Negative busbar/Shunt

[jenevers]

Just checking that the negative busbar cable should be connected to the NON battery side of the shunt.

Correct?

[Paul C]

Correct

[jenevers]

Correct

Thanks. And ALL negative cables, whether charging cables or cables taking power FROM the batteries can be connected to the negative busbar? And the same principle for the positive busbar ??

[Paul C]

Positive busbar is different because:

 

1) You wouldn't put a shunt in the positive busbar

2) You probably have 2 banks of batteries - engine/start and domestic

3) You need isolation switches

4) Some items are fused and connected separately to the isolation switches

 

Given the above, it soon transpires that there's not that many connections which need to go to exatly the same place, so often there is less/no need for a positive bus bar.

[jenevers]

Positive busbar is different because:

 

1) You wouldn't put a shunt in the positive busbar

2) You probably have 2 banks of batteries - engine/start and domestic

3) You need isolation switches

4) Some items are fused and connected separately to the isolation switches

 

Given the above, it soon transpires that there's not that many connections which need to go to exatly the same place, so often there is less/no need for a positive bus bar.

1) true

2) correct. The starter battery is connected to the alternator via a Durite 140 amp voltage controlled relay and will only receive a charge from the alternator when the domestic batteries are full

3) I have an isolation switch between the 80 amp battery charger and the domestic batteries. I don't think I need one between the alternator and the domestic batteries....... Do I?

4) is it OK to have " in line" fuses between solar panel controller, power steering cables etc before they are connected to the positive busbar?

[jenevers]

Hmm. Just taken the paneling off the wiring between the batteries and charger, to get at the shunt, and discovered that the isolation switch is on the negative cable! There isn't an isolation switch on the positive cable but there is a mega fuse.

Looks to me like the isolation switch is on the wrong cable, am I right?

[Keeping Up]

It is not uncommon for the isolation switch to be in the negative, indeed it used to be the standard practice. The main reason was that if you have only a single alternator, then just one isolator switch will disconnect both domestic and starter batteries which as well as being cheaper is also a very good safety feature. It also has other advantages including the fact that when it is switched off the accidental dropping of either one of the positive or negative battery cables on to the hull while connecting or disconnecting a battery will not cause a big bang. Gibbo - a once highly respected electrical expert on this Forum - years ago published on his website a list of reasons why the switch should be on the positive side; the only one of those reasons which I (a retired electrical expert) agree with is the one which states that the positive side is where nowadays people expect to find it. Personally by choice I have my switches (both of them now that I have a two-alternator setup) in the negatives and megafuses in the positive.

[jenevers]

It is not uncommon for the isolation switch to be in the negative, indeed it used to be the standard practice. The main reason was that if you have only a single alternator, then just one isolator switch will disconnect both domestic and starter batteries which as well as being cheaper is also a very good safety feature. It also has other advantages including the fact that when it is switched off the accidental dropping of either one of the positive or negative battery cables on to the hull while connecting or disconnecting a battery will not cause a big bang. Gibbo - a once highly respected electrical expert on this Forum - years ago published on his website a list of reasons why the switch should be on the positive side; the only one of those reasons which I (a retired electrical expert) agree with is the one which states that the positive side is where nowadays people expect to find it. Personally by choice I have my switches (both of them now that I have a two-alternator setup) in the negatives and megafuses in the positive.

Well that makes me feel better! I thought I was going to have to change the isolation switch around on top of the other wiring that needs doing.

However, I also discovered that the cables from the battery charger are connected across the 4 x 180ah parallel wired, batteries in the accepted diagonal method, ie the positive cable is connected to no 1 battery and the negative cable to no 4 battery BUT from the alternator it's the opposite way around, ie the positive cable is connected to no 4 battery and the negative to no 1 battery. Can anyone se a problem with that?

[by'eck]

It is not uncommon for the isolation switch to be in the negative, indeed it used to be the standard practice. The main reason was that if you have only a single alternator, then just one isolator switch will disconnect both domestic and starter batteries which as well as being cheaper is also a very good safety feature. It also has other advantages including the fact that when it is switched off the accidental dropping of either one of the positive or negative battery cables on to the hull while connecting or disconnecting a battery will not cause a big bang. Gibbo - a once highly respected electrical expert on this Forum - years ago published on his website a list of reasons why the switch should be on the positive side; the only one of those reasons which I (a retired electrical expert) agree with is the one which states that the positive side is where nowadays people expect to find it. Personally by choice I have my switches (both of them now that I have a two-alternator setup) in the negatives and megafuses in the positive.

 

One reason not to have the battery isolator in the negative feed, and to me the most important/relevant, is that some low current equipment requires at permanent feed from the batteries, alarms, car radios, heating timer, bilge pump etc. These devices will need a direct feed to battery negative to allow them to operate when the single negative isolator switch is turned off. All fine, but what happens when the thin negative wire typically used to supply such devices is asked to carry the current from a more power hungry device it controls, since the normal path has been interrupted by operation of the isolator?

 

Related to this are devices such as car radios that have the casing (electrically in contact with the hull) at battery negative. The thin cable that bypasses the isolator to feed the radio memory/clock will provide a path for any other device which (maybe unknowingly) has a return via its casing, and likely also connected to the hull.

 

This is not wild and unlikely speculatiion, I found it happening on my last boat which was fitted with negative isolation, and which had a GRP hull BTW.