Generator battery charging

[Ross Macdonald]

I have two 1993 Onan 15 KW MdL3 generators aboard. Never had any issues and have always done proper maintenance on them. All of sudden my stbd genset is no longer charging the battery that starts it. I just put in a new group 31 AGM battery for each genset. I put a meter on them and they are fully charged. My manual guage indicates no charging when genset is running. My manual is worthless regarding this issue. There are no alternators on these sets. So my guess is there has to be some diodes and or a voltage regulator in the box on the start/stop panel. I pulled the cover and it was above my pay grade. I had a mechanic friend try to find replacement parts for these things and he found everything but the voltage regulator. (bad news is Onan is phasing out all of the replacement parts for gens of this type and vintage.) So I'm guessing my alternate plan B would be to get a small charger, mount on the aft bulkhead and run it to both batteries. The one drawback to my setup is the only way the batteries can be charged now is by the generator running. It has never been a problem though because I run both gens once a week while in the slip (with a load) and the AGMs hold a charge for quite a while. Any thoughts on this?

[Boatsb]

When the gen is running what is the batt voltage? Could it just be the gauge?

[rsmith]

"The one drawback to my setup is the only way the batteries can be charged now is by the generator running"

Both my gens quit charging the batts years ago. there were oil press switches that kept the 110v charger from coming on when the gens were running. I disconnected the charging circs from the gens and eliminated the press. switches. As long as the gens are running there is 110v for the charger at the same time I got rid of the second gen batt. But I dont anchor or use a inverter and anytime the boat is underway one of the gen's are running. We do lay offshore and drift for swordfish at night but we have all the tower lights on and need the gen power.

[Ross Macdonald]

You might be on to something with the oil pressure gauge. When this started happening, the oil pressure gauge pegged all the way over. So maybe it's a related issue. The temp gauge is normal and of course the battery voltage gauge is showing 12 volts while running. Normally when the gen fires up, it puts out about 13.6 volts to begin charging. I'm going to have a tech look at it this week, and I'll post the findings. Thanks.

[yachtsmanbill]

I am under the impression that Onan uses a secondary winding on the armature for charging and for starting. At least on my MCCK 6.5 its that way with no starter motor. ws

[GJH]

My 1991 MDL 20 has no charging capability. I have the circuit drawings and the engine manual, which suffices. It was a replacement by a PO from a 32v Kohler, which got its start from the starboard battery bank. When I bought the boat, it was being started off one of the 8Ds in the thruster bank, and killing that battery on a regular basis. I gave it its own 12v battery and a little 6 amp $80 Pro Mariner charger, so now it charges that battery when running.

[Ross Macdonald]

Thanks for the updates. I guess my MDL3 has some charging method because the volt meters show charging. It looks like I will use the charger method. I found a good charger that will charge both 12V batteries at West Marine for about $120.00. The brand is a Prosport. Just plug into a receptacle in the ER and run Pos. and Neg. cables with ring terminals to each battery. This should do the trick and will charge both batteries from either genset or shorepower. Ross

[GJH]

I have the little 6 amp ProSport (by Pro Mariner) single bank at has been fine for 3 years. I hard-wired it.

[Ross Macdonald]

Well, I finally installed a 12 amp ProMariner charger for the genset batteries. Everything seems to be working according to the manual. But I have a question that isn't covered in the book. As I stated before, I have the 2 Onan MDL3s. The starboard one won't charge it's battery, but the port one still does. So when either set is running now the charger is charging both batteries. Since the port set still can charge its battery, am I ok to leave the charger on? I checked the batteries with a meter while both sets were running and they both are showing about 13.2 volts. So I guess I'm a little concerned I might overcharge the port battery. Any thoughts? Ross

[Canuck Dennis]

13.6 is usual starting batts

[davidwigler]

You should have no problem at all leaving a smart AC charger on while the generator is charging also. The smart charger reduces current to avoid overcharging. It's just like having your main engine alternators running while your house charger is on. No problem.

You could consider using one starting battery for both generators.

[jim rosenthal]

I'm still wondering why the one genset won't charge it's own starting battery. Will you let us know when you figure it out? The oil pressure switch or some other safety interlock sounds like a promising culprit.

[Ross Macdonald]

You guessed it...The oil pressure switch controlled the charging. Replaced it and it works perfectly. But I still have the small charger hooked to both AGM genset batteries. So far all is good...Ross