Busbars and batteries and shunts

[Col_T]

So, I'm getting ready to install a BMV battery monitor, which means installing a shunt, and I'll take the opportunity to tidy-up the battery wiring. The inter-connect cables are 70mm CSA, so no issues there, but there are too many +ve and -ve connections to sit on a single battery post, hence the busbars.

 

The shunt will go in the -ve line, and be the only thing connected directly to the domestic batteries, except for the inter-connects and the -ve connection to starter battery. The other end of the shunt will connect to the -ve busbar, and here is where the question lies. My instinct is to connect the -ve from the solar charger to the post on the busbar that connects to the shunt, then connect the -ves from the alternator and battery charger to the next post, and then all other -ve cables to the remaining posts. Sound okay, or is there a batter way of wiring this?

[Jen-in-Wellies]

Assuming the bus bar design will suit (no picture, or link), I'd stick the connection to the shunt in the middle, then attach the other negative wires each side, with the highest current ones closest to the shunt connection. In order probably inverter, if fitted, closest (100's of A at times), followed by alternator batt charger and solar, (10's of A's for long periods), followed by domestic 12V (usually <10A most of the time). Minimises resistance and connection numbers.

Jen

Edited May 24, 2020 by Jen-in-Wellies

[Tony Brooks]

Agreed but as volt drop on charging circuits (especially high current ones) is critical I am not sure that I would not put the alternator negative on the shunt rather than the busbar. That looses two extra connections I think at the expense of having two connections on one stud.

Edited May 24, 2020 by Tony Brooks

[WotEver]

3 minutes ago, Tony Brooks said:

Agreed but as volt drop on charging circuits (especially high current ones) is critical I am not sure that I would not put the alternator negative on the shunt rather than the busbar.

That’s what I’d do

[Col_T]

Thanks for the comments, gentlemen. 

 

The busbar bar is one of these - https://www.12voltplanet.co.uk/4-point-negative-distribution-block-busbar.html - slightly over-specced at 210A, especially as the advice is to connect the alternator direct onto the busbar side of the shunt! It's not terribly clear from the photo, but the leftmost stud is slightly smaller than the other three, so @Jen-in-Wellies idea of connecting the shunt to the middle stud will work well - I'll leave the smaller stud unoccupied.

 

I also have an equivalent busbar for the +ve side. @Tony Brooks, would the advice be to connect the alternator +ve direct to the battery, and then everything else via the busbar?

 

For clarity, the shunt will be connected to the -ve of battery no. 1 of the five battery bank, with the +ve busbar connected to the +ve of battery no. 5.

[Iain_S]

1 hour ago, Col_T said:

So, I'm getting ready to install a BMV battery monitor, which means installing a shunt, and I'll take the opportunity to tidy-up the battery wiring. The inter-connect cables are 70mm CSA, so no issues there, but there are too many +ve and -ve connections to sit on a single battery post, hence the busbars.

 

The shunt will go in the -ve line, and be the only thing connected directly to the domestic batteries, except for the inter-connects and the -ve connection to starter battery. The other end of the shunt will connect to the -ve busbar, and here is where the question lies. My instinct is to connect the -ve from the solar charger to the post on the busbar that connects to the shunt, then connect the -ves from the alternator and battery charger to the next post, and then all other -ve cables to the remaining posts. Sound okay, or is there a batter way of wiring this?

The -ve  connection to the starter battery goes on the "boat" side of the shunt. Otherwise, most of the charging current from the alternator will bypass the shunt.

[Tony Brooks]

1 minute ago, Iain_S said:

The -ve  connection to the starter battery goes on the "boat" side of the shunt. Otherwise, most of the charging current from the alternator will bypass the shunt.

Why?

The negative connection from the engine battery is normally connected to the domestic battery so if its a single alternator boat all the charge must pass through the shunt. I accept the ammeter will not then tell you exactly how much charge is going into the domestic bank but as the engine battery spends most of its time fully charged I don't see that as a problem.  If its a twin alternator boat the fact it won't matter. The only think to consider is on a single alternator boat how accurate you want the Amps and its derived readings to be.

[Tony Brooks]

23 minutes ago, Col_T said:

Thanks for the comments, gentlemen. 

 

The busbar bar is one of these - https://www.12voltplanet.co.uk/4-point-negative-distribution-block-busbar.html - slightly over-specced at 210A, especially as the advice is to connect the alternator direct onto the busbar side of the shunt! It's not terribly clear from the photo, but the leftmost stud is slightly smaller than the other three, so @Jen-in-Wellies idea of connecting the shunt to the middle stud will work well - I'll leave the smaller stud unoccupied.

 

I also have an equivalent busbar for the +ve side. @Tony Brooks, would the advice be to connect the alternator +ve direct to the battery, and then everything else via the busbar?

 

For clarity, the shunt will be connected to the -ve of battery no. 1 of the five battery bank, with the +ve busbar connected to the +ve of battery no. 5.

 

To be strictly BSS compliant the alternator output (+) should go to the load side of the master switch, not the battery side and certainly not to the battery. several on here, e included, think that is not an ideal situation so on my boat the Alternator positive would be fitted to the battery side of the master switch but with a lead long enough to easily connect to the load side in case the BSS examiner noticed. However that is not a  recommendation, its just what I would do.

[David Mack]

3 hours ago, Col_T said:

there are too many +ve and -ve connections to sit on a single battery post, hence the busbars.

 

I don't understand this bit. From the domestic battery bank positive you have a single wire leading to the isolator, and from the negative you have a single wire leading to the shunt. So if you wire your bank as below, you have a maximum of two connections to each battery post.

[Col_T]

Thank for that, @Tony Brooks, and apologies for my sloppy English. The domestic alternator +ve goes to an isolator switch and then a fuse before connecting to the domestic battery +ve terminal. As it happens, both the engine alternator and battery charger +ves go via their own isolator switch and fuse before reaching the relevant battery. It was how the boat was wired when we bought it and has passed two BSS examinations so far. Having clarified that, would you recommend the domestic +ve route to be alternator => isolator => fuse => busbar => battery or is it better to leave out the busbar?

 

Also, and just for completeness, it's a two alternator boat.

[Col_T]

9 minutes ago, David Mack said:

 

I don't understand this bit. From the domestic battery bank positive you have a single wire leading to the isolator, and from the negative you have a single wire leading to the shunt. So if you wire your bank as below, you have a maximum of two connections to each battery post.

As presently wired, before fitting shunt and/or busbars, the nice and neat "To installation" in your diagram comprises multiple connections to the batteries. From memory, there are two connections to the domestics fuse board, and one for each of the MV heater, battery charger, solar controller and domestic alternator. The hope is that the wiring will be as neat and simple as your diagram after fitting the shunt and busbar!

[Tony Brooks]

1 hour ago, Col_T said:

Thank for that, @Tony Brooks, and apologies for my sloppy English. The domestic alternator +ve goes to an isolator switch and then a fuse before connecting to the domestic battery +ve terminal. As it happens, both the engine alternator and battery charger +ves go via their own isolator switch and fuse before reaching the relevant battery. It was how the boat was wired when we bought it and has passed two BSS examinations so far. Having clarified that, would you recommend the domestic +ve route to be alternator => isolator => fuse => busbar => battery or is it better to leave out the busbar?

 

Also, and just for completeness, it's a two alternator boat.

I don't fully grasp it ether but it the alternator positive goes to an isolator switch with the other side of the switch connected to the battery then it sounds as if it is BSS compliant although the alternator may be wrecked if that switch is turned off with the engine running.

[WotEver]

10 minutes ago, Tony Brooks said:

I don't fully grasp it ether but it the alternator positive goes to an isolator switch with the other side of the switch connected to the battery then it sounds as if it is BSS compliant although the alternator may be wrecked if that switch is turned off with the engine running.

Likewise I wouldn't put a fuse in that circuit either.  It's not required by the BSS and if it blew while the alternator was charging very low batteries it would wreck the alternator.

[Tony Brooks]

4 minutes ago, WotEver said:

Likewise I wouldn't put a fuse in that circuit either.  It's not required by the BSS and if it blew while the alternator was charging very low batteries it would wreck the alternator.

Agreed unless that cable also feeds something else or the cable run to the switch is very long and vulnerable to damage. If it were I would rewire to keep the alternator supply off that cable.

[Detling]

If you leave the engine -'ve direct to the domestic -'ve and not via the shunt you have wasted your money on the BMV as the only thing it will tell you correctly is the voltage. It has to measure all current in from the alternator -'ve and any solar and charger, as well as all current out to domestic or inverter loads, anything less and it can only tell lies all the time, for amphours used,  amps flowing and Soc (usually wrong anyway).

[Tony Brooks]

but if the two banks negatives are joined and then either bank main negative is connected to the shunt all the current must pass through the shunt.

[WotEver]

5 minutes ago, Tony Brooks said:

but if the two banks negatives are joined and then either bank main negative is connected to the shunt all the current must pass through the shunt.

Yes, but that's not what Col said.  He said he'd moved everything off the domestics negative except the starter negative.  He needs to move that too.

 

Basically, @Col_T, the only connection to the domestic negative post should be the shunt.

(Unless you have a Smartgauge)

[Tony Brooks]

Agreed

[Col_T]

Okay, so what we have is 5 off 110Ah batteries - we'll call them by number, where battery 1 is the furthest from the bulk-headed mounted charger, and battery 5 is closest to the charger and immediately next to the starter battery.

 

What I presently intend is that the -ve posts of all 5 batteries will be connected by interlink cables, so that battery 1 is linked to battery 2, which is linked to battery 3, which is linked to battery 4, which is linked to battery 5, which is linked to the engine starter battery. After that, nothing will be connected to the -ve post of any battery except battery 1 -ve, which will have the shunt connected to it. 

 

There is no Smartgauge to complicate things, so does the above sound okay? If not, and @WotEver and @Tony Brooks really do mean that the domestic and starter negatives should not be connected, then could someone explain why they are connected now, and why they shouldn't be connected when the shunt goes in because, with apologies for being thick, I genuinely don't understand!

 

 

[WotEver]

1 minute ago, Col_T said:

does the above sound okay?

No.

1 minute ago, Col_T said:

If not, and @WotEver and @Tony Brooks really do mean that the domestic and starter negatives should not be connected, then could someone explain why they are connected now, and why they shouldn't be connected when the shunt goes in because, with apologies for being thick, I genuinely don't understand!

They'll still be connected, just on the other side of the shunt.  The reason for moving the link is so that all the power going into and out of the domestic bank goes through the shunt.  If you keep the starter negative connected to the domestics then circuits can bypass the shunt (via starter negative-->engine bond-->alternator for example).

[Tony Brooks]

5 minutes ago, Col_T said:

Okay, so what we have is 5 off 110Ah batteries - we'll call them by number, where battery 1 is the furthest from the bulk-headed mounted charger, and battery 5 is closest to the charger and immediately next to the starter battery.

 

What I presently intend is that the -ve posts of all 5 batteries will be connected by interlink cables, so that battery 1 is linked to battery 2, which is linked to battery 3, which is linked to battery 4, which is linked to battery 5, which is linked to the engine starter battery. After that, nothing will be connected to the -ve post of any battery except battery 1 -ve, which will have the shunt connected to it. 

 

There is no Smartgauge to complicate things, so does the above sound okay? If not, and @WotEver and @Tony Brooks really do mean that the domestic and starter negatives should not be connected, then could someone explain why they are connected now, and why they shouldn't be connected when the shunt goes in because, with apologies for being thick, I genuinely don't understand!

 

 

 

I don't think either of us said that. I think the two bank negatives shoudl be connected and I think Wotever said the same.

 

Its where they are connected.

Edited May 24, 2020 by Tony Brooks

[Col_T]

Aha - if only I had read what you had actually written, rather than what I thought you had written! That'll be another cable to make up rather than re-use - bother.

 

Thanks for all your patience.

 

Stay well.

[nicknorman]

I’m a bit late to this one but if the point needs reinforcing there must be no direct connection between the engine battery negative / engine negative, and the domestic battery negative. Only via the shunt. So the only connection to the battery negative is the shunt. Nothing else at all (except a smartgauge, if you have one).

[canals are us?]

Interesting thread. I wired my battery bank and tidied it up and added inverter and battery monitor and busbars as not enough room for the add ons on the main battery posts.

 

Still need to fit a cover over the positive take off! Has a plywood lid to stop anything falling onto it.

 

James.

Edited May 24, 2020 by canals are us?