Onan stumped the chump

[Captain Bill]

I'm stumped. I have an Onan MDKAU 5KW genset which has a 12V charging circuit to charge the start the start battery. When I got the boat the 12V charging circuit was disconnected at the regulator. I connected it up when I discovered it and found it was putting out 15.6V which is way too high for my AGM starting batteries. I assumed the regulator was bad and disconnected it because I wasn't in a place where I could wait for a new regulator. I have been at a dock for and on the hard for 2 months so I ordered a new regulator. I just started it and the new regulator started at 14.4 but climbed to 15.6 within a couple of minutes. The regulator is a potted and sealed unit with only +, -, and 2 AC connections so no way to adjust it. Does anyone have any ideas why the voltage would be so high other than a bad regulator?

[a64pilot]

Do you connect it to a 12V battery?
I've seen this before on an open circuit, but the instant it was connected, it worked just fine.


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[Captain Bill]

Yes, the B+ wire is connected to the Battery. I did a continuity check between the Battery connection bolt and the B+ wire at the regulator. I also checked the ground wire.

[dave948]

What is the state of charge on the start battery and are your nothing is wrong with the battery?

The VR2 (DC charging regulator) gets it AC power from the VR1 (AC regulator) . If you are having problems with VR1 it could be causing your problems.

[Emmalina]

Unless you use a separate start battery for the gen set I would leave it unplugged. Mine was connected to my engine start battery and trying to charge a 150aH battery was consuming v belts in a big way. Now as the gen connects to my multi charger there is little point in getting involved with trying to charge using this small alternator set up ! I also saw a boat where the little onan alternator had melted trying to charge big battery banks. FYI I did replace my reg on the onan after a lightining strike but have never reconnected it ! The kubota one is a lot lot cheaper !

[RicknSue]

The battery is what causes crazy high jumps like that.
Back in the day , that was actually a test they taught young mechanics to spot sulphated batteries. My guess is that the regulator doesn't know it's an agm, so it's pumping out as hard as it can. The battery may be smoked too( although it works ok) , I've brought 'dead' agm batteries back to life by pounding the charge to them ( way higher than anything will ever tell you is ok) for hours, and they have lasted for years after that.
Hook it to another battery, see what it does ( a lead acid would be the best test).

[Captain Bill]

The batteries are only 6 weeks old and start the main engines as well. They were fully charged when I started the genset. They act perfectly normal when charging off the the ACR. The genset charging circuit is all on the engine side and does not charge the house bank. The genset start sequence runs the glow plugs for up to 20 seconds depending on engine temp. This pulls very little out of the batteries. The only thing charging the batteries was the genset charging circuit.

There is not a separate alternator on this genset, there is only a separate coil in the main alternator. According to the wiring diagram this coil directly feeds the Battery charge voltage regulator and does not go through separate regulator first. The wires come out of the stator and go directly to the regulator so it looks to me like the diagram is correct. I want to power my blower off this circuit. The blower draws 3.8 amps so only a little over 6 amps will be going into the batteries.

[Zephyrcat229]

Well if you were feeling bold you could throw a small step down transformer on it at the range you want for agm's.

Cheap, easy, worth a try if you're of the proper disposition.

If not there is probably some spendy fix (agm voltage regulator) through onan or aftermarket if that's more your bent.

[lvictorlucas]

A temporary solution until you find the problem is to just get some high current power diodes and place them in series forward biased as required to bring the voltage down. Typically about 0.7v each for silicon or 0.5v for Shockley depending on the current and temperature.

[skipmac]

Hi Bill,

Where are you checking to see the high voltage, at the regulator output or at the battery terminals or is it the same at both?


Skip

[lvictorlucas]

Originally Posted by Zephyrcat229

Well if you were feeling bold you could throw a small step down transformer on it at the range you want for agm's.

Cheap, easy, worth a try if you're of the proper disposition.

If not there is probably some spendy fix (agm voltage regulator) through onan or aftermarket if that's more your bent.

Usual transformers ONLY work on AC. Most people mean DC to DC converters or Hall effect devices when they say DC transformers. Actual DC transformers are very expensive custom devices with application in only scientific applications as far as I know.

[Cadence]

I'd check with Onan. I'd bet they could tell you. I've seen some garbage advise here.

It may just be specifying the type of battery when ordering a regulator? Or maybe you have nothing to worry about?

[skipmac]

Originally Posted by Cadence

I'd check with Onan. I'd bet they could tell you. I've seen some garbage advise here.

It may just be specifying the type of battery when ordering a regulator? Or maybe you have nothing to worry about?

I would say charging a new AGM (which are not cheap) with 15.6V is something to worry about.

[Cadence]

Originally Posted by skipmac

I would say charging a new AGM (which are not cheap) with 15.6V is something to worry about.

As I said ask Onan. They may or may not have a regulator for AGM batteries. Depending on the age of the genset may be a factor? Jury rigging something sounds like a bad idea. When someone finds a DC transformer let me know or diodes to put in line?

[skipmac]

Originally Posted by Cadence

As I said ask Onan. They may or may not have a regulator for AGM batteries. Depending on the age of the genset may be a factor? Jury rigging something sounds like a bad idea. When someone finds a DC transformer let me know or diodes to put in line?

Unless it's a temporary, emergency get you home fix, kludges, make do's and jury rigs are a bad idea 99% of the time.

DC transformer? Unless someone invented something new, these are only sold at the same store that sells magnetic diesel fuel filters and 200 mpg carburetors.

Inline diodes that drop 0.7V? I do have a box full of various sizes in my parts bin at home.