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Author Topic:   Two Motors, One Battery: Damage to Regulator
ofdog posted 01-13-2006 05:01 PM ET (US)   Profile for ofdog   Send Email to ofdog  
I am running on one battery a 1995 Mariner 135-HP and a 2004 Mercury 9.9 kicker. Last season every thing was fine this season, third trip out my cranking battery was dead when I got to the water. When I got home I tested it, and I have a constant drain on the battery. I have isolated the short to the 135 Mariner itself. It is not the wiring. It appears to be a short in the charging system itself (regulator perhaps). Is there something I should have installed when I added my kicker motor to keep it from sending power back into the Mariner, or was this just something that just went bad in its time? Any past experiance will be appreciated.
derf posted 01-13-2006 06:40 PM ET (US)     Profile for derf  Send Email to derf     
Yes. Charging systems were generally made to charge one load by themselves. Depending on how it is wired, you could do all sorts of things to damage charging systems if you connect two together. The same thing goes for disconnecting the load (battery) from the charging system while the engine is running.

If you have two motors and two batteries it is a good idea to use one battery per motor. If one battery goes down, you still have the second that you can switch. You can tie all the negatives together and install a switch for your accessories.
If you have one battery and two motors, just use the regular Perko switch and wire the two motors where the batteries would go and the battery where the motor would go in a conventional setup.

ofdog posted 01-13-2006 07:39 PM ET (US)     Profile for ofdog  Send Email to ofdog     
Thanks for the info. It is much appreciated, I will give it a try! ofdog
jimh posted 01-14-2006 11:07 AM ET (US)     Profile for jimh  Send Email to jimh     
When two motors are connected to one battery, there is a potential for problems if both motors are running at the same time.

I would avoid running both motors in situations where they are connected to the same battery and thus have their charging outputs connected together. However, my impression is that this is often done by many boaters, and I don't think it results in the immediate destruction of the charging circuits. On the other hand, I think over time it does cause problems, based on my own experience.

I don't think the regulator should have been affected by this. I would be more more concerned with the stator coils.

When both motors are running simultaneously, they will both be trying to charge the battery. If the charger output of one motor is higher than the other, it could raise the battery voltage above the charger output of the other motor. In that case, the other motor's charging circuit is effectively electrically disconnected from the battery while it is still running. That is a situation in which some damage could occur.

My aversion to connecting together in-common and in hard-wired fashion multiple charger outputs from multiple outboard motors stems from my personal experience. Let me explain.

I bought a boat with twin outboard engines with simple charging circuits in them. They were hooked together in the battery distribution system. At the time of purchase, there was a problem with one of the engine tachometers. The tachometer is always a good indicator of the charging circuit as generally they share the same coil as their source of alternating current. The problem ultimately turned out to be a bad coil under the flywheel. The coil was overheated and had developed an open circuit. Getting this fixed cost about $300. It is quite a messy repair, as you have to yank the flywheel off to get to the coil. This is not a simple job. After you put the flywheel back on you have to tighten it with very high torque. If you were a purist you would then verify the timing marks and re-time the engine ignition timing.

About two hours of operation with the repaired engine in common with the other engine, and the coil blew on the other engine. This cost about $300 to fix, too.

At this point I was suspicious that the cause of these coils going bad was related to the fact they were tied in common by the battery distribution wiring. I have a fairly deep understanding of electricity, rectifiers, alternating current, etc., and I could see how this could happen and cause damage.

My options were to either re-assemble the boat's wiring the way it had been all along, and see if either of the engines failed, or to wire it up with more electrical separation between the two. Having already invested $600 into this project, I decided that it was a better choice not to wire it back the way it was to test if I was wrong about my conclusion. I wired them up separately, and they operated for many years without a problem or failure. Yes, it could be coincidence. If someone wants to spend some money they could test the theory, but, for me, I was happy with the way it worked and I lacked the financial resources and time to keep testing the in-common wiring arrangement until it was proven not to be the cause of the two rapid failures in succession.

Based on this, I cannot, in good conscience, suggest that other people just hook the charging outputs of their outboard motors together and see what happens.

If anyone has a set up where they have been operating with their twin engines with simple charging outputs wired in common and have had no failure, I encourage them to speak up. But I cannot say that because I had two failures in short succession.

In most electrical distribution systems, it is very common for there to be one source and many loads. It is not very common at all for there to be several sources combined into a single load. For example, I have never seen anyone build a Y-cord onto a table lamp and plug the lamp into two outlets in their living room. I have never seen people take two audio amplifiers and connect them to a single loudspeaker, and set the volume to different levels and see what happens.

Generally within electrical devices that are supplying power to a load where several sources are hooked in parallel there are usually build-out resistors or some other method of balancing the load among the paralleled sources. The build out resistance is usually chosen to be greater than the output impedance of the source, so that the effect is to drive the output impedance higher and toward a more uniform value. If you don't do this, one source does all the work and the other source loafs along. I think this what was going in my twin outboard situation above, and in that case, although somewhat counter-intuitive, the source not taking any load may have burned itself up from eddy currents developed in the coils. Whenever you have wire in the shape of a coil, strange effects can occur that are not anticipated in simple DC analysis. The stator of an outboard motor is a coil with an alternative current being generated in it.

It may very well be that certain devices which are designed to charge batteries can permit themselves to be connected with other certain devices designed to charge batteries, and that no harm will result from this arrangement. I just think that it is prudent in the case of certain outboard motor charging circuits that they not indiscriminately be connected together in duplicate. And that is why I recommend against it.

It might very well be that a particular outboard motor has a charging circuit that can tolerate being tied in common with its twin, but this does not mean all outboard motors have such a charging circuit. It is no guarantee that someone won't end up spending $600 to learn this again, either.

6992WHALER posted 01-14-2006 10:29 PM ET (US)     Profile for 6992WHALER  Send Email to 6992WHALER     
The previous owner of my twin-engine whaler had by passed the battery switch resulting in both batteries be charged by to both engines. Having both engines charge the same bank of batteries burned out one engine's coil [under-flywheel charging coil or stator].

The mechanics I talked to about this made it pretty clear that you never have two engines charge the same battery at the same time, because you will damage at least one of the charging systems.

jimh posted 01-14-2006 11:31 PM ET (US)     Profile for jimh  Send Email to jimh     
By way of confirmation of my experience and recommendation above, I am glad to hear of your problem and the recommendation of your mechanic. This tends to confirm my opinion--don't have two engine charging circuits connected together while both engines are running. And I think this is especially true if one of them is running at high speed and the other is just idling. In that situation the output voltage from the charging circuits might be at the greatest difference in voltage.
seahorse posted 01-15-2006 06:03 PM ET (US)     Profile for seahorse  Send Email to seahorse     
According to a factory engineer, a regulator switches current as it "samples" the system voltage many times per second, depending on rpm. When two engines are running on the same battery ciruit, the regulators are still sampling the voltage but are now turning on and off (or shunting to ground and back) much faster than normal because one will sense the voltage increase caused by the other. Bottom line is the switching is increased in fequency and the internal temps of the switching circuits increase which usually shortens their life.
placerville posted 01-16-2006 02:55 AM ET (US)     Profile for placerville  Send Email to placerville     
I'm one of the one-third owners of the 21 Revenge mentioned in the other forum. We have twin 70 hp Johnsons, two batteries and two Perkos switches. I haven't yet traced out the wires but If I understand you guys correctly, you should have one Perko switch on "1", the other Perko switch on "2". Never have both switches on "both" or both switches on the same number?
Does that mean if one battery is dead then you can only run one motor? I guess you could use one battery to start both engines and then flip one switch.
Revenge 25 posted 01-16-2006 09:30 PM ET (US)     Profile for Revenge 25  Send Email to Revenge 25     
I blew two stators by having two outboards charging one battery bank (Perko switch on both). I am a bit of a slow learner.

I now have the motors completely separate with no switch. No switch = no user error.

jtwhaler posted 01-22-2006 05:03 PM ET (US)     Profile for jtwhaler  Send Email to jtwhaler     
Jimm and all: I may have had too much time on my hands, what with a sore back and a rainy and cold day on Lake Corpus Christi, Mathis, TX and my classic whaler setting on the trailer. Nonetheless, here is an interesting site conserning Marine Battery Charging.
John as jtwhaler
jimh posted 01-22-2006 08:05 PM ET (US)     Profile for jimh  Send Email to jimh     
John--Not much information there regarding TWO motors connected to ONE battery.
jtwhaler posted 01-23-2006 06:22 PM ET (US)     Profile for jtwhaler  Send Email to jtwhaler     
Sorry I wasted your time Jimh.
rtk posted 01-23-2006 07:25 PM ET (US)     Profile for rtk  Send Email to rtk     
Matt, are the switches simple ON OFF switches, or are both switches the OFF 1 BOTH 2 switches?

If they are the OFF 1 BOTH 2 switch, you will need to see if there are actually battery leads connected to all the poles of the switch. If that is the fact, than you will need to do some investigating to see where the wires actually go to figure out how to set the switches when running. The OFF 1 BOTH 2 switch is made to have two batteries connected to it so you can choose a single battery or parallel two batteries, and really is not used for a single battery/single engine application.


LHG posted 01-23-2006 07:26 PM ET (US)     Profile for LHG    
Having operated twin engine Whalers since 1986, I can say I am not a fan of battery switch setups in a simple boat like a Whaler, and in spite of extended cruising, have found them not necessary, even for house power. To me it is a lot of HD wiring clutter in boats that don't have much space for this, more corrosion and terminal problems, expense, confusion, and risk of problems like are described here.

I keep my system at it simplest. One battery for one engine, and no interconnection at all, and the house wiring comes off the starboard batt/engine, as Whaler designed it to be. And for additional flexibility, I use the combo batteries, giving me more discharge/re-charge capability if needed. I always have a set of jumper cables available in the car, for a possible dead battery after months of no use, or on board if on an extended trip. One battery can be used to jump the other. Simple, and nothing else is needed. In all these 20 years, and with two twin engine boats set up this way, I think I have needed the jumpers maybe 4 times.

The other key is not trying to squeeze every once of life out of a battery. As they do in autos, they always tend to give you notice of impending weakness. Just get a replacement at that time and enjoy worry free boating.

jimh posted 01-23-2006 07:52 PM ET (US)     Profile for jimh  Send Email to jimh     
John--There are so many website with recommendations about wiring boat batteries that you could make a cottage industry of cataloguing them all. This current discussion regarding the problems associated with using two outboard motors connected in parallel to charge a single battery is about the only intelligent discussion of this topic I think you will find.

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