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Author Topic:   Dual Motor, Dual Battery Starting and Charging
wwknapp posted 10-18-2004 08:44 PM ET (US)   Profile for wwknapp   Send Email to wwknapp  
I'm working on a dual motor setup on my Montauk. Not twins; one is a Yamaha F50, the other a Yamaha F9.9 high-thrust. Both motors have electric start and battery charging. Both will be set up for remote running from the console, except maybe for tilt for the 9.9. I can't figure a method to use just one battery without a lot of manual switching, so two batteries it is, no more due to weight. Nominally each motor will have it's assigned battery.

The requirements:

--isolate the batteries so house load won't discharge both;

--start either motor from either battery. Probably each motor will be normally hooked to it's own battery. House load probably on the kicker battery. This seems easy enough to accomplish with manual switches;

--charge both batteries from either motor, while preserving the necessary isolation of the two motors from each other.

This is where it gets sticky. The configuration must work safely with either motor alone or both running. Either motor could be the only one running for extended times, though there could be times both would be running. There are newer design combiners or isolators that will have voltage drops of a tiny fraction of a volt. Some are entirely solid state, the state of the art has improved. So far I've not found any specifically designed for the two motor situation. They are really set up to protect the starting battery from discharge with charging less well thought out when you add that second motor, although they work very well with one motor. Currently one of the Hellroaring Battery Isolator-Combiners seems the best choice, but I'm not sure.

Doing this entirely with manual switches seems to lead to too many ways to mess it all up with a setting error.

I've looked through the archives, there is info on protecting starting from house loads. And some info on dual motors. But none seems to address the charging problem directly. Outboards cannot have their charging circuits connected together without messing them up.

I know there are folks who have boats with twin engines, and ones that have boats with a kicker that can charge batteries. Short of going in and modifying the outboards internally, and must work with the outboards I have, anyone have a system that's automatic and reasonably foolproof?

jimh posted 10-18-2004 10:39 PM ET (US)     Profile for jimh  Send Email to jimh     
My article in the Reference section:

Dual Battery Wiring

wwknapp posted 10-18-2004 11:38 PM ET (US)     Profile for wwknapp  Send Email to wwknapp     

Your article was already read, and many more like it. It covers pure mechanical, manual switching, but examine the isolation of the two outboards if charging both batteries. With both motors running you can only charge one battery with each. If charging with either motor by itself, the other motor is exposed to the one charging if both batteries are charged. It can only be done by shutting down the other motor and switching it completely off with a battery switch.

I'm aware of these systems, it's where I started, and will be the basic system in the boat. But the more I thought about it the more it was obvious it could be quite annoying in ordinary day to day running as I'd do it. The 2nd motor is not just a trolling or emergency motor, but will be used lots of other times when it's the appropriate motor. What I'm after is the next step beyond. Once you have installed a basic system like one of your three dual motor systems, then you add a automatic combiner or other methods to that. The switches in your basic system only have to be changed for emergency operation, the automatic system does the rest.

Until fairly recently combiners were usually solonoids operated by a sensing circuit. subject to the usual failure rate of such mechanical equipment, low but not insignificant. There are now entirely solid state combiners that have voltage drops across the combiner that are well less than 0.01 volts. This overcomes the problem of diode isolators where the voltage drop was so much as to not fully charge the batteries. The systems I've been interested in also incorporate running and failure lights so can be monitored on the console. Though solid state should be pretty reliable.

I asked only to see if anyone has modernized their system to include any of this technology and what their experience has been. Some new larger motors are now coming with this sort of thing built in, so I expect with time BW will start using it. The technology is catching up, I'm not sure if it's up to it, but I intend to find out. Before I start mucking around shutting down motors setting switches every time I want to change motors. Just to insure the batteries both stay charged.

andygere posted 10-19-2004 12:47 AM ET (US)     Profile for andygere  Send Email to andygere     
wwknapp, you pose an excellent question, and one that I have been considering for a while. My Outrage is set up exactly as you describe, with both main and kicker motors having electric start and charging systems. The original rigger of the boat set it up with 2 batteries and the typical Off-1-Both-2 switch, which requires manual switching to ensure that the #1 starting battery is not discharged by house loads when it's not running. In particular, I often troll for several hours on the kicker, with full electronics, including the power hungry radar, in operation. I'd like a system that optimizes charging while insuring against a dead starting battery. I've read Jim's excellent article, as well as those in the West Advisor (from the West Marine Catalog) but still haven't worked out the ideal system. It seems that a solid state battery combiner must be integral to the system. I look forward to any input others may have on this topic, as I suspect this problem has been solved already.
Moe posted 10-19-2004 10:54 AM ET (US)     Profile for Moe    
> Start either motor from either battery. Probably each motor will be normally hooked to it's own battery.

With each motor directly connected to its own battery, as in the schematics on the Hellroaring site, and in JimH's "New" Dual Battery Configuration, you CANNOT start either motor from EITHER battery. The best you can do is parallel the good battery with the bad battery and start either motor from BOTH batteries, while the bad battery is drawing the good battery down. And in these schematics, you will note you cannot disconnect the batteries in the event of an electrical fire. Are you okay with these limitations?


wwknapp posted 10-19-2004 01:48 PM ET (US)     Profile for wwknapp  Send Email to wwknapp     

Maybe I should have made it crystal clear. Each motor would have a battery that was nominally it's battery. But, it should be obvious that you do that connection through a 4 way switch for each motor. Each motor is on the common lug of a 4 way switch and both switches are connected to both batteries. On normal running one switch (and it's motor) is set on battery 1, and one on battery 2. That's pretty much the basics no matter the circuit details. That system would allow you not only to hook either motor to either battery but would allow you to completely disconnect either motor from the system when it's shut down. Simply turn it's switch to off when it's shut down and it's isolated (never while running, but with the switch off it would not crank) It covers the cranking options pretty well.

Note that system by itself is subject to human error. You have to keep your wits about you. And, while it can be used safely to charge both batteries with either motor by disconnecting the off motor and manually using the switch on the running motor, that increases the risk of human error. With that basic system in place I'm trying to reduce that risk and inconvenience by automating the charging. Then the switches only have to be changed under emergency conditions or if a battery fails entirely.

Note I'll probably have the house wiring on a third 4 way or at least a way of choosing which battery it's using without moving wires. I've not yet finalized my thinking on the manual part of the design. It will depend partially on what I find in automated equipment.

Yes, it's important to deal with the possible shorted electrical. Primarily by prevention, well done and well weatherized connections and wire everywhere, lots of fuses and circuit breakers. That's a whole separate subject, in my Montauk there is a awful rats nest of improperly done wiring, and it started with BW as many components are their original ones. This project includes a full rewire of the boat to eliminate all the potential electrical leakage now present and replace all the corroded components. And organize it all so future modifications can be done properly. It pains me to look at that mess, let alone work on it. Everything works just fine, but is awful to work on.

One of the flaws I see in hellroaring's circuit diagrams is fuses that can too easily disable the system. Under storm conditions you want a system that's reliable, you certainly don't want to be changing fuses. Even if a short develops you need to get home or at least manage the boat. Though in my case at least one motor can be rope cranked and does not depend on the batteries to run. I'm still thinking through the main circuit protection, but it probably won't include fuses in the main bus. Switches, and maybe circuit breakers.


wwknapp posted 10-19-2004 05:06 PM ET (US)     Profile for wwknapp  Send Email to wwknapp     

I've been back forth, up down, and crosswise through the site with the searcher over the past month or so. Best I've found is some mention of a auto connect device being used on a single motor installation. And a great deal of blind faith that the house battery is holding enough charge from a plug in charge before the trip. A lot of folks are using battery setups they don't understand.

I've read the T-8 Kicker article very carefully, more times than I care to remember. Though less of it than might be imagined applies directly to my installation, it's just pretty work. Just went back and checked it, on the subject of dual battery connections here's the sum total of it's information (has not changed since I last looked):
"I currently have the Yamaha battery cables wired to battery #2 and the Evinrude wired to battery #1, with a 4-way battery switch. I may delve further into this arrangement at a later date."
This sounds like the sort of setup that Moe was worried about. I'm doing that delving. While there is a lot of information in the site if you have the hours I spent looking, it's not complete in many areas. This is especially true when it comes to dual motor battery charging.

I should note that the system I'll probably end up with may have a added benefit. I use solar panels to maintain batteries on my two boats, one per battery. With the current single battery, no problem. With a standard manual dual setup I'd have to have two solar panels. It's looking like the auto circuits can take the solar panel input and distribute it to both batteries. Same would happen with a plug in charger. That alone will offset some of the extra cost of the connector.

Motor controller hookup which was a problem on the T8 installation took me almost no time to figure out as I'm using two motors of the same brand, and the same brand controller. And little time to set up, most of which was spent converting one side of a new dual Morse control to "push to go forward" from it's stock "pull to go forward". Small Yamaha motors use "push to go forward", and larger use "pull to go forward". I have one of each. Most, but not all controllers are convertable. In my case I'm replacing the original single Morse controller with a dual.

I've not built the steering linkage yet, but it looks straightforward too. Mounting is easy, put it where BW designed it to be put. Bolted on, of course, with some protection for the hull.

50 lbs of size 27 battery will be removed from that corner of the stern, making my net weight change back there 50 lbs and less clutter. And two size 24 batteries will end up in the bottom of the console, actually sitting in cutouts in the console floor on the deck. Barely stick through the elevated floor. Probably the new blue top optimus batteries, combined starting/deep cycle.

BTW, the T8 remote weighs more than the T9.9 remote, probably because of the tilt motor setup. Still mumbling on the tilt, I may end up designing a tilter for it. See how it goes, will build at least a proper tilt support.

I'm sorting out the completion of the partial remote conversion that was on the T9.9. That's definitely not covered in info on this site (or even directly in Yamaha's rigging manuals). It involves some rewiring of the motor. The motor was originally a H model with tiller handle, not a R model, previous owner did a poor job of installing a remote kit in the motor, but none of the rest of the conversion, except remoting the start button. Most of that will have to be replaced, shift interlocks, both mechanical and electrical were not removed and floated where they could have locked up the shift anytime, nor was the on motor kill tether or kill button removed. That stuff is all now out of the engine, awaiting arrival of parts. Most of which are replacements for corroded ones, not conversion parts. Kill tether, start switch, oil pressure light all will be at the console. Battery and 7-pin wiring harness will be installed in the motor to bring everything to the console.

The only thing still up in the air a bit is the battery setup. I've got a month or so before that will have to be decided. Though the sooner the better as far as accumulating pieces.

newport jack posted 10-19-2004 05:28 PM ET (US)     Profile for newport jack    
On one of my previous boats (a 1989 Sea Ray Laguna) I had twin 100hp mercs, and Sea Ray kept everything separate, including separate battery chargers, the neat thing they did was have an emergency battery tie system , which was basically a starter solenoid hooked to a momentary toggle switch on dash, negatives were bonded together. I used it once when I drained a battery , just hit the switch and engine started up. I'm thinking about doing the same thing on my current twin engine whaler! I'm not a big fan of the battery switches, other then as a disconnect, just seems like another possible weak link. Are you sure you want the two battery system, your redunt system would still be the rope start kicker which could charge up a dead battery as you head in. Lot less wiring, less potental problems and you can get one big kick but Optima battery. I agree with moving the battery away from the transome...Jack
jimh posted 10-19-2004 08:37 PM ET (US)     Profile for jimh  Send Email to jimh     
In reply to some comments about wiring:

There are probably a couple of million boats with outboard motors where the battery is connected directly to the motor without a disconnect switch. The incidence of these boats catching fire from electrical problems is quite low. Most of the connections to the battery are made with wing-nuts, and you can disconnect a battery in a few seconds if you need to. Or include a high-current fuse that can be mechanically removed in a hurry. I do not think there is a word of Coast Guard regulation or boating-industry practice that recommends anything special in this regard.

I think it is important to minimize the connection of two charging systems together.

Most "isolators" will have diodes in them. The better ones have special diodes with very low forward-biased voltage drop across them, but there will still be about 0.3-volts drop at best. This means that the battery will never quite see the full voltage output from the charging system.

There are some devices often called "combiners" which act as switches to connect batteries in parallel when certain conditions are favorable for charging them both at once.

Some outboard motors have been designed with charging systems that can accommodate dual batteries and successfully charge them independently. If you have such a motor you can consider different connection arrangements.

There is no upper limit to how complicated you can make the 12-volt primary wiring on your boat. You can include disconnects, switches, fuses, patch panels, connection busses, relays, solenoids, and so on, to whatever level of complexity you desire. You can, if you wish, make it so complicated that it becomes less reliable and harder to operate.

In general I would recommend that you bond the negative terminals of the batteries together with a conductor that is large enough to carry the starting motor current. Having a switch wired to parallel the batteries for starting or having a jumper cable to jump them together for starting (the positive terminals only) will get you going in 99-percent of the cases. In the other 1-percent you can just physically swap batteries if one is totally dead.

I have about 45-years of experience in wiring things together, but I have found that simple is better in most cases, and especially so in a boat. I have been guided by the boating experience of others, and in this regard many long-time outboard boat owners have been operating their Boston Whaler boats for 25-30 years without the benefit of elaborate electrical switching and disconnection between the motor and the battery.

When there is just a single piece of wire without splices or connections between the battery and the motor, you have the most reliable installation. If you add a dozen terminal lugs, connections, switches, solenoids, and tie-points, you just add potential places for failure.

I know there might be one day out of a million where some boater looks back and sees smoke coming from the cowling of his engine, and pushes a little button on the helm console which drops out three solenoids or relays, and stops the current flow. But those are rather long odds, and keeping it simple often has its own reward.

Moe posted 10-19-2004 10:01 PM ET (US)     Profile for Moe    
West Marine Hyperlink

BEP Marine Hyperlink

Apparently, they don't believe paralleling the output of two outboard's voltage regulators is a problem.


wwknapp posted 10-19-2004 11:45 PM ET (US)     Profile for wwknapp  Send Email to wwknapp     

If the purpose of putting the 9.9 on my Montauk was only to get back in case of main system failure, then you are right, a electric start and charging are not necessary. However, that's not the purpose of this setup. It has several additional reasons. First is to go in horsepower limited waters. Not just small passages, but spend a day or more in such waters. You are not allowed to run the main motor at all, not even for charging. At least down here they don't require you remove it! Second is for extended shallower water running, the 9.9 draws less water, and has less power to tear itself up. Making it the choice for swamps and such like. Again, this could be for days, I'm a aquatic ecologist. Obviously the motor is also a backup for the main, and for something like trolling, but during any extended periods when we are running slowly it can save a lot of fuel. All those reasons together mean the 9.9 has to be able to function the same way as a main motor as far as charging house batteries and so on. It will see a lot more use than most kickers. Also, the T9.9 does not have a set up rope starter, you remove the cowl, and the plastic flywheel cover, which does not take tools. Then you use a regular plain starter rope on the grooves in the flywheel. All works , but not something you want to do often. The 50 can be done the same way, but I've not tried that.

Yes, the Optima has the capability to deliver large amperage, but only will do so if needed. I've a Optima on my Ford Ranger, red top, and have been happy with it. The reason for going with Optima is high energy density for weight and bulk, and no liquid to leak.

The starter solenoid to do the both jump is fairly common in RV's. I've used systems like that, and they are certainly more foolproof than the both setting on the battery switches, the bad for running setting is only there as long as you keep holding the switch and you don't change your main running settings. You reminded me to consider that for the both starting option. Thanks.


Currently the function of isolators and combiners is being merged somewhat. In some solid state units by actual field test the voltage drop is 0.001 volts. You are behind the times, though isolators like you describe still exist and are cheaper. Instead of a blanket statement you might want to tell folks that they need to look into the specs.

These systems don't add splices or connectors in the main path to the motor any more than was added by the manual battery switch, which is still the backbone of the system. They add additional interconnects in that system, branch circuits. Similar to what all the house connections do. They will not change your available starting current which will be based on the state of battery charge. They might mean you will have a higher average charge state on all your batteries, additional safety factor.

Yes, it's additional complexity, and I did spend some time deciding if that was ok. Since such a system has considerable benefit, and has the ability to show failure in a reliable way, and does not change the manual system you always have the manual system to fall back to in the event of failure. I have some experience with similar electrical setups in RV's, so am not going into this cold.

I've spent as long messing around boats. And wiring. My boating experience is different from yours as it comes in the main from ocean going, round the world type sailboats. Where you don't dash back to the harbor if conditions go bad as it can be weeks away. I'm well used to thinking in terms of potential failure modes.

I do agree with you it's important to minimize the connecting of two charging systems together. However, this is not a large boat (Montauk). The system has to work well without adding a lot of weight or bulk.

I also think the chance of smoke pouring out of something is fairly remote in a well done installation. If you do it sloppy where things can short, you deserve what you get. A greater danger when setting up all the connections is allowing leakage currents, which cause electrolysis. It takes very little leakage to do that, a amount that won't be noticed until the corrosion get's going.


I think if you look at the links you gave carefully you will find neither of them is connecting the two starting batteries directly together. Their systems are the solinoid based ones I pointed to earlier. And, as is important in a Montauk, occupy more space than what I'm looking at. I expect that actual measurement will show that the two paths from the starting battery to the house battery will have enough resistance in all the steps to isolate the motors. If not it could be a risk.

I have not been able to find data on how much load a stator must have to protect it. If it's tiny, as I suspect, a load resistor in each motor may be all it takes to protect the system. The systems I'm getting back from the company experts seem to sometimes do this simply by using 15-25' lengths of cable for one of the connections. The cable provides enough load resistance. If it's that simple life is easier. But until I'm sure, I'm not going to be the one to risk a stator. Though it's beginning to sound like I may be the first one in the group to move into these newer setups. I asked if anyone was using them, so far a blank on that.


jimh posted 10-20-2004 07:26 AM ET (US)     Profile for jimh  Send Email to jimh     
Any connection where the voltage drop is less than 0.2 volts is just a mechanical connection without isolation, unless there has been some substantive breakthrough in physics that somehow I slept through.
jimh posted 10-20-2004 07:51 AM ET (US)     Profile for jimh  Send Email to jimh     
I would be interested to see the schematic diagram of the outboard motors where the charging system provides dual outputs. I wonder if they are diode isolated?

The (long) link Moe gives above refers to the device as a Voltage Sensitive Relay (VSR) which connects the batteries together for parallel charging. Compare that description with mine:

"There are some devices often called "combiners" which act as switches to connect batteries in parallel when certain conditions are favorable for charging them both at once."

I don't think there is anything about a VSR that isn't contained in my description.

If there are really solid state devices which provide isolation and voltage drops of 0.001 volts, could we have a hyperlink to some descriptive information about them?

That BEP MARINE switch cluster looks quite interesting, and it would be a good solution to the two-motor/three-battery installation. It is a bit expensive and maybe overkill on a small outboard boat. Apparently there can be some problems with the relays chattering if the battery loads and charging current are not well matched. (See .)

It looks like BEP MARINE is a breath of fresh air for marine battery distribution and charging products. I had not heard of them before nor seen their products in use anywhere on outboard powered boats. Their website is worth a visit:

Moe posted 10-20-2004 09:03 AM ET (US)     Profile for Moe    
The BEP VSR is slightly different from West Marine's description of a combiner. West says that if voltage on EITHER side of the combiner rises to the charging level, the relay closes.

This would mean that if there were isolated starting and house batteries, the outboard was connected to the start battery, and a solar panel to the house battery, the relay would close if EITHER side of the combiner rose to charging voltage, so either charging source could charge both batteries. The VSR appears only to sense the voltage on one side.

In the dual outboard/dual battery installation, if both outboards are outputting charging voltage, both VSRs will close the relay and the outboards and batteries will all be connected together (as long as the switches are all closed). The same is true for the combiner.

Interconnecting multiple house battery charging sources is done all the time in the RV world. The sources may include the tow vehicle/motorhome alternator, solar panels, and the onboard charger/converter. These all have built-in protection from reverse current from another source in the form of diodes on their outputs. The rectifier on the output of a stator also protects it. My Ford SuperDuty has a relay in the line running from the alternator to the trailer connector to allow the truck to charge RV house batteries when the engine is running, and prevent the RV from discharging the truck batteries when it isn't running.

Moe posted 10-20-2004 11:32 AM ET (US)     Profile for Moe    
Upon closer examination, I see that the BEP VSR is available in a dual-sensing (DS) version, which would close the relay if either side is at charging voltage, like West's combiner description.

These DS versions are not used in the 717 distribution cluster, which is about 5" X 8".


jimh posted 10-20-2004 01:52 PM ET (US)     Profile for jimh  Send Email to jimh     
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 inside 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.

So this is my dilemma. Because I have had the experience of two rather inconvenient and expensive failures of outboard motor charging circuits that were connected in common, my integrity prevents me from just going around willy-nilly telling people that it is OK to hook them up that way.

I know that people can construct situations where my proposed arrangement of engines and batteries can create a limitation or even perhaps a hazard, but I have not had anyone write or call to tell me they blew up the charging coil on one of their outboard after following my recommendation.

This, again, could be just a coincidence.

wwknapp posted 10-20-2004 09:18 PM ET (US)     Profile for wwknapp  Send Email to wwknapp     
The funny thing is this is a group that eagerly grabs the latest in things like fuel systems in outboard motors, but from what I see is far behind on electrical systems. It's really not that complex. This group is about a classic boat, but hardly keeps the motors or what's on the console classic. The electrical system that's running it can be modernized too.

Rather than try and respond to each of the luddite ramblings, I'm going to describe the system that's coming together with help from the Hellroaring people. Actually I send them my questions and worries and they send back all the ways they have dealt with them in other boats. With suggestions for improvements I've not thought of. This has gone back and forth via email several rounds as the design details have been worked out. Here's Helroaring's link:
I looked at several similar systems, settled on Hellroaring as theirs seemed the most completely thought out. Talking to them I've found nothing they have not already thought about and done.

First off I have two motors, both of which can and will act as a main motor and maintain the charge on the batteries, depending on what I'm doing. Dual batteries will be used, Optimus blue tops in size 24 battery boxes. On some occasions I may have both motors running. The battery connection from each of these will be hooked to the common connection on separate 4-way battery switches. The 1 & 2 positions on each motor's switch will be connected to the 1 & 2 batteries in the console in the Montauk. All the main wiring of the battery system will be mounted inside of the console, with some exceptions that will be described. In addition to each motor having it's own switch access to both batteries, there will be a house switch which will give the ability to select either battery as the house battery. In normal usage the switches of each motor will be set to separate usage, each motor has a battery to itself. House will be set according to which motor is the current one to have it's battery most protected, on the other battery. If one battery fails entirly I'll be limited to one motor at a time, and the house load on it's battery, the system allows that to be either battery. So far this is a standard manual switching system varient, most kinds of battery failures can be handled with just this, as far as starting engines, though not conveniently. It does, by itself provide isolation of house load from the other battery, if set right. Set these switches wrong and you can cause all kinds of problems, any of the switches set to both and everything is tied together. (we'll try to avoid that error, see below) This is as far as most go.

Now, first we add a marine solenoid, this will connect the two batteries when activated. It will be activated by a momentary switch which will be accessable on the console. As long as you hold the switch the two batteries will be tied together for starting boost, when you let go your original main switch settings prevail, whatever isolation you set is back. You never need to set both on the main switches. I will probably work out wiring so the solenoid's energizing voltage can come from whichever battery has a charge.

Now, we add a Helroaring BIC-95150A solid state combiner between the two batteries on it's own wiring. This is modified so it's voltage sense is for either battery using a dual sense kit which is a fancy name for a few bit's added to it's terminals. Hellroaring has told me they so far are doing it as a kit, but in the future it will probably come as a separate model. When either battery reaches 13.4 volts the two will be connected together. (it's possible to do a little fiddling with the threshold voltage, but probably not necessary) The combiner is capable of doing it's job with only 0.001 volts drop, this is considered too little to maintain a load balance, so the combiner is actually wired with fairly long wires of a carefully chosen gauge when installed, to provide a little loading between the batteries. In my case this will probably be 10-12 gauge about 15 - 20 feet, neither motor puts out all that much amps. If both batteries are below 13.2 volts, the combiner drops out. At this point either outboard alone will be able to charge both batteries without any changing switches or other human intervention.

But, this will also combine the batteries when both motors are running and charging. The load wire is probably enough to provide enough isolation, but there is another bit to add to take care of this. Add a small amount of wiring that will ground the sense terminal of the BIC-95150A if both motors are running, probably wired off the aux terminals in the motor keyswitches. Since it now senses zero volts, it won't combine. This wiring also does not require human intervention once installed. And since both motors are running, they can each charge their own battery. There is no need to combine charging. Switch back to a single motor and combining is back, either way.

The system has two surge suppressors in it, TVS-20V-1.5K transient suppressors. These provide protection from spikes in the system or overvoltages above 20 volts. Both for the solid state of the combiner, but also good for the other electronics on the boat.

I'll also add the remote kit, on the console. That provides LED indication of the state of the system.

Now, it's the end of the day, I've towed my boat home, I hook a single solar panel to the system, either battery, and both batteries are maintained until I next go out. Only a connection plug needs to be added. If I wire everything right, I could even set the house, and two motor battery switches to off and this would work as the combiner is a independent setup, or can be. Any charger within the current and voltage limitations of the system can be used. It's strongly preferable that any such charger have automatic dropping to trickle. Dual batteries, only one charger needed. I use solar panels, but a single trickle charger could be added inside the console run off a shore power connection.

Please note this is a system still being designed, I've given you enough to sort out your own, though not all the details I have, and I've not got some of the small details done, and of course it's not all installed to try. In time this is probably what's going to be on my boat as described. Certainly by next spring whatever is the final design will be done. When it's all running, in normal use I can pretty much ignore charging and such like. No switches to change, except maybe the house switch.


jimh posted 10-21-2004 08:49 AM ET (US)     Profile for jimh  Send Email to jimh     
Walt, Many thanks for that link to

as it looks like they have some interesting products for use in marine applications.

I am not opposed to modern technology, but the simple lead-acid battery and charger system does seem to be sufficient for the great majority of boaters. Certainly in some patterns of use there might be room for great improvement in a boat's battery distribution and charging system. If you use your boat like I do--start the motor once or twice a day and run it for hours--the simple and old-fashioned technology still works well. If you want to troll for hours with electrical trolling motors, have an electrical anchor windlass, or have other high demand electrical loads, you may need to pay more attention to the battery and its charging.

When you work out the new electrical system on your boat, please let me know. Perhaps we can do an article about it.

wwknapp posted 10-21-2004 06:32 PM ET (US)     Profile for wwknapp  Send Email to wwknapp     

I ended up with Hellroaring after considerable looking at all the products I could find. Theirs seemed the best for what I wanted to do. Working with them they seem to be well informed.

I realize my usage is probably different than many. I don't just fish, in fact I rarely fish (I prefer fly fishing on small streams). So, the idea I'm putting on a trolling motor is not it. In the sort of mix of boating I do I expect to see lots of days where I will shift between motors, and others where either motor may be the only one used. I also use equipment that's not usual in my nature recording activities. It does require power, primarily battery charging as it's designed to operate independently in the field. That will not be a big demand on the boat batteries but it will be some. It's also mostly a nighttime activity. On top of the usual console instruments, and sometimes camping in the boat overnight.

These new systems are not just for complex needs. Battery life is highly dependent on consistent charging. A good automated charging section on top of the emergency switches can save batteries. If you are talking as I am about something like blue top Optimus, that's a good chunk of change in the batteries. Even the single motor dual battery folks might want to look at this. Trying to maintain accurate charge by manual switching is not going to be the best for batteries.

The trick is to have a good manual backbone to the system, and the automatic stuff is on top of that on it's own wiring. Don't try to make the automated system all there is. At least that's my thinking, and how I'm doing it.

Yes, once I've got it all installed, and have some running experience with it I'll provide updates, photos and such like. A few have expressed interest. I may have a few other details on this installation that might be of interest as well as the batteries. Even in electrical it's including rewiring the boat. It's not just throwing a motor back there and calling it good. This is a installation that integrates a auxillary motor into the console controls, fully runnable from the console. Unless I design and build a tilt unit I won't have the tilt that's in the T8 installation, but otherwise this is intended to be much the same functionally.


andygere posted 10-21-2004 08:07 PM ET (US)     Profile for andygere  Send Email to andygere     
Walt's comments on battery use are of interest, as are Jimh's. My battery use varies, but often goes as follows:

1) Halibut or Rock fishing: Start up the main engine, run a short distance at planing speeds(<15 minutes) and drift fish with some electronics/lights energized and the engine off. Restart the main engine every 20-40 minutes, motor a few minutes at low speed, then shut down the main engine and repeat. This may go on for 2-4 hours or more, then run a short distance for 10-15 minutes, put the boat away and leave it for a week or longer.

2) Salmon fishing: Start up the main engine, run a longer distance (6-15 miles) at planing speeds, then shut the main engine down and start the 15 h.p. kicker (with electric start and charging system), run at low speeds for several hours with electronics/lights energized (often including radar), then shut the kicker down and return on the main engine at planing speeds. While fishing, a pair of electric downriggers, wired directly to respective port and starboard batteries, are cycled numerous times, running continuously for a minute or more per cycle. Again, boat may sit a week or longer between uses.

3) Cruising: Start up the main engine, run longer distances (25+ miles) at planing speeds, arrive at destination, shut down main engine. At marina, house loads include cabin lights, halogen deck lights, and minor electronics/cell phone charging, etc.

In case 1, I probably use more charge from my batteries then I restore, primarily by cranking the engine numerous times, but not running for long enough periods to fully restore the charge. In case 2, I think I sometimes use more charge, particularly if I use the radar on continuous mode for a long time. I'm not sure what the output of the kicker's charging system is, but I'm sure it's not much. This is somewhat offset by a longer run back to the dock, but I doubt it's really enough. In case 3, I think I restore more charge than I use, as long as use of the halogen deck light is limited. I installed the light primarily so I can see while cleaning up the boat after evening fishing trips, and it works great for that but I'm always worried about killing the batteries by using it for too long. I pull the boat from it's slip about 3 times per year, and always trickle charge each battery before relaunching (and they usually need it). I have had a few times when one of my 1-2 year old batteries needs to be combined on the "both" switch to start the main engine, particularly when the boat has been idle for a few weeks. I have the typical 1-2-both-off switch, but I'm not entirely sure how the charging system of the kicker is wired to the switch. I am fanatical about keeping the terminal connections on the batteries clean, less so about those on the back of the switch. Since my cables do not inspire confidence where they have been exposed to weather, I plan to rewire the system this winter, and hope to come up with a system that provides the best possible charging scheme for all the conditions described above. I am considering an on-board charger that could be plugged into the shore power outlet at my marina and provide a continous charge to the batteries. This another piece of gear to stick in the back of the boat somehere (or perhaps the console) but it sure would be convenient. I am intrigued by Walt's system, as well as those offered by Bep Marine. I am also a big fan of simpler is better, but hate the idea of getting stranded. I do have the advantage of having a kicker motor that also has a recoil starter, but I'd prefer not to slog a long way on the kicker if I can avoid it, and I'd like to get the best life out of my batteries that I can. Any thoughts on boat usage vs. battery life vs. charging systems are welcomed.

wwknapp posted 10-21-2004 11:32 PM ET (US)     Profile for wwknapp  Send Email to wwknapp     

I suspect your are right and your system is often undercharged. The fact you have needed to use both batteries to start on such new batteries is another indication. Chronic undercharging is a battery killer, just as chronic overcharging is.

Of course no amount of automation will compensate if your charge sources are not on long enough relative to the demand. The idea of shore power charge is one way to make it up as long as the deficit is not too bad during the trips. Note that you will need two chargers, or one that's designed to service two batteries unless you have the auto sensing system. Or are willing to risk turning the main switch to both, probably not a good way to operate as you will forget eventually. In any case make sure and have a charger that senses battery charge and drops back.

Another way, which I prefer is to use a solar panel. With the long time between trips it does not take all that large of one. Again you have the problem of two isolated batteries. I noted recently that volkswagon is putting a solar panel on each of their more expensive cars when shipped, and those appear to be removed when the car is sold. Anyway, ebay is full of those at fairly low prices. I've not been able to find out how weatherproof they are.

I've got a small IsoSolar panel on my pontoon boat that's maintained it's battery for over 4 years now. And that's one of the cheap batteries you expect to go in a year or two, supplied when I bought the boat. If I have it right that panel is only 7 or 8 watts. The panel looks like new. My Montauk has one of those twice that size on it, as I happened to have that one when I got the Montauk. I hook up the panels when I arrive home from a trip, just part of cleaning up. Then pull them as part of getting ready for the next trip. I always start a trip at full charge.

Probably a first step would be to get some onboard monitoring so you can watch what sort of charge your batteries have. Not a lot of money involved in that, and it should be there anyway. Find out where you are at.

I'd sure find out how that kicker is wired. If it's wrong it might not even have a good charger anymore. The 9.9 I've got can supply 10 amps at full throttle, but it's got more capacity than most motors it's size. And as it's throttled down the output drops, at low speed it's only 4-5 amps.


andygere posted 10-22-2004 12:06 AM ET (US)     Profile for andygere  Send Email to andygere     
Walt, thanks for the info. I've considered solar, but since I pay for a slip with electrical service, it just seems easier to plug it in. Once in a while I fish 2 days in a row (rare now that I have my 2 little ones) and an AC charger would get them to full charge overnight. Guest seems to have several new models out, and I'm starting to read up on them now.

My assumption on the kicker charging system is that it simply outputs to the common lug of the 3 way switch. I am careful to run only one motor at a time to avoid the problem Jimh had with failed coils. The cables are mostly bundled in wire wrap, so I can't trace them without taking it all apart. I decided to wait until I wouldn't be using the boat for a while to tackle the project all at once. Overall, I haven't had major problems with this system, but the wire is old and the insulation is weathered in a few areas. On my old Montauk, a cable failed (water through a pinhole in the cable jacket) and I was amazed at how bad it got and how quickly. The failed section was nothing but green goo when I cut the insulation away.

I do have a surrogate on-board monitoring system, which is the voltage display on my Garmin fishfinder. This is subject to whatever voltage drop there is in my house wiring system, but it gives a pretty good indication of the state of charge for the battery that is switched on. In general, I start the main engine on #1, and use #2 for house loads (house circuit comes from #2). When running the kicker, I switch to #2 to keep #1 fresh for starting the main engine. Switching is a pain, and sometimes I do forget, thus my interest in a better system.

wwknapp posted 10-22-2004 02:23 AM ET (US)     Profile for wwknapp  Send Email to wwknapp     

Solar is my contribution to saving fossil fuel I guess. Or more in my case power to the boats would have to run across the driveway, so solar is more practical for me. No cord strung across the countryside. Anywhere the boat is parked is fine for the solar which is just tossed on the boat where it will get sun.

If you look carefully at the description of my system you will find it can expose one motor to the other's charging voltage. But only when the motor is off, and then through the load balancing wiring. I'm assuming they are not that delicate so that's safe at least. The surge suppressors on the system will keep them from being exposed to too high a voltage surge. So, yes if you never ever run both motors at the same time it should be fine.

In my case the one I thought of is caught in a sudden storm with the aux running. I'd really not want to shut it down before starting the main so as to maintain boat control. Thus, for that alone I had to have the ability to run both. There will, I'm sure, be other instances, though less serious. I've thought of other times where it could be convenient.

Running shore power, you might want to look up galvanic isolators. Shore power ground may have a ground plane that's quite different from the water. The isolators prevent that from chomping through various metal bits on the boat. Apparently one symptom of that one is all the outboard anodes going fast, though that is not a reliable indicator.

Electrolysis is everywhere. A leak into a battery cable is a good source. One end or another is going to dissolve. A major portion of rewiring is insuring no exposed conductors anywhere. There is saltwater corrosion in some of the electrical stuff inside the console on mine. To all be replaced. BW did it pretty good originally, they coated every connection. But less careful folks have been in there. Thus the full rewire. BW did fail to seal around the panels on the console, and it's obvious that's a leak source onto the wiring, dripping on the main house bus for one. Panel redesign is part of the project, and will include gaskets for the panels. Plus trying to secondarily waterproof everything. My inspiration is thinking about how much water might be sloshing around in there if taking on waves. I hope I never get into that sort of thing, but I want my boat to handle it. It does not hurt when outfitting boats to have lots of paranoia.

That switching is a pain and can be forgotten is why I'm trying to avoid it.


jimh posted 10-28-2004 01:34 PM ET (US)     Profile for jimh  Send Email to jimh     
[Fixed long hyperlinks.]

I entered this discussion generally opposed to excessive complication of the primary electrical wiring on a small boat, but during the dicussion I was impressed with several of the resources mentioned, particularly the switching equipment from BEP Marine.

I still have my doubts about the inter-connecting of unregulated charging outputs from outboard motors. My thinking is that if one motor has significantly higher output voltage than the other, the battery load will be floated to a higher voltage than the output voltage of the second motor charging circuit. When this happens, the effect is electrically the equivalent of disconnecting the battery from that second charging circuit. There is practically universal agreement that one must never operate a OFF-1-2-BOTH switch while an engine is running to prevent momentarily disconnecting the battery from the engine, yet this can occur when two motor's charging circuits are connected in parallel. One motor drives the voltage level above the other motor, and the diodes in the second motor become reverse biased, effectively disconnecting the second motor from the battery as far as the charger circuit is concerned.

With twin engines, this could happen if you had one engine on idle and the other running quite a bit faster. This would tend to cause the voltage outputs to be quite different (from unregulated charging circuits). I think this may have played a factor in the problem I described above.

In more sophisticated motors where the charging circuit output is closely regulated, this may not happen to the same degree. If the two charging outputs are regulated to within about 0.5 - 1.0 volts of each other, there is little chance that the battery could be driven to a higher voltage by one engine to the point where the second engine sees it as disconnected.

wwknapp posted 10-29-2004 03:31 AM ET (US)     Profile for wwknapp  Send Email to wwknapp     

I think that the greater risk in switching a dual manual switch to off on a running motor has to do with the high voltage surge that occurs as the contact is broken. Certainly in the days when diodes were things other than silicon that would have zapped them. Even now if the manufacturers are scrimping on the diodes it would be a danger. Note that the warning applies even if only one motor is involved.

On top of that the generater system of the outboards is generally that of a magneto, not a alternator. The open circuit voltages generated in the stator coil can be quite high. The repair manual for the 9.9 I'm working on lists measured voltages as high as 77 volts under test conditions on the open coil. There's probably parts in the regulator that are not too happy with that voltage.

Most of it's poor design. Sloppy because they could get away with it. With so much electronics being put on boats they really need to rethink how they do it.

The situation with two motors seems to be as you describe, but I don't think it's actual measured voltages would turn out the same as the open case. It's a more complex problem that I choose to simply assume I want to avoid. While still getting full benefit of the charging systems of both motors.

I do understand your reluctance about adding equipment. I believe the trick is to do it as layers, so the failure of one layer can be spotted and isolated. The benefits of better battery charging and just plain convenience are too great not to consider it. I'm still headed toward the Hellroaring system, though I've not bought the parts yet. I believe theirs is the more technically advanced.


LHG posted 10-29-2004 02:44 PM ET (US)     Profile for LHG    
[Suggested that the discussion was not yielding useful information and that the wiring used by a particular boat manufacturer (Boston Whaler) for installation with particular engines be consulted and used instead.]
wwknapp posted 10-30-2004 02:31 AM ET (US)     Profile for wwknapp  Send Email to wwknapp     

A large boat with two large engines is actually not the same case as a whaler (any size) with a main and smaller auxiliar engine. In the case of those large twins the boat often won't even run right unless both engines are running. This simplifies the handling of dual batteries as nearly all the time each engine charges the battery it's hooked to. The condition where one engine handles all the charging duties can be practically ignored. It generally only involves a single run, back to the dock for repairs.

With a main and aux, particularly as I originally asked it, the situation is quite different. Either of the engines might have the load of charging all batteries for days if not weeks at a time. On top of propelling the boat. This is not the temp condition implied by kicker thinking. This is two alternate mains. Which can, at times be both running. But, generally only one is running.

Smaller engines often do their electrical duties different from the huge engines that some use. I'd not at all assume that the two could be treated the same. If nothing else the large engines like that have a much greater charging capacity, you can be very wasteful of electricity and it won't hurt. When you are running off a available max charging rate of 6 or 10 amps, then you need to have that charging well applied to be enough.

In any case I believe that BW takes the cheap way out and just puts in the minimal manual system. Which, by posts I've seen in the various forums, is not even understood by many owners. If someone can find a proper main and aux design from BW that properly handles charging without touching a switch, I'll be more than happy to study it.

Big motor owners seem to have only one thing they do better than small motor owners, that's consume large amounts of gas. If anything the smaller boat owners probably have less money to afford repairs and could teach a lot to the big boat owners.

Some of my problem is I come from a experience background where a little trip on your boat might be around the world. No sea tow to pick you up if things go wrong. I'm used to not depending on outside help.

BTW, I've talked to a number of local Yamaha mechanics. They seem to know only enough to follow the diagrams in their rigging manuals, which are simple manual systems. They, however, do understand that the main and aux problem is different than the twin motor installation.

I have noticed a strong tendency in the forums for the large boat owners to jump on any discussion about smaller boats and take it over to expound at length about their boats, be my guest. It looks pretty much like the info I was looking for is not in the group. Certainly not the experience. I asked looking for folks who had tried the stuff I'm looking at, and found I had to explain it several times. That's not something I'd have to do with someone who uses it.


jimh posted 07-17-2005 06:07 PM ET (US)     Profile for jimh  Send Email to jimh     
newt posted 06-13-2011 09:25 AM ET (US)     Profile for newt  Send Email to newt     
I am resurrecting this old thread to add my own recent experience which is similar to what Jimh writes about.

My twin engines and dual batteries are wired as shown here: .

I recently noticed some intermittent erratic tachometer readings on my starboard engine and had planned to pull the boat to replace the regulator/rectifier.

My boat is in rack storage this year. Typically, they guys at the marina pull and launch the boat and leave it at the dock with all batteries switched off. Last time out however, they were running late, so I met them on the dock, left the motors running and took off for a day of fishing. Several hours into the trip, I lost the PORT tachometer - the needle dropped to zero and stayed there. I checked the battery switches (something I should have done before I left the dock), and both were set to "both". I had been running around for several hours with the batteries combined and both engines running.

Yesterday I replaced both regulators/rectifiers and the problems are solved. Bottom line is do not run two engines on a single (or combined dual) battery!

ASIDE: A Google search for information regarding twin engines and dual batteries points to Continuouswave very high on the search results. Another good resource that popped up is below, but scroll down and note the source...

jimh posted 06-14-2011 10:57 PM ET (US)     Profile for jimh  Send Email to jimh     
Many thanks for the update on your rectifier and regulator problems which appear to have occurred after your boat was operated (inadvertently) with the charging outputs of two engines tied together and charging a common battery or bank of batteries.

The failure that occurred in your case was very similar to the failure that occurred on my (previous) boat when its battery and engine charging circuits were wired in a similar manner. I have been cautioning against such a configuration ever since then. I regret that you had a similar failure, but by sharing your experience and adding it to mine, we can perhaps help other boaters avoid this problem in the future by avoiding having to engine-operated charging circuits wired in parallel and charging a common battery or bank of batteries. I don't know for certain that it will cook every engine's rectifier, but it appears to have cooked yours and mine.

Jerry Townsend posted 06-16-2011 10:10 PM ET (US)     Profile for Jerry Townsend  Send Email to Jerry Townsend     
Why not just put diodes in each charging line?

I just noticed this tread - and looked at the Hellroaring website. One thing that is quite apparent is that there is a lot of heat generated in their product - as indicated by the large cooling fin surface area.

I only have one engine and battery on my 17 Outrage. Quite some time ago, someone was discussing the same subject - and I thought how I would set my boat up for multiple batteries and/or engines. Pretty simple - takes a multiple (number of batteries plus one that is open) position switch mounted on the console, solenoids (number of batteries) and diodes in each charging line. Negatives on both batteries are common.

Select either battery to operate or start from or select the open switch position - off. But charging both batteries simultaneously would require big diodes - at least. --- Jerry/Idaho

newt posted 06-17-2011 01:05 PM ET (US)     Profile for newt  Send Email to newt     

Installing diodes sounds complicated. I haven't the slightest clue how to size or purchase one. I am more inclined to screw a stop onto the battery switches so that they can never be set to both without some added effort, or as LHG practices, simply remove the link between the batteries - let each engine run on it's own battery.

While I like the ability to combine the batteries, I have never, in 10 years of boating, had the need to do so.

jimh posted 06-18-2011 08:00 AM ET (US)     Profile for jimh  Send Email to jimh     
Installing a diode in the output from the charging circuit is not something I would recommend. When current flows across a diode there is a voltage drop. The voltage drop varies and depends on the nature of the diode. Typically the voltage drop will be at least 0.6-volts. If the battery charging circuit is not very well regulated, it may put out excess voltage, and, if that is the case, a drop of 0.6-volts is not especially awful. However, if the battery charging circuit is a modern design and has a well-regulated output voltage, losing 0.6-volts from the charging voltage will mean that any attached battery will not be able to reach a full charge.

Also, when current flows though a diode, because of the voltage drop, there is heat created. If the charging current is small, say 5-Amperes, then the heat created is small. For example, if a diode had a 1-Volt drop and a 5-Ampere current there would be 5-Watts of heat created. To most people 5-Watts of heat does not sound like a lot of heat energy, but in a small area and the heat being created for a long time, even 5-Watts can produce significant temperature rise. Because of this any diode will need a heat sink. Fitting a diode and a heat sink under the cowling of an outboard may be a challenge.

[Editor's note: several comments related to another article which did not contain any information on this topic have been deleted from this archived thread in order to avoid confusion about the main topic of discussion--jimh]

Jerry Townsend posted 06-18-2011 10:55 PM ET (US)     Profile for Jerry Townsend  Send Email to Jerry Townsend     
Jim - thanks for the information. I think/suspect I have read similar words - and probably yours - on related threads before.

I was not aware of the voltage drop across diodes. And indeed, 5 watts, albeit with a 1 volt drop, is not insignificant.

But, are all of the problems with two engines operating at the same times? I can see where in that case and the batteries are switched, there would be a problem.

You caution about breaking the charging circuit, even momentarily. Now, is there a problem with only one engine running and switching batteries? In this case, the charging line would be momentarily opened - but there is still a load, albeit small on the generating system. --- Jerry/Idaho

jimh posted 06-25-2011 12:12 PM ET (US)     Profile for jimh  Send Email to jimh     
The ability of a charging system to be connected in parallel with a second charging system and suffer no harm from that connection is most likely dependent on the design of the charging system. I can't say for certain that all charging systems are intolerant of being wired directly in parallel with another charging system, not can I say all are tolerant. It seems to depend on the charging system.

In the case of the charging system on my c.1987 Yamaha 70-HP engines, I can tell you with reasonable certainty that those charging system were intolerant of being wired in parallel. This is based on each motor having a failure of its charging system when wired in parallel with another. It looks like the strongest unit survives, as first motor A blew its charging coil when in parallel with motor B, and then, once a was repaired, motor B blew its charging coil when in parallel with motor A. After two expensive repairs I stopped connecting them in parallel, and for over six years there have been no further failures of the charging coils.

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