Connections to a multiple-battery bank

[Peregrine]

On the SmartGauge website, Chris Gibbons has a page about connecting multiple batteries, in which he recommends a wiring arrangement he attributes to this forum's smileypete. His arguments are detailed and suitably convincing, and I'm trying to work out how to follow his advice on Erin Mae.

 

However, he ends the article with: "And finally, finally, we keep getting asked where the chargers should be connected to. We didn't address this question because it seemed so blatantly obvious where they should be connected that it never occurred to us that anyone might be unsure. The chargers should always be connected to the same points as the loads. Without exception. " In contrast with the rest of the page, there is no argument here, just a statement. So I'd like to know the rationale for this statement. I can see why you would want the +ve and -ve charging leads connected to opposite ends of the bank, but not why they should have be to the same points as the load leads.

 

The reason is that, as far as I can see, the +ve connection from my Victron MultiPlus inverter/charger goes to one end of my 4-battery bank, via its own isolator, and the alternator and solar charging sources are connected to the same point. However, the feed from the batteries to the 12v distribution panel is taken (via a different isolator) from the other end. Now I recognise that the Victron / battery lead is both for charging (from a landline), and for supply (to the inverter when cruising), so I think this needs to be tidied up regardless. Nevertheless, I want to be as sure as I can be about the principles here before I start getting into what I think will be quite a complex re-wiring exercise.

[Paul C]

Because when the charger is on and some other electrical loads on the boat are on, its the shortest path. Or rather, shorter path. You couldn't connect the charger straight to the fusebox or circuit breaker panel because loads need to go via the isolator switch, which is close to the battery - so it MUST go between the battery and the isolator switch. In a "normal" boat with battery bank wiring interconnects, one lead coming from the chosen terminal to the inverter, pragmatically there would be 3 possible places to connect it using an existing connection:

 

1) On the isolator switch's terminal

2) On the battery post terminal which the isolator switch is connected to

3) On another battery post terminal

 

If it were on the isolator switch's terminal then there would be another additional terminal and length of wire for the charger's current to flow through - which would not be good - so the obvious choice is (2), on the battery post terminal which the isolator switch (and hence, all the other electrics of the boat, one would hope*) are connected to.

 

* Fused connections eg solar panels, items on the permissable list of items which can stay on with the battery isolator off; and other chargers eg alternator should also be connected here for similar reasons.

[BEngo]

The idea is that the charger input current (a and voltage drops) is perfectly shared between the batteries as the load output current/connection voltage drops is under the recommended arrangement.. If the battery was always either only being charged or only supplying the load then you could use the opposite diagonal to the load connections and get the same result. However, most modern chargers also service the load simultaneously. Servicing the load is the most important thing

to avoid (some part of) the battery being discharged or more slowly charged than it could be.

 

Connect the charger inputs to where the load goes out and this will happen automatically and the battery will have the remaining charger capacity, evenly distributed across the component batteries IYSWIM.

 

N

[Tony Brooks]

That sounds as if something is wrong. any chance of a diagram.

 

* unless the regs have changed in the last few years I think this is wrong re the alternator although I agree I would prefer I that way. The last time I looked the BSS required the alternator charging lead to be isolated by the master switch although I know many are not.

Edited May 9, 2015 by Tony Brooks

[Paul C]

That sounds as if something is wrong. any chance of a diagram.

 

* unless the regs have changed in the last few years I think this is wrong re the alternator although I agree I would prefer I that way. The last time I looked the BSS required the alternator charging lead to be isolated by the master switch although I know many are not.

 

The regulations would suggest that an alternator must go through the isolator switch, where a battery charger, solar or wind, doesn't need to (just needs to be fused) but it also mentions "any other equipment where the manufacturer’s instructions indicate or specifically require direct connection to a battery" so an alternator might be able to be connected direct, with supporting documentation. Its an interesting one.

[Peregrine]

That sounds as if something is wrong. any chance of a diagram.

 

* unless the regs have changed in the last few years I think this is wrong re the alternator although I agree I would prefer I that way. The last time I looked the BSS required the alternator charging lead to be isolated by the master switch although I know many are not.

 

Tony, I'm not sure if you meant that my current system is wrong or that Paul C's suggestion was wrong, and whether you meant a diagram from him or of the actual way my wiring has been done.

 

 

The regulations would suggest that an alternator must go through the isolator switch, where a battery charger, solar or wind, doesn't need to (just needs to be fused) but it also mentions "any other equipment where the manufacturer’s instructions indicate or specifically require direct connection to a battery" so an alternator might be able to be connected direct, with supporting documentation. Its an interesting one.

 

I have a Stirling AB12160 alternator-battery gizmo combining the output from two alternators and governing the battery charging. So when I said "alternator and solar charging sources" I really meant the output from the AB12160's terminal, which goes to the same battery post as the inverter link, without an extra isolator. The solar controller output goes via a fuse to the AB12160's terminal, to make use of the hefty charging cable already in place.

[Tony Brooks]

I meant that on the face of it your set up sounds not to be the optimum however the way the engine battery negative usually connects to the domestic bank and both banks are next to each other has caused confusion on more than one occasion in the past. I asked for a diagram just to be sure I understand what you are describing and there are no misconceptions.

 

It sounds as if your 12V feed should be moved to the other end but not if its the engine system 12V feed.

[Peregrine]

I meant that on the face of it your set up sounds not to be the optimum however the way the engine battery negative usually connects to the domestic bank and both banks are next to each other has caused confusion on more than one occasion in the past. I asked for a diagram just to be sure I understand what you are describing and there are no misconceptions.

 

It sounds as if your 12V feed should be moved to the other end but not if its the engine system 12V feed.

 

Thanks for the clarification. We've had to come away from Erin Mae for a couple of weeks, so I can't trace out an accurate diagram just now. I do know that it doesn't quite match the generally excellent official Aqualine wiring diagram, and that not all the differences are due to the installation two years ago of the AB12160 unit.

 

I agree about the potential confusion with the wiring for the engine starter battery – cable identification is one of the issues, especially of some the negative leads – but this is only about the domestic bank. The two isolator switches are either part of a double unit or else very close together, and are sited close to one end of the battery box – the wrong end (!) if, as seems probable, I need to re-route from the domestic switch down to the far end of the bank.

[smileypete]

For up to four or five batteries, the classic 'ladder' with diagonal takeoffs should be fine with decent size cable and good connections.

 

As mentioned, there's also this way but it means getting an extra terminal onto a batt post which isn't always practical:

 

Sometimes the need for a bus bar can be minimised by carefully allocating connections to the available terminals on batteries, isolators and fuse holders.

 

cheers, Pete.

~smpt~

Edited May 10, 2015 by smileypete

[Peregrine]

For up to four or five batteries, the classic 'ladder' with diagonal takeoffs should be fine with decent size cable and good connections.

 

As mentioned, there's also this way but it means getting an extra terminal onto a batt post which isn't always practical:

 

Sometimes the need for a bus bar can be minimised by carefully allocating connections to the available terminals on batteries, isolators and fuse holders.

 

cheers, Pete.

~smpt~

 

 

Well, well, Pete. I've been wondering how to implement Chris Gibbons' representation of your scheme, in a situation where I think it may be difficult to "stretch" the current charge lead (from my Stirling gizmo) to the +ve post on the second battery in the bank – because that's how Chris shows it. Last night in bed it suddenly occurred to me that the cables could be arranged so the take-off points are from the end batteries. I've just spent 15 minutes drawing it up, ready to post in this thread – and you've beaten me to it! I wonder why the SmartGauge site shows it as it does. Perhaps that was how you originally drew it up.

 

Your point about extra terminals on a battery post is well taken, especially since there are some sensors attached, and some negative cables I shall have to trace when we get back to Erin Mae. But my batteries have an extra screw terminal as well as the stud, so that will help. The main thing will be to see whether, with the lead from the isolator switch moved to the "proper" end of the bank, I end up with the ladder / diagonal setup, and that will depend on what my -ve cables are actually doing. If they're OK, I may just leave them, as you suggest.

[nicknorman]

If the load being taken from the batteries to the boat is "on the diagonal" so as to minimise any differences between the batteries during discharge, and the charger is across the diagonal so as to minimise any differences between the batteries during charge, I fail to see how it can matter if these aforementioned diagonals are the opposite ones. For a setup designed to run an inverter (ie thick wires) and bearing in mind that when the charger is on, the inverter isn't, and hence 12v loads into the boat will be light compared to the capability, I can't see any significant possibility for any battery load mismatching when on charge and normal 12v boat services are on.

 

There is an awful lot of hype, excitement and sheepism on this forum about this subject, but if you ever actually do any testing of load and charge mismatch resulting from non-optimal configurations, provided the interconnects are adequately sized and in reasonable condition, such mismatches are too small to be detected and thus completely inconsequential. Relax and spend your time doing something useful like examining your navel, rather than messing about moving the wiring that is perfectly OK as it is.

[spadefoot]

I tend to agree Nicknorman, people likes to tweek tho'. You should see what geeks get up to with perfectly reasonable hi-fi rather than just listening to it (maybe you know already)

[Peregrine]

If the load being taken from the batteries to the boat is "on the diagonal" so as to minimise any differences between the batteries during discharge, and the charger is across the diagonal so as to minimise any differences between the batteries during charge, I fail to see how it can matter if these aforementioned diagonals are the opposite ones.

 

This was my initial query, based on Chris Gibbons' statement. Except in relation to how the SmartGauge actually works, that website is chock full of explanation. So coming across his final but unargued assertion, I wanted to understand the rationale.

 

It was when I examined Erin Mae's wiring in more detail that I realised that the actual configuration means that the 12v supply is taken from one end of the bank, while the supply to the inverter is taken from the other end – one of them is not on a "diagonal". As I understand it, that's a situation which could cause an imbalance and premature battery ageing, and probably ought to be changed.

[nicknorman]

This was my initial query, based on Chris Gibbons' statement. Except in relation to how the SmartGauge actually works, that website is chock full of explanation. So coming across his final but unargued assertion, I wanted to understand the rationale.

 

It was when I examined Erin Mae's wiring in more detail that I realised that the actual configuration means that the 12v supply is taken from one end of the bank, while the supply to the inverter is taken from the other end – one of them is not on a "diagonal". As I understand it, that's a situation which could cause an imbalance and premature battery ageing, and probably ought to be changed.

Debateable. Our bank was from new not wired diagonally with the alternator at one end and the 2500w inverter charger at the other, however the interconnects were 70mm^2. After reading the SG website I did some tests and with 175A coming in from our alternator, or with 200A going into the inverter, I was unable to detect and difference in the current going to/supplied from each of our 4 x 110AH batteries, and unable to detect any difference between the batteries' terminal voltage within the meter's accuracy of about 5mV. My conclusion was therefore that whilst there is a theoretical mismatch, in practice it's too small to worry about.

[Peregrine]

Our bank was from new not wired diagonally with the alternator at one end and the 2500w inverter charger at the other, however the interconnects were 70mm^2. After reading the SG website I did some tests and with 175A coming in from our alternator, or with 200A going into the inverter,

 

Doesn't this mean that what you're commenting on is having a charging source and an inverter load at opposite ends (which was certainly my original question) and concluding that it's fine? Whereas what we've moved on to discussing is whether it's OK or not to have two loads (12v circuit and inverter load) placed, one at one end and the other at the other end of the bank.

[smileypete]

Well, well, Pete. I've been wondering how to implement Chris Gibbons' representation of your scheme, in a situation where I think it may be difficult to "stretch" the current charge lead (from my Stirling gizmo) to the +ve post on the second battery in the bank – because that's how Chris shows it. Last night in bed it suddenly occurred to me that the cables could be arranged so the take-off points are from the end batteries. I've just spent 15 minutes drawing it up, ready to post in this thread – and you've beaten me to it! I wonder why the SmartGauge site shows it as it does. Perhaps that was how you originally drew it up.

 

The above is indeed my original diagram from a topic on here sometime in 2008, but not sure why Gibbo didn't just copy it for his site and crop the logo off...

 

cheers, Pete.

~smpt~

Edited May 10, 2015 by smileypete

[nicknorman]

 

Doesn't this mean that what you're commenting on is having a charging source and an inverter load at opposite ends (which was certainly my original question) and concluding that it's fine? Whereas what we've moved on to discussing is whether it's OK or not to have two loads (12v circuit and inverter load) placed, one at one end and the other at the other end of the bank.

 

Not quite. In both cases, the -ve was at one end. So the alternator was not across the diagonal whilst the inverter was. Even with the full 175A from the alternator going in at one end, the current was shared by the batteries equally, and I could detect no significant difference in their terminal voltages. Of course there is some voltage drop in the cable but it is pretty small.

With more normal loads such as 10-20 amps going into the boat's services the issue is even less significant. When charging, I'm sure there is a very slight difference between the current going into each battery when charging at high current but it is very small and anyway, as the batteries approach fully charged the current falls off and the cable resistance becomes less and less significant so that by the time the batteries are fully charged they will all be fully charged. Perhaps if you were in the habit of continuously cycling the batteries between say 70% and 50%, some differences might be manifest but that is not a good way to run batteries anyway.

[Peregrine]

 

Not quite. In both cases, the -ve was at one end. So the alternator was not across the diagonal whilst the inverter was. Even with the full 175A from the alternator going in at one end, the current was shared by the batteries equally, and I could detect no significant difference in their terminal voltages. Of course there is some voltage drop in the cable but it is pretty small.

With more normal loads such as 10-20 amps going into the boat's services the issue is even less significant. When charging, I'm sure there is a very slight difference between the current going into each battery when charging at high current but it is very small and anyway, as the batteries approach fully charged the current falls off and the cable resistance becomes less and less significant so that by the time the batteries are fully charged they will all be fully charged. Perhaps if you were in the habit of continuously cycling the batteries between say 70% and 50%, some differences might be manifest but that is not a good way to run batteries anyway.

 

I've no argument with this. It always seemed to me that, even if there was an issue in relation to charging, it would be small, and you confirm this. I originally wanted to see a rationale for Chris' assertion about charge and load needing to be connected to the same terminals. So far, very little on this thread has supported it.

 

But my final point / question was not about charging at all. It was about load. As far as I can see, my boat runs the 12v circuits off one end of the bank, and the inverter off the other end. What I've gleaned suggests that's not sensible, and that's what any re-wiring would primarily address.

[smileypete]

But my final point / question was not about charging at all. It was about load. As far as I can see, my boat runs the 12v circuits off one end of the bank, and the inverter off the other end. What I've gleaned suggests that's not sensible, and that's what any re-wiring would primarily address.

 

Maybe just keep an eye on the SG's if they're 'open' batts, tis wise to do this anyway.

 

cheers, Pete.

~smpt~

[Peregrine]

 

Maybe just keep an eye on the SG's if they're 'open' batts, tis wise to do this anyway.

 

cheers, Pete.

~smpt~

 

They're sealed, Pete.