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Old 09-01-2023, 08:53   #1
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Negative Battery Switch - is it necessary?

Some (older) boats come wired with both positive and negative battery switches: the negative switches the Ground Bus for all battery banks, plus one Positive Switch per battery bank (starter + house).

-Can anyone think of a good reason to keep the negative battery switch?
-Is there anything in ABYC that would ever require it?
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Old 09-01-2023, 09:11   #2
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Re: Negative Battery Switch - is it necessary?

It depends on how the rest of the boat is wired, but isolating the negative supply can reduce the chance of stray current corrosion problems and fire. I would not change the manufacturer’s installation.

It is not required by the ABYC, but is is compulsory for some other marine standards.
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Old 09-01-2023, 09:13   #3
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Re: Negative Battery Switch - is it necessary?

I would love to have a negative switch. The point is to isolate the batteries completely from any load. This is very helpful when storing the boat on the hard for any length of time, where continuous shore power isn't available to keep a charger on. There are always parasitic loads. The only caveat would be if you're stored in the water, you still want to keep power to the bilge pumps, so this might not be the best option then.
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Old 09-01-2023, 11:11   #4
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Re: Negative Battery Switch - is it necessary?

As long as you can isolate (shut off) the pos side, there is no circuit through which any load can feed so a neg switch is superfluous. Bilge pumps should be direct wired to the batteries so this isn’t a consideration.

About the only reason for a neg switch is when you have multiple isolated sets of batteries to preclude ground loops and induced stray current due to different ground potential but that is an unlikely scenario.
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Old 09-01-2023, 12:02   #5
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Re: Negative Battery Switch - is it necessary?

Quote:
Originally Posted by S/V Illusion View Post
As long as you can isolate (shut off) the pos side, there is no circuit through which any load can feed so a neg switch is superfluous. Bilge pumps should be direct wired to the batteries so this isn’t a consideration.
.
If the bilge pumps are directly wired to the battery bank, the positive side cannot be isolated by definition. So this is indeed a consideration. It is no longer then possible to isolate (shut off) the positive side when leaving the boat. There is a circuit through which any load can feed.

Without the negative battery switch all (or at least most) of the other devices on the boat (autopilots, instruments, pumps etc etc) are permanently connected to negative pole of the battery even when all switches and circuit breakers are off.

If the bilge pumps are to be left working it is not possible to disconnect their negative (or their positive) supply, but at least the negative battery switch disconnects the vast majority of yacht equipment, greatly reducing the possibility of a path for stray current corrosion and also reducing the chance of fire.
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Old 09-01-2023, 15:07   #6
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Re: Negative Battery Switch - is it necessary?

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Originally Posted by noelex 77 View Post
If the bilge pumps are directly wired to the battery bank, the positive side cannot be isolated by definition. So this is indeed a consideration. It is no longer then possible to isolate (shut off) the positive side when leaving the boat. There is a circuit through which any load can feed.
No.
As I said, bilge pumps should be direct wired to the batteries. That implies not through the panel, buss bar or any switches thereby isolating that circuit from every other electrical circuit onboard

Quote:
Without the negative battery switch all (or at least most) of the other devices on the boat (autopilots, instruments, pumps etc etc) are permanently connected to negative pole of the battery even when all switches and circuit breakers are off.
Which means they are all at ground potential - regardless of the use of a switch on that side and which therefore means not in the circuit.

Quote:
If the bilge pumps are to be left working it is not possible to disconnect their negative (or their positive) supply, but at least the negative battery switch disconnects the vast majority of yacht equipment, greatly reducing the possibility of a path for stray current corrosion and also reducing the chance of fire.
That’s what I wrote earlier
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Old 09-01-2023, 15:19   #7
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Re: Negative Battery Switch - is it necessary?

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Originally Posted by noelex 77 View Post
It depends on how the rest of the boat is wired, but isolating the negative supply can reduce the chance of stray current corrosion problems and fire. I would not change the manufacturer’s installation.

It is not required by the ABYC, but is is compulsory for some other marine standards.
I've never seen this arrangement in a boat. Could you please note one or two of the standards that require this.
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Old 09-01-2023, 15:22   #8
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Re: Negative Battery Switch - is it necessary?

Quote:
Originally Posted by S/V Illusion View Post
No.
As I said, bilge pumps should be direct wired to the batteries. That implies not through the panel, buss bar or any switches thereby isolating that circuit from every other electrical circuit onboard
The negative connection is not isolated. It is connected to every other electrical device on that bank unless there is a specific switch on the negative connection.

Nauticat installed such a switch on Glowplug’s vessel and he is asking if it advantagous to remove this.
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Old 09-01-2023, 15:28   #9
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Re: Negative Battery Switch - is it necessary?

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Originally Posted by noelex 77 View Post
The negative connection is not isolated. It is connected to every other electrical device on that bank unless there is a specific switch on the negative connection.

Nauticat installed such a switch on Glowplug’s vessel and he is asking if it advantagous to remove this.
Yes, I know what he asked but I am simply trying to correct an error.

To state it differently, if only the bilge pumps are “live” and everything else on the boat is wired through the panel which has no power when a battery switch disconnects the pos side, everything is isolated from that one “live” circuit. Having a neg switch does nothing as there is nothing to shut off.
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Old 09-01-2023, 15:31   #10
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Re: Negative Battery Switch - is it necessary?

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I've never seen this arrangement in a boat. Could you please note one or two of the standards that require this.
It is quite common on many European boats to have a battery switch to turn off the negative supply. I am surprised you have not encountered this.

Below is the first photo thrown up by Google when searching for “Jeanneau battery switches”. The black battery switch isolates the negative supply.

One example of a standard that requires this is the Australian standard for charter yachts.
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Old 09-01-2023, 16:06   #11
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Re: Negative Battery Switch - is it necessary?

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Originally Posted by S/V Illusion View Post
Yes, I know what he asked but I am simply trying to correct an error.

To state it differently, if only the bilge pumps are “live” and everything else on the boat is wired through the panel which has no power when a battery switch disconnects the pos side, everything is isolated from that one “live” circuit. Having a neg switch does nothing as there is nothing to shut off.
You are forgetting about the possibility of salt water acting as an electrolyte and making a connection. With a minor insulation fault salt water can complete the circuit between the “live” positive supply of the bilge pump and the negative supply of any of the other electrical devices on the boat.

This is the most common and fundamental mechanism of DC stray current corrosion. It only takes a very small induced voltage difference to produce extensive corrosion on parts such as the saildrive or rudder shaft. Even a circuit with high resistance is of concern.

Most marine electrical devices have their negative circuits permanently connected to the battery even when all the switches and circuit breakers are turned off. Conventional (single pole) switches only act on the positive supply.

Switching off and isolating the negative circuit to everything that is not needed greatly diminishes this risk.

The extra protection afforded by isolating the negative supply is not essential, but it is valuable. It would be a mistake to remove a system wired up like this from the factory.
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Old 09-01-2023, 16:38   #12
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Re: Negative Battery Switch - is it necessary?

I commonly convert the neg switches to a pos Parellel switch instead. Move the grounds off to a bus. They are Normally wired wrong anyways as people just add wires to either side of the switch, whichever has more space.
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Old 09-01-2023, 18:11   #13
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Re: Negative Battery Switch - is it necessary?

[QUOTE=noelex 77;3728282]You are forgetting about the possibility of salt water acting as an electrolyte and making a connection. With a minor insulation fault salt water can complete the circuit between the “live” positive supply of the bilge pump and the negative supply of any of the other electrical devices on the boat.

Could you explain this a little more for me?
I was warned that a bad connection on the bilge pump (actually on the negative side - I thought) could cause an accelerated corrosion problem (e.g. sacrificial shaft anode). Perhaps the positive supply to the bilge pump completing a circuit via the bilgewater? Anyhow, I just seek some clarification - the salt water raises the ground potential of other devices? I do not fully understand how that creates a circuit or is your point that the current intended to go through the bilge pump and its wired connection to battery ground instead also has a parallel circuit involving bilge water? (not really involving other devices). Is there a simple test for such a fault?
Many thanks.
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Old 09-01-2023, 19:30   #14
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Re: Negative Battery Switch - is it necessary?

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Could you explain this a little more for me?
I was warned that a bad connection on the bilge pump (actually on the negative side - I thought) could cause an accelerated corrosion problem (e.g. sacrificial shaft anode). Perhaps the positive supply to the bilge pump completing a circuit via the bilgewater? Anyhow, I just seek some clarification .

Salt water conducts electrical current and can act as the connection between the positive and negative sides of the battery. This circuit potentially creates rapid corrosion on metal parts that are immersed in seawater. Sail-drives and rudder shafts are the most common components involved. This is DC stray current corrosion.

In the example quoted in this thread the bilge pump is permanently connected to both the positive and negative sides of the battery. Other electrical devices have their positive supply disconnected much of the time either by the switch used to turn the device or by the main battery switch.

However, the negative supply is normally left permanently connected to all devices. This creates a potential circuit between the bilge pump positive and the negative of other devices even if these devices are “off”. Selecting “off” only disconnects the positive connection.

Turning off the negative supply removes this potential circuit and hence reduces the chance of stray current corrosion.

This is a well known and accepted solution. It also reduces the fire risk. Unfortunately it is more expensive to wire a boat this way hence this practice is confined to more expensive boats, or where the manufacturer anticipates potential sales in areas where this is a legal requirement. I would not recommend removing this option in an existing boat.
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Old 09-01-2023, 20:09   #15
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Re: Negative Battery Switch - is it necessary?

Quote:
Originally Posted by noelex 77 View Post
Salt water conducts electrical current and can act as the connection between the positive and negative sides of the battery. This circuit potentially creates rapid corrosion on metal parts that are immersed in seawater. Sail-drives and rudder shafts are the most common components involved. This is DC stray current corrosion.

In the example quoted in this thread the bilge pump is permanently connected to both the positive and negative sides of the battery. Other electrical devices have their positive supply disconnected much of the time either by the switch used to turn the device or by the main battery switch.

However, the negative supply is normally left permanently connected to all devices. This creates a potential circuit between the bilge pump positive and the negative of other devices even if these devices are “off”. Selecting “off” only disconnects the positive connection.

Turning off the negative supply removes this potential circuit and hence reduces the chance of stray current corrosion.

This is a well known and accepted solution. It also reduces the fire risk. Unfortunately it is more expensive to wire a boat this way hence this practice is confined to more expensive boats, or where the manufacturer anticipates potential sales in areas where this is a legal requirement. I would not recommend removing this option in an existing boat.
For stray current corrosion though the current needs to be passed through the component in question to cause corrosion, correct?

So it needs to be an insulation failure on the bilge pump positive before the automatic switch, and it needs to be sitting in bilge water that is electrically grounded to exterior seawater (plausible, especially with a bolt on keel), so that the current can then use the sail drive or prop shaft as a path back into the boat to battery negative. Is this right?

The reason I ask is the first scenario I imagined was water trapped in a conduit. That could create stray current (or a short), but I don’t think would result in any corrosion issues.
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