the circuit and it reads 13.2 ohms across the outer lugs. It is in good shape, no where near the 6 ohms listed in the specification in the manual.

Custom power resistor

Left outer to center lug is 3.9 ohms
Right outer to center lug is 9.5 ohms

adjustment points were set at

Left outer to center lug is 3.5 ohms
Right outer to center lug is 7.3 ohms

I plan to put the highest resistance on the low charge side and see what it does.
When I ran it completely disconnected, batt voltage reads 12.4

Resistor as it was installed

closest screw terminal with double wires on the 2 step charge regulator relay has the low charge wire which I likely will end up taking off.

 

[BertKu]

Sdowney,
You have in anyway 2 stage charging. If it is true, that the low current , second stage is only 1.57 Amp and your voltage is truly moving up to 17 Volt ( i.e. with a higher than 1 Amp current at 17 Volt, not just measuring some leaking voltage). Just use one 2N3055, 2 x 1 watt zener-diodes to the combined voltage to the base you are comfortable with, a resistor of 22 Ohm 1 watt, and you never have a problem anymore. Your batteries will last. Just keep an eye on the gassing voltage at the temperature you think your boat will always be in.

refer: http://www.powerstream.com/SLA.htm

I have only a few hundred of those chargers exported to different parts of the world and in term of USA consumption it is peanuts, but I haven't had one complain from anybody.
I haven’t exported to Australia, they know it all better.
If you get stuck, I gladly post you those few components with compliments

 

[pistnbroke]

having studied the generator diagram I dont think its anything to do with the resistor ...I think you have a problem with the regulator ..

Berku ..whats that silly australian comment about ????
ie
I haven’t exported to Australia, they know it all better.

 

[Frosty]

The picture shows very very corroded contacts. I would suggest cleaning them as they may well be the problem.

It also looks as if the resistor is adjustable as the contact brackets can be moved up or down the resistor coil.

A very clever Idea but cleanliness will be essential.

 

[pistnbroke]

Strange isnt it when that old fashioned diagram appeared everyone faded away..no do this or do that ......

Now ....this device must have some relay to act as what in auto terms is a cutout so we need a diode @ 30 A to replace that and an auto style regulator..

What I would do is pull out all the old crap and modernise it .. If the shunt field current is below 4A then a modern auto regulator could be fitted direct replacement ..dare I say lucas 8TR ...
One diode replaces the Cut out . Leave the field diode in place as this ensure DC from the residual field during initial excitation. Neg earth assumed

Now Berku its over to you.....

 

[BertKu]

First at all, I think you are very brave to tackle an old, rusty, oxidiced unit to make it work again. Normally a voltage regulator is optimised for the type and make of the alternator. The latest regulators are for the sealed lead acid batteries which is slightly different than the older versions. If I were in your shoes, and got stuck by not be able to obtain older type of components, such as sliding resistors, I would just use as I suggested, a few 2N3055 with either zenerdiodes or 7815 regulator should the Voltage be sufficient. You have than the gassing voltage level adjustment in full control.

 

[Frosty]

I think thats what every body is thinking.

 

[pistnbroke]

Whilst I am not a fan of series regulators as they dissipate much power. In this case as the output current flows in the series field winding it would have the advantage that as the charge current drops so would the series field current reducing the output volts. This would reduce dissipaton in the transistors. If the output volatage before the regulator was way too high ..say 20v + then the old heavy duty resistor could be placed across the series field coil to reduce its effect ...

Now we just want a reort on the finished job !!!

 

[sdowney717]

Ok, it was bugging me a lot why it should not work as originally made.

So I cleaned everything nicely, even solder tinned the resistor lugs.
I adjust the upper band as high as the resistance can go around 9 ohms for the low charge circuit. Disconnected the gen battery bank off the convertor-charger.

Amazingly the voltage climbed just to 13.54v as it ran for about 15 minutes.
So it is working, only thing I did was clean up all connections and set upper resistor from 7.5 to 9 ohms.


I then disconnected the upper low charge wire off the power resistor and reconnected my convertor- charger to charge the Onan battery. The front Onan ammeter actually then shows a slight drain as it runs which makes sense. Boat convertor-charger does three banks and senses load and adjusts voltage as needed. With the onan running, the charger-convertor puts 14.12 v on that bank.

I then measured the series field voltage coming into the center lug of the resistor and it reads 45vdc. Likely cause it is floating with zero load, the two step charge regulator opens the points because the convertor-charger is providing sufficient voltage to both run the gen and charge the onan battery.

So I want to leave it like this letting the convertor charger do the charging. Reason being I dont want the gen batt to run down, I want it being trickle charged from shore-grid power. With full batteries on the other 2 banks, I get 13.5 to 13.7 vdc which is good. So I think as the onan battery gets juiced fully by the convertor-charger, it might drop to that trickle charge level, we will see.

I left the high charge 2 step regulator wire hooked up. I figure if the convertor-charger died, then I could still charge the onan battery.

When run with both low and high charge wires disconnected, the Onan battery was 12.5 vdc.

SO I completely removed the low charge wire from both terminals as dont want to risk a short. In some future day, no one will know it should be there, so perhaps leave a note it is not hooked up? It might be that if the convertor-charger went down, and the onan was running that the 2 step charge regulator would eat its points trying to maintain a set value. Because it did not have the low charge wire boosting the voltage and keeping the points open all the time. In that case you could install a small relay or circuit components that if convertor charger is off, allow power to come in from the low charge circuit of the Onan series field.

Convertor-charger is on all the time so it would have to break to do that. I had an old Buick with gen not alternator which had a 3 relay coil voltage regulator. When the engine was running and the cover removed, it spark floated the points continually inside.

Then Onan had gone thru three batteries in 10 years.
When I got the boat it was not hooked up or running and seem to recall the prior owner saying it was eating batteries. Boat also had much messy wiring running all over, which I went thru and cleaned up and redid. Added galvanic isolater, new power cables, boat had 4 shore power inlets and a lot of romex, etc... I think a lot of people are just simply lost and cant begin to fix all the electric issues on an old boat. I replaced much of the old romex with multistrand wiring. Anyway too much to tell but it was like a rats nest. Some people would still think so.


Another Onan owner suggested to me that if the Onan had something heavy running such as a light or bilge blower, it might have dropped the charge voltage down.
I dont know about that as when the Onan was charging all 3 batteries when the convertor-charger was hooked up, it overwhelmed that and voltage being pushed was 15.5vdc. When I disconnected the convertor - charger onan wire, it jumped to 17 vdc. I seem to recall that connecting wire was not hooked up when I got the boat.

Just doing the KISS method.

I just remembered this, The convertor-charger does sense when the prop engines are running, it shuts itself off so as not to compete with the engine alternators.
So that means the third bank output is likely to get turned off. I will check that and see.