Balancing the domestic bank.

[Guest]

I've been pondering this a bit more over the last couple of days after recent battery issues and posts.

 

Currently our domestic bank is wired as so -

 

 

This is not as per the wiring diagram for the boat when it was new which shows two distinct banks one charged by the starter alternator and the other charged by the domestic alternator (confirmed by the following in the diagram on the boat) -

 

Charging circuit-

 

The starter alternator charges the starter battery and a bank of 2 domestic batteries through a split charge diode, and the domestic alternator charges a bank of 5 domestic batteries.

 

The labels 1,2 and Inverter is how the positives are labelled with a pen mark. I'm taking 1 is the lead from the starter alternator and 2 is the domestic. The one labelled inverter is obviously the inverter. (Or is one of 1 or 2 from the charger side of the combi.)

 

Looking at the smartgauge web site it would seem I need to get mine wired as follows :-

 

 

A couple of things concern me -

 

1 - I will lose the large 70mm² cable that currently connects the 2 banks together. This looks like it has been added at some point after the boat was built.

 

2 - Taking the +ve load from and putting all the charge from both alternators in at one single point seems contradictory to creating a balanced bank.

 

Thoughts please......

[Gibbo]

Diagram 2 is a better solution than diagram 1. By far.

 

It would be even better if you move each connection point in by one battery.

 

All feeds into and out of the battery should be at the same points. Taking out of one point, and putting back into a different point makes a complete mess of just about everything.

 

Have you just got a single battery bank that does both domestics and starting?

 

I don't understand point 1 - what 70mm cable that connects what to what?

[Guest]

Have you just got a single battery bank that does both domestics and starting?

Sorry should have said - we have a separate starter battery

 

 

I don't understand point 1 - what 70mm cable that connects what to what?

 

If you look at the first diagram the batteries look like they are 'grouped' into one group of 2 (on the left) and one group of 3 (on the right). A 70mm² cable then connects the bottom right +ve to the +ve of the bottom left battery.

[Gibbo]

If you look at the first diagram the batteries look like they are 'grouped' into one group of 2 (on the left) and one group of 3 (on the right). A 70mm² cable then connects the bottom right +ve to the +ve of the bottom left battery.

 

Right, so I assume the other cables are somewhat smaller?

 

In that case, you've really got a horribly out of balance bank. Rewiring as per your original diagram 2 (but with the take off points moved in one battery) will be a vast improvement. Note that all the interconnecting leads really need to be the same length and thickness for it work properly.

 

There are even better methods available but the improvement becomes very small and usually they're lots more work.

[Guest]

Right, so I assume the other cables are somewhat smaller?

 

In that case, you've really got a horribly out of balance bank. Rewiring as per your original diagram 2 (but with the take off points moved in one battery) will be a vast improvement. Note that all the interconnecting leads really need to be the same length and thickness for it work properly.

 

There are even better methods available but the improvement becomes very small and usually they're lots more work.

 

Yes the other connecting leads are smaller, not sure what size though as they are not labelled like the bigger one is.

 

Thanks.....

 

ed to add - just checking Chris you don't mean move the take off points to the second battery in but rather move them direct to the first battery on the left.

Edited June 18, 2012 by MJG

[smileypete]

1 - I will lose the large 70mm² cable that currently connects the 2 banks together. This looks like it has been added at some point after the boat was built.

 

2 - Taking the +ve load from and putting all the charge from both alternators in at one single point seems contradictory to creating a balanced bank.

 

Thoughts please......

Though the connections aren't balanced well, I'd check the batt SGs (specific gravity) with a hydrometer if possible

 

As long as the batt SGs aren't diverging too much or too quickly, I think I'd leave things as they are.

 

If they do diverge some equalisation will help level them up. Best not to worry too much, they're only batteries

 

cheers, Pete.

~smpt~

[nicknorman]

Hi Gibbo

 

I totally take the point about the bank wiring to share the load and charge evenly between the batteries in theory. However surely the degree to which a good configuration is better than a poor one, or not, depends on the inter-battery connections? In an ideal case of zero resistance cable and connections, it would not make any difference. In the case of thinly wired corroded connections I can see it could make a big difference. On our boat, the bank is connected together using short runs of 70mm^2 cable with connnections clean and tight. Although I have modified our bank of 4 x 110AH to be more like your ideal diagrams (because I was redoing it anyway to install a Mastsershunt), to be honest I am not sure it has made any difference. Both before and after, putting a DVM on the actual posts of adjacent batteries whilst charging at 150A or so, or discharging at 150A or so from the inverter, the voltage difference is less than 10mV and (IIRC) less than 5mV (Can't remember the exact value, just that it was effectively zero.). Puttint a clamp meter on the inter-battery leads suggests an equal sharing of current within the accuracy of the meter.

 

Therefore perhaps the advice to Martin should be to ensure beefy and clean inter-battery connections rather than necessarily exactly complying with the ideal configuration?

[Gibbo]

I totally take the point about the bank wiring to share the load and charge evenly between the batteries in theory. However surely the degree to which a good configuration is better than a poor one, or not, depends on the inter-battery connections?

 

Correct.

 

In an ideal case of zero resistance cable and connections, it would not make any difference.

 

Correct.

 

In the case of thinly wired corroded connections I can see it could make a big difference.

 

It does indeed.

 

...to be honest I am not sure it has made any difference. Both before and after, putting a DVM on the actual posts of adjacent batteries whilst charging at 150A or so, or discharging at 150A or so from the inverter, the voltage difference is less than 10mV

 

Your good connections and 70mm cable obviously help. Most installations use something closer to 25 or 35mm cable for interconnects. On duff installations I've seen 0.5 volts difference between adjacent batteries.

 

Almost every time someone on here has complained that one battery has died out of a bank, it almost always turns out they have the take off points at one end and it is almost always that battery that has died.

 

It isn't just theoretical. It is practical.

[matty40s]

 

 

 

It would also be far more balanced if one of these had been used..

 

[Gibbo]

Yes the other connecting leads are smaller, not sure what size though as they are not labelled like the bigger one is.

 

Thanks.....

 

ed to add - just checking Chris you don't mean move the take off points to the second battery in but rather move them direct to the first battery on the left.

 

No. At the moment you have one on batt 1 and one on batt 5. Move them to one on batt 2, and one on batt 4.

[Guest]

It would also be far more balanced if one of these had been used..

 

 

No need - the diagram was well understood by those that need to....

 

No. At the moment you have one on batt 1 and one on batt 5. Move them to one on batt 2, and one on batt 4.

 

But then the take off points will not be diagonally opposed...will they?

[Mat B]

.....

putting a DVM on the actual posts of adjacent batteries whilst charging at 150A or so, or discharging at 150A or so from the inverter, the voltage difference is less than 10mV and (IIRC) less than 5mV (Can't remember the exact value, just that it was effectively zero.). Puttint a clamp meter on the inter-battery leads suggests an equal sharing of current within the accuracy of the meter.

 

......

 

I tried the same experiment (Charging at 30A, 3 batteries) and got 7mV on most connections, 12mV on one. Is the difference worth worrying about??

 

m@

[Chalky]

 

It isn't just theoretical. It is practical.

 

 

Works in theory as well. If you have a detailed enough and accurate enough battery model you can see it in simulations.

 

You can also see some effect if you model the battery cables accurately, treat the batteries as capacitors and apply a step input. The capacitors charge at nearly the same rate, but there is a slight difference due to the differences in cable resistance. (LTspice is good for playing with and is free) As Gibbo says, drawing 2 is better.

Edited June 18, 2012 by Chalky

[Gibbo]

But then the take off points will not be diagonally opposed...will they?

 

Yes they will.

[Guest]

Yes they will.

 

OK - I think I see what you are saying, why I'm querying this though is because it is different from the diagram on the SG site which shows the take off points from the very first and very last batteries in the bank....what you are advocating above differs from this, and I'm curious to know why.

 

Is it to do with the number of batteries in my installation being an odd number....

[Gibbo]

OK - I think I see what you are saying, why I'm querying this though is because it is different from the diagram on the SG site which shows the take off points from the very first and very last batteries in the bank....what you are advocating above differs from this, and I'm curious to know why.

 

Is it to do with the number of batteries in my installation being an odd number....

 

Yes. The "diagonally opposite on outside corners" works perfectly with 2 batteries. It's pretty damned close to perfect on 3 batteries. On 4 batteries it's still damned good but not perfect. Good enough in practice though.

 

But once you get to 5 batteries it starts to deviate from perfection a bit more and bringing the connection points in by a battery (or by 2 or more batteries with even bigger banks) helps get closer to perfect balance than taking the outside corners.

[jenlyn]

Yes. The "diagonally opposite on outside corners" works perfectly with 2 batteries. It's pretty damned close to perfect on 3 batteries. On 4 batteries it's still damned good but not perfect. Good enough in practice though.

 

But once you get to 5 batteries it starts to deviate from perfection a bit more and bringing the connection points in by a battery (or by 2 or more batteries with even bigger banks) helps get closer to perfect balance than taking the outside corners.

I followed the SG site drawings too, on a 7 bank. Won't be boating much today then, go down below and change it around, tut.

[Gibbo]

I followed the SG site drawings too, on a 7 bank. Won't be boating much today then, go down below and change it around, tut.

 

Why? The drawings show 4 batteries not 7. The text explains what to do with more batteries...

 

 

With a large number of batteries it may be necessary to go to the 3rd method shown above.

 

Even with 8 batteries it is possible to get reasonable balancing by placing the main "take off" feeds from somewhere down the chain instead of from the end batteries. Remember, count the number of links each battery needs to run through to reach the final loads and get these as equal as possible.

 

[Guest]

Yes. The "diagonally opposite on outside corners" works perfectly with 2 batteries. It's pretty damned close to perfect on 3 batteries. On 4 batteries it's still damned good but not perfect. Good enough in practice though.

 

But once you get to 5 batteries it starts to deviate from perfection a bit more and bringing the connection points in by a battery (or by 2 or more batteries with even bigger banks) helps get closer to perfect balance than taking the outside corners.

 

Got it thanks...

[micky44]

Hi hope no one minds me jumping on to this tread, i am looking at for most efficent wiring for my 24 volt battery bank, i have found this diagram.

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My question is would it be better to take the three neg to a buzz bar and also the three poss to a seperate buzz bar , (cable lenghts all the same) then come off each buzz bar with 1 cable to the inverter. hope this makes sense as i am a complete numpty with electricity.

[Justme]

With 3 bats I would just move either the + or the - to the other end of the bank.

[bottle]

Just to confirm agree , with, Justme

 

I know you have six batteries but effectively it is three as the links to parallel them make it a bank of three.

 

Keep all links as short as possible and equal length

Edited June 24, 2012 by bottle

[micky44]

thank you for the advice, so no real advantage with regards to me fitting buzz bars ?on this set up.

[bottle]

Correct.

 

Just make sure the cables are of sufficient and equal size mm²

 

I have similar to you (6 batteries in 24v configuration) and all cables are 50mm²

[plainsman]

Right, so I assume the other cables are somewhat smaller?

 

In that case, you've really got a horribly out of balance bank. Rewiring as per your original diagram 2 (but with the take off points moved in one battery) will be a vast improvement. Note that all the interconnecting leads really need to be the same length and thickness for it work properly.

 

There are even better methods available but the improvement becomes very small and usually they're lots more work.

 

Sorry to interrupt with my query, on the SG info site it shows a 4 battery bank with the diagonal arrangement and a neg link to the starter battery. If we have a 5 battery bank and move the take off (and charge in) points are in by one battery, am I correct in assuming the link to the starter batt neg is similarly moved in by one battery? This is for a single alternator arrangement.