Dual battery set up help

[Mischief Maker]

Question if I remove the negative wire from motor off the battery than measure from negative on battery to Frame of boat I get 12v it's not until I join negative up again I get same potential again is this right?

[stevemid]

Sounds to me like a + side battery shorted out to the hull. Does it change with isolation switches one by one in the off position?


I stand behind everything I say, except usually I'm prone.

[Mischief Maker]

I might pull the fuse out which feed accessorie bus bar and see what happens

[stevemid]

Yes we'll start with nothing connected and measure the voltage to the hull as you connect things


I stand behind everything I say, except usually I'm prone.

[Mischief Maker]

Turned isolator off, than disconnected fuse that feeds switch panel, dropped voltage from 12 to 9v from negative to boat frame. This boat will be the end of me haha

[stevemid]

Get the blood hounds out. If you have to physically disconnect the batteries. Don't forget the solar panels as a 12v source!


I stand behind everything I say, except usually I'm prone.

[scottar]

Question if I remove the negative wire from motor off the battery than measure from negative on battery to Frame of boat I get 12v it's not until I join negative up again I get same potential again is this right?

It could well be normal. The link could be occurring via the engine ECU or some other electronic item. What you are seeing is a common result with electronic items when there is a negative feed fault ie - the chassis shows 12 volts. In your case the fault is that you have removed the terminal. The only wild card is the solar panels.

[stevemid]

Scott I've only ever had glass boats but is this truly normal? Sounds wrong.



I stand behind everything I say, except usually I'm prone.

[Mischief Maker]

So you rekon disconnect solar panels as we'll, all seems to work ok just bit worried when I disconnected the neg cable from motor and got that reading

[Mischief Maker]

I also had battery isolator turned off, and for some reason the red arc wouldn't turn on until neg motor cable was out back on start battery

[Mischief Maker]

* isolator was back on when tried turning red arc on

[scottar]

Yes it can be totally normal and is due to resistors etc used to bias transistors etc in the power supply of electronic items. It is not unusual for radios and as such is definately the case for motors to use the chassis as a ground which is connected to negative. Any electronic items on the motor will therefore have a negative connection connected to the chassis either directly or via the negative wiring. It is quite common for power supplies in these items to have biasing resistors between the voltage input and ground. A resistor is effectively a piece of wire that that when a voltage or potential is placed across it produces a voltage drop relative to the current being drawn. If the current is zero because you have removed the negative, the voltage drop across the resistor will be zero and as such it will have the input voltage at the end which is usually at negative potential when the circuit is connected.

[stevemid]

So the 12v reading with the neg battery cable off was a total wild goose chase. I get that. Mischief, it sounds like putting it back on is the answer to that one

Scottar what did you mean when you said "the only wild card is the solar panels?"

[scottar]

The solar makes its own power independant of the batteries and there is a remote possibility that there could be a circuit back to the hull via a damaged cable. I don't think there is in this case because everthing does work as required when it is all hooked up apart from the meter. I suspect the issue with that will be the location of the shunt or charging sources interfering with each other but without being able to actually lay hands on it all it will be a bit tricky to sort out

[stevemid]

The solar makes its own power independant of the batteries and there is a remote possibility that there could be a circuit back to the hull via a damaged cable. I don't think there is in this case because everthing does work as required when it is all hooked up apart from the meter. I suspect the issue with that will be the location of the shunt or charging sources interfering with each other but without being able to actually lay hands on it all it will be a bit tricky to sort out

Ya I had the same suspicions. With the shunt, Mischief has his Aux Battery negatives joined together but the batteries are on opposite sides of the boat. That would make the shunt closer (probably) to one battery that the other. I'm not sure of the cable sizes but I wondered if that could be a problem. Still curious about the battery inventory and whether they are same age/type etc.