Two Motors - Common Grounding ?

[US1Fountain]

My 2 motors curently each have their own neg cable from a battery directly to their motor stud. (2 motors, 2 batteries, 2 cables) There is a cable that connects both motors together at the motor studs. I have no neg busbar and the batteries are not conected together.

I am doing some rewireing and adding a neg busbar, fuseblock, etc.

The SB battery is being reconfiged to use as a house/SB motor only start circut. Other battery will be the port motor start only. I will be taking the neg cable from the SB motor and connecting to the bus, then from the buss to the battery. To that bus, will be my AC safety and DC ground common point.

Question, should I also run the port motor battery neg cable to that new bus, and then back out to the port motor, for an all neg common grounding connection, or do I leave the port battery neg cable going directly to the port motor ground stud? I know electrically it will work the same either way, it's the galvanic and grounding safety circut I'm having troubles understanding.

With both battery banks neg cable and motor grounding studs neg cables going to the bus and then the common motor grounding cable, these will make for 2 circuts that tie both motors together.

Thanks,
Jerry

[goboatingnow]

NO, thats will create ground loops, leave the Port battery as is with the negative starp across to the SB battery. Then just run a negative from the SB battery to the negative bus.

Note that the setup is less then optimal, really a seperate house battery is way better. Firstly youll have lots of starting transients and load dumps on the "house circuit and secondly starter batteries and house batteries are different specs.

ABYC requires a connection between DC grouns and protective earth, I understand it , but I hate it. I prefer to use RCDs and no link

Dave

[delmarrey]

What about your accessories? Which battery do they run from?

The reason I ask is if the batteries (-) are connected and only one motor is running then it could possibly be charging both batteries thru the accessory circuit, unless there are diodes in the system to stop the cross over. W/o the batteries connected the charge will only go to the running motors battery.

The accessory system wiring may not be able to carry the load if a battery were low.

[US1Fountain]

Let me try this. I realize it's a very crude drawing, but I have no software nor knowledge on how to use one if I did, so if we could go easy on my schematic abilities it'd be appreciated. Only grounds are shown for simplicity since this is what is being addressed.

Battery 1 is for start only for the P motor only.
Battery 2 & 3 will be the SB start and the house bank
Cable 2 is the common connection cable.

All access/house loads will go to the bank of 2/3, connected to pos terminal of battery 2
There is no connection on the POS terminals between Bat 1 and Battery bank 2/3, except thru a combining switch for emergency, otherwise 1 is isolated from 2/3.
P alt will charge Bat 1, S alt will charge Bats 2/3.
I have an ACR that is not shown that will handle any cross bank charging if required. Both motors are always ran together.

goboatingnow, if I understand you correctly, you are saying to leave cable 1 as is, but to add cable 4, correct?

I understand the house vs starting battery usage/separation, but since '95, this boat has just had 2 batteries used for both starting and house loads, plus I do not have any of the electronics that most on here do to be concerned of the transients and load dumps. For my use I am fine using the house bank to start the 1 motor. Your opinion is noted and appreciated.

Thanks for all the advise offered.
Jerry

[US1Fountain]

This is the current setup for the grounds just to show where I'm starting from.

[btrayfors]

Lose wire #4. It's redundant, and would cause a ground loop.

Bill

[US1Fountain]

Thanks.
So I will disregard the #4 wire, and the #1 wire will stay as is, connected directly to the port motor stud from battery, w/o going thru the neg bus.

Would you agree that it is best to relocate the AC safety wire and the DC ground wires from the 2 motor studs and connect to the newly installed neg buss for a single point connection?

pictures are worth a 1000 words at times.

[CharlieJ]

E-11 Fig. 18- Grounding.pdf

This can be a confusing discussion! Here is a very clear diagram from the latest ABYC E-11 Standard.


Hope this helps.
Charlie

[Fishspearit]

Originally Posted by btrayfors

Lose wire #4. It's redundant, and would cause a ground loop.

Bill

I don't usually disagree or doubt anything Bill says, but I think you need to keep wire #4 for the same reason you have the two motor grounds tied together (with cable #2). That reason, I've always been told, is that if the battery ground comes loose it will pull the current through the fuel lines to the other engine where it then makes the ground connection. Without wire #4 your grounds don't connect. Though that probably just means you're engine won't start, rather than trying to pull the current through the fuel line.

[US1Fountain]

Yes, I refer to that pic many times. The difference is how the batteries are connected, 2 cables to each motor stud that confuse me. Doesn't that pic represent a ground loop?


In my application, the grounding buss and plates do not apply to my boat. I have the standard Mercathode system and anodes on my drives, So I ignore the lower half grounding representations.

Thanks

[delmarrey]

Originally Posted by CharlieJ

Attachment 25107

This can be a confusing discussion! Here is a very clear diagram from the latest ABYC E-11 Standard.


Hope this helps.
Charlie

This one is fine and dandy for a three battery system. The OP only has two batteries.

[US1Fountain]

Fuel lines are rubber, if that makes any difference.

When you say I have 2 batteries, is that saying 2 battery banks (which I will have) or just 2 batteries in total, (which I am changing from)?

Jerry


For clarification, the 1st picture is where I am trying to end up, with 2 dedicated battery banks, neg. busbar, moving the AC Safety (green wire)and DC ground (black wire) to the common connection point at the newly installed busbar. The AC ground can stay at teh motor stud if nothing is to be gained from moving it, but I think the DC ground needs to be moved.

[CharlieJ]

delmarrey-
It is a generalized wiring diagram used to illustrate the general principles involved.

I still hope it helps.
Charlie

[Fishspearit]

Originally Posted by US1Fountain

Fuel lines are rubber, if that makes any difference.

No, it doesn't.

My point was just that the cable connecting the grounds of both motors, which most people don't understand the reasoning for, is that if the ground comes off(or develops bad connection) on one motor then the only source for current back to the negative of a battery(while starting the engine) is through the fuel itself(liquids generally being very good conductors) to the other engine which has a good ground connection to the negative of a battery. You can probably imagine the issues involved with a gas engine pulling 100amps or so through the fuel lines....

So that ground cable cross connect, which most people think of as just a redundant ground wire, serves an important safety purpose.

The only problem I see with your drawing #1 is that it's probably best to go straight from the battery ground to the engine ground rather than through a buss bar connection because of the large currents being pulled through. It would work fine the way it is in your drawing, just one more place to develop a high resistance connection down the road and leave you wondering why your engine is not starting.

Keeping wire #4 also enables you to start your port engine by using the combining switch you mentioned you had for the two battery banks, easier in a pinch than crawling down in the bilge with jumper cables...

[US1Fountain]

Thanks for explaining the purpose of the wire #2. I see what is being said about the ground wire that ties both banks together. I didn't include it in this drawing, (forgot).

So this is the latest (referred to as pic #3) understanding with the both motor grounds going directly to the battery grounds bypassing the busbar. I have no concern going thru the busbar as in the 2nd picture if it meets code. But if not, then what I'm unsure of is do I run wire #4 to battery #2 as shown, or does it need to go to the SB motor ground stud? Going to the motor stud requires current flow to travel back thru wire #3 for an additional 8' run. Simply going to the same stud on the battery that the motor cable is connected to, that stud will be the joining connection, eliminating the need to size the motor cable for the house load and the additional 8' run.

The problem I'm having when referring to the ABYC schematic, that it uses 2 start batteries and 1 house. I'm using 1 bank for both a start and house purpose.

My main concern here is if I am creating some type of issue that is a safety issue or chance of introducing DC current into my motors and to the drives thus eating them away.

Thanks for all the help guys, very much appreciated.