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Author Topic:   Common battery ground cable - twin engines
L H G posted 07-06-2009 04:15 PM ET (US)   Profile for L H G  
On my twin Merc installations, Mercury calls for a battery cable, same size wire as the engine cables, connecting the ground terminals of the two batteries. I have done this, but have often wondered why this is required. Anybody know?
Bulldog posted 07-06-2009 04:48 PM ET (US)     Profile for Bulldog  Send Email to Bulldog     
Larry I assume you don't have a battery switch then that would allow both batteries on at once, so I'm going with, perhaps Mercury wants your fuel system grounded to the battery negative, and this ties both together....Jack
L H G posted 07-06-2009 05:11 PM ET (US)     Profile for L H G    
Thanks Jack, but I'm not sure that's the answer. The Mercury rigging diagrams do not show any connection to the fuel system ground, which on all of my Whalers is completely separate from everything else. There is a dyna-plate gound on the transom, and green wiring to the tank.

You are correct in that I don't use battery switches, but the diagrams don't show that either.

jimh posted 07-06-2009 05:18 PM ET (US)     Profile for jimh  Send Email to jimh     
WIth dual batteries, the negative terminals ought to be connected together with a large conductor so that in the event there is an attempt to start an engine off the other engine's battery, the current can flow on the common cable.
L H G posted 07-06-2009 05:46 PM ET (US)     Profile for L H G    
Thanks, Jim for the reply, but i am still confused. Are you saying that if I wanted to jump one engine off the other battery, I would only need to jump the positive terminals? Since jumper cables have both pos and neg anyway, it seems like a lot of extra work and expense just for that purpose.

As you know, for years and years, I have operated my 18 Outrage with the twin 115's without such a neg cable between the batteries, but on my 25, they required it, so I did it. And now, on my re-power with twin 90's, they are also saying it should be done.

deepwater posted 07-06-2009 06:09 PM ET (US)     Profile for deepwater  Send Email to deepwater     
Maybe they require separate grounding to ensure proper grounding so that the computerized things will work in each separate motor,,Just a thought
jimh posted 07-06-2009 09:34 PM ET (US)     Profile for jimh  Send Email to jimh     
By bonding the negative terminals of the two batteries together, you establish a common point. This also effectively bonds the blocks of the two engines together. The size of the conductor is really the topic of discussion.

If there is no current flow between the two motors and their associated electrical systems, the conductor that bonds them together at the battery negative terminals will never have any current flow. You could use a conductor of small size, as no current would ever flow. The bonding would just be to keep the two systems at the same voltage reference. For safety sake, however, it is a good idea to make the bonding conductor the same size as the largest conductor. This guarantees that in some circumstance, perhaps something not foreseen or not intentional, should current flow on the bonding conductor the wire will be large enough to handle it.

In your jump-start situation, yes, you'd only need to run the positive lead.

My guess is that Mercury is expecting you'll have some sort of switch in the primary battery distribution system so that you can select batteries.

TransAm posted 07-07-2009 07:36 AM ET (US)     Profile for TransAm  Send Email to TransAm     
I'm still a little confused.

jim writes:"The bonding would just be to keep the two systems at the same voltage reference."

Is this saying that both batteries will draw down and/or re-charge at the same rate, and therefore maintain the same rate of charge?

seahorse posted 07-07-2009 08:11 AM ET (US)     Profile for seahorse  Send Email to seahorse     

According to American Boat and Yacht Council (ABYC) rules:

"Multiple Engine Installation - If a boat has more than one engine with a grounded cranking motor, which includes auxiliary generator engine(s), the engines shall be connected to each other by a common conductor that can carry the cranking motor current of each of the grounded cranking motor circuits."

L H G posted 07-07-2009 04:20 PM ET (US)     Profile for L H G    
Thanks to all for your replies. Now I'm even more confused as to what the actual functional application of all this is. Seahorse has shown us one thing, at least, that the ground between the two batteries, which as Jim points out, effectively grounds the two engines together, is a Code requirement. But my question is still, what does this accomplish in the absense of a battery selection switch?

I don't use those switches in the interest of simplified wiring in these open Outrages, and have found that a simple jumper cable does the job if I should have a dead battery. I use the combo starting/deep cycle batteries, and in 20 years, on 2 twin engine boats, have only had to jump start an engine twice. I have also operated one of these boats, with twin 115 Mercs, without any such cable for 23 years, and have experienced no problems that I am aware of.

seahorse posted 07-07-2009 06:33 PM ET (US)     Profile for seahorse  Send Email to seahorse     

Twin batteries for twin engines have to be at the same potential in salt or brackish water to prevent electrolysis damage. If the engines were not commonly grounded, then one may be a fraction of a volt higher than the other and a transfer of electrons would take place, meaning pieces of one motor would be donated to the other.

You can see it dramatically even in fresh water when a bass boat with a 24 volt trolling motor has the outboard hooked up to the "upper" potential battery so the aluminum outboard is 12 volts higher than the ground of the aluminum trolling motor. Usually bubbles appear quickly in those cases.

L H G posted 07-07-2009 07:20 PM ET (US)     Profile for L H G    
Finally. Thank you Seahorse, as that sounds like the answer. I wish these engine installation booklets would just give people the reasons for the requirements.

Although it doesn't affect me, would people with electric start pony motors need to have this negative ground cable between their batteries also?

seahorse posted 07-07-2009 07:39 PM ET (US)     Profile for seahorse  Send Email to seahorse     


Both Evinrude and Yamaha rigging manuals specify installing the correct sized ground wire between multiple batteries and they reference ABYC practices and procedures.

Jerry Townsend posted 07-07-2009 09:32 PM ET (US)     Profile for Jerry Townsend  Send Email to Jerry Townsend     
Having a single common ground is preferrable when dealing with electronics - as interfacing electronics, each using a different ground can have a fluctuating voltage (albeit a low voltage) via what is known as a "ground loop". Having a single common ground eliminates this problem. With sensitive equipment/instrumentation this fluctuating low voltage will cause problems.

Now, not being an electronics engineer, I don't know why Mercury make their recommendation regarding twin installations - unless they were trying to eliminate a ground loop as mentioned above. ---- Jerry/Idaho

rmart posted 07-08-2009 10:36 AM ET (US)     Profile for rmart  Send Email to rmart     
Larry, according to this this link the reason for the common ground is to prevent a fire that could be caused by the starting current passing through a wire of insufficient size.

jimh posted 07-08-2009 04:09 PM ET (US)     Profile for jimh  Send Email to jimh     
With twin engines, there are likely to be several circuit paths between the two engine blocks. There may be common grounds used in various devices which are associated with the two engines. For example, some of the instrumentation might be rigged with a common ground. These conductors are small wires, appropriate only for small currents.

We know that during engine starting very high currents flow. The beautiful thing about electricity is that it always tries to take the path of least resistance. Although there may not be any anticipated situation in which a high current might flow between the two battery negative terminals, all it would take might be a loose connection somewhere to create a situation where high current would then try to flow on some other unanticipated path. The effect of this might be for current to try to flow on little conductors associated with other circuits and wiring. You might turn the key on the starter of one engine and suddenly burn up a bunch of wiring in the other engine's tachometer gauges, or something like that.

Follow the recommended practice, and bond the two battery negative terminals together with a conductor who size is at least equal to the battery starting cable size.

[Posted from northern Lake Michigan on a long and slow connection.)

pglein posted 07-08-2009 05:11 PM ET (US)     Profile for pglein  Send Email to pglein     
There simply isn't enough information in your initial post for anyone to understand what it is you actually want to know. You say that you have two batteries, but you don't explain to what those respective batteris are connected. There are a lot of different possible ways to design a multiple battery system, especially when you throw in the variable of multiple engines. One battery could start both engines, or you could have each battery dedicated to one engine. One engine could be for house loads only, while the other is used for starting. There could be an isolator/combiner or there could not be.... the list goes on.

But I can tell you this, in general, it is ALWAYS customary to have ALL your batteries linked by large gauge wire, of the shortest length practical, on the negative terminals to create a common ground. This way, no matter what you do with the positive side of the circuits, there will always be a sufficiently sized path for the electricity to flow on the negative side.

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