Battery inter-connect cables

[Col_T]

Reading various threads, and other websites, has given me the idea that all battery interconnect cables should be the same length. Is it okay to have the length of positive interconnect cables different to the length of negative interconnects??

The reason for asking is that I am considering connecting the batteries to bus-bars (one for positive, one for negative) but, because of limited space, both bus-bars would need to be on the same side of the batteries. This results in the interconnect cable from each positive terminal being longer than the interconnect from the corresponding negative terminal as the positive cable would have to go over the battery.

 

[Detling]

Should be ok try to get all positives the same length and all the negative the same so that all batteries are equally used. 

[MtB]

Provided all your interconnects are all nice and fat, the lengths don't matter a toss in practice.

[WotEver]

15 minutes ago, Mike the Boilerman said:

Provided all your interconnects are all nice and fat, the lengths don't matter a toss in practice.

Define fat. 

Define toss. 

But I'm just kidding really - in principal you're right although some folk might think 10mm^2 is 'fat'. 

[nicknorman]

But anyway, having different lengths for the positive vs the negative interconnects has no significance even if they are not fat.

[MtB]

1 hour ago, nicknorman said:

But anyway, having different lengths for the positive vs the negative interconnects has no significance even if they are not fat.

 

Yes if the cables a are not fat (i.e. thin, for people struggling to get what 'not fat' means  ), the relative lengths are the least of your problems.

Provided your cables are fat (or fekkin fat), by far the weakest point is the quailty of the terminations. They need to be really well made and have negligible electrical resistance. 

 

Edit to add a missing word!

Edited August 2, 2017 by Mike the Boilerman

[DHutch]

To answer the question, no it doesn't matter if the leads on the positive side match the negative.

The intention of 'having all interconnects the same length' is to ensure no one battery has a lower total line of resistance than the other.

#

So for instance if all the positives are a foot and all the negatives are a two foot, all batteries see a the same number of joins and the same total length of cable. You could also then half of them with the positive as the longer cable and the negative shorter. As said, if the cable is thick enough as to be considered zero resistance, it all drops out in the wash, but if the load is a 3kva inverter that thickness is considerable.

You can also do various configurations linking the battery terminals together more locally terminal to terminal, but I won't go into detail if that's not your plan.

How many batteries do you have? 12 batteries connecting to a 12 system? 

 

Daniel

 

 

[MtB]

36 minutes ago, DHutch said:

To answer the question, no it doesn't matter if the leads on the positive side match the negative.

The intention of 'having all interconnects the same length' is to ensure no one battery has a lower total line of resistance than the other.

 

 

A theoretical ideal which in practice is of no consequence whatsoever.

Why would it matter if one battery is discharged to 50% and another is discharged to 50.001%? These are the sort of immeasurably small differences non-matching cable lengths are likely to make.

But for people who like to obsess pointlessly over imaginary problems, this a a harmless one to choose.

[Robbo]

If you have a even number of batteries you can double up batteries so you reduce the amount of cabling to the bus bar well if required.

[Col_T]

Thanks for all the replies, chaps, very much appreciated.

To answer a couple of questions:-

• the battery bank comprises 5 off 110 Ah sealed lead acids connected to a 1500w Sterling Pro-Combi charger / inverter,
• the boat is 12 / 240v.

I don't have the manual to hand, but I think that the Pro-Combi manual specifies 35mm^2 cable to connect that unit to the batteries so I'm rather hoping that is what has been fitted, as any new cabling would be in that size. As an aside, would quality cable have specs marked in the covering??

As far as other inter-connect options are concerned, I know that the SmartGauge site details some but those are all for even numbers of batteries, which ain't what I've got. The possible requirement for bus-bars came from me trying to diagram the electric circuits and deciding that it didn't really make sense, but then that might be because my understanding of electrics is still very, very basic! To keep things clear, I'll start a new thread for that.

Once again, thanks for the help.

 

 

[Robbo]

For 5 batteries I wouldn’t bother with connecting individually to a busbar.  I would just use around 70mm2 interlinks beteen them.  95mm2 if the inverter is above 2-3kw.

if your Combi is anything around 2-3kw use min 70mm2 to connect the Combi. 95mm2 if it’s 3kw.   This all depends on how far away it is as well tho.

just seen you have 1.5kw, I would use min 50mm2 for both interlink and connecting the Combi, if the Combi is over 2-3m away up to 70mm2.

Edited August 3, 2017 by Robbo

[Col_T]

Thanks for this, Robbo.

Is there any logic behind the 70mm2 recommendation or is it a case of experience saying that seems right? Also, I guess that re-wiring the inter-connects, right up to the charger / inverter, removes any advantage offered by the other inter-connect options detailed on the SmartGauge site??

[WotEver]

4 minutes ago, Col_T said:

Thanks for this, Robbo.

Is there any logic behind the 70mm2 recommendation or is it a case of experience saying that seems right? Also, I guess that re-wiring the inter-connects, right up to the charger / inverter, removes any advantage offered by the other inter-connect options detailed on the SmartGauge site??

As MtB has pointed out, with fat enough cables the SmartGauge wiring diagrams become pretty much irrelevant. 70mm² is large enough to make any voltage drops insignificant for your installation (as long as the connections are all sound). 

Edited August 3, 2017 by WotEver
Added the 'for your installation' bit so that no-one thinks it'd necessarily be correct for them

[jddevel]

Not wishing to highjack this but although I`ve put this in equipment topics thought it might be relevant to this as well. Hope I`m not to out of order!!!

Back to this subject for me. Using the Smart gauge method three(http://www.smartgauge.co.uk/batt_con.html) of balancing batteries ( I`ve 5 leisure so seems suitable) the batteries "housed"  at 90 degrees to the starboard hull on a horizontal shelf on the starboard side and running from cabin bulkhead to stern in the engine room.  The upstand running along the battery shelf would allow me to fix the two bus bars along this upstand in the middle and bring -neg or +pos cables across the top of the batteries to busbars. I have four questions:

1) Is my idea for the location of the busbars acceptable -they are 650amp and housed.They are from Furneaux Riddall 

2) Going to use a 450A fuse (I`ve a 3000 inverter) so can I fix that immediately adjacent to the +pos busbar on the shelf in the 95mm cable take off to the electric cupboard which is the other side of the bulkhead.

3) What size of cable should be used from the batteries to the busbars. Obviously the cable lengths which will be cut to equal length for each positive or negative terminal will differ. Either -neg or +pos being longer dependent on layout.

4) As the batteries can be placed with either positive or negative closest to the hull side is there a recommended preference.

Sorry for all this so early in the morning and I hope I`ve made myself clear with all the questions.

[MoominPapa]

8 hours ago, Mike the Boilerman said:

 

A theoretical ideal which in practice is of no consequence whatsoever.

Why would it matter if one battery is discharged to 50% and another is discharged to 50.001%? These are the sort of immeasurably small differences non-matching cable lengths are likely to make.

But for people who like to obsess pointlessly over imaginary problems, this a a harmless one to choose.

Have you read Gibbo's writings on this? I turns out (because battery internal resistance is so low) that rather small absolute differences in interconnect resistance can have quite large effects on the distribution of current through the network. 50& and 50.001% sure don't matter, but 25% and 75% sure does.

 

Intuition is not a good guide to what will happen, I recall that even Gibbo was surprised at the theoretical and measured results he got when he investigated.

 

MP.

 

[rusty69]

5 minutes ago, MoominPapa said:

Have you read Gibbo's writings on this? I turns out (because battery internal resistance is so low) that rather small absolute differences in interconnect resistance can have quite large effects on the distribution of current through the network. 50& and 50.001% sure don't matter, but 25% and 75% sure does.

 

Intuition is not a good guide to what will happen, I recall that even Gibbo was surprised at the theoretical and measured results he got when he investigated.

 

MP.

 

Mtb also wrote :-

 

"Provided your cables are fat (or fekkin fat), by far the weakest point is the quailty of the terminations. They need to be really well made and have negligible electrical resistance. "

[MoominPapa]

1 minute ago, rusty69 said:

Mtb also wrote :-

 

"Provided your cables are fat (or fekkin fat), by far the weakest point is the quailty of the terminations. They need to be really well made and have negligible electrical resistance. "

Yeah, but that misses the point too. Provided that it's reasonably low, the absolute resistance of each battery circuit doesn't really matter, what matters is that all the battery circuits have the same resistance within a very small amount. Even if the terminations have as much resistance as a foot of cable, then a two foot cable (with two terminations) has 3 units of resistance and a two foot cable (also two terminations) has four units of resistance. That's a still big imbalance in battery current.

MP.

 

 

[MtB]

13 minutes ago, MoominPapa said:

Have you read Gibbo's writings on this? I turns out (because battery internal resistance is so low) that rather small absolute differences in interconnect resistance can have quite large effects on the distribution of current through the network. 50& and 50.001% sure don't matter, but 25% and 75% sure does.

 

Intuition is not a good guide to what will happen, I recall that even Gibbo was surprised at the theoretical and measured results he got when he investigated.

 

MP.

 

 

Have you got a link to where Gibbo says differences as large as 25% and 75% SoCs can be the result of different interconnect lengths please?

I'm staggered.

 

[MoominPapa]

5 minutes ago, Mike the Boilerman said:

 

Have you got a link to where Gibbo says differences as large as 25% and 75% SoCs can be the result of different interconnect lengths please?

I'm staggered.

 

http://www.smartgauge.co.uk/batt_con.html

Quote

The bottom battery provides 35.9 amps of this.
The next battery up provides 26.2 amps.
The next battery up provides 20.4 amps.
The top battery provides 17.8 amps.

Not quite 25% and 75%, but still substantial.

 

The point is that it's not sufficient to make the resistance of each battery circuit low, they have to be the same for each battery. That involves good crimps and connectors (for reproducible resistance, rather than low resistance) and equal lengths of cables.

 

MP.

 

 

 

[MtB]

Thanks MP. Will have a good read of that later. 

My gut feeling is still the resistances of the terminations are bigger than that of the unbroken lengths of cable.

(And will vary widely from one to the next, even though all are very low.)

[dmr]

The bottom battery provides 35.9 amps of this.
The next battery up provides 26.2 amps.
The next battery up provides 20.4 amps.
The top battery provides 17.8 amps.

 

I have also read this and find it a bit surprising. I do wonder if this is the result of concocting a laboratory experiment that is a little detached from reality. I suspect it is possible to see differences like this in the short term but surely batteries are self balancing? so that if one is working extra hard it will soon discharge just a little bit extra till the loading is equalised. If this was not the case then the concept of wiring batteries in parallel would not work.

..................Dave

[dor]

A small resistance is far more likely in the cable connections than in the cable itself.  Rather than obsessing too much about the length of cable, it is important that each battery has the same number of connections between its posts and the main fuse to the distribution board.  This is why it is recommended to connect the negative to the opposite corner to the positive take-off.  Of course, if each battery is individually connected to a bus bar, then this setup will also have the same number of connections.  If the negative was twice the length of the positive cables going to the bus bars, then each battery will still have the same path to it. I can't see how having different lengths of positive and negative connectors would be an issue, even if the resistance of the cables was measurable over such short distances, providing all batteries are connected by the same lengths.

[MtB]

I'm with DMR on this. As the SoC of the battery delivering 35.9A falls, so will its terminal voltage relative to the others and less current will flow. 

Gibbo may have measured those values instantaneously, but I'd more interested to see multiple sets of data taken at say one minute intervals for 10 or fifteen minutes. I would expect the currents to roughly equalise, but at the end of the test the SoCs of the batteries will vary very slightly.

The furthest battery away will have the highest SoC but the differences between furthest and nearest I would expect to be very small. I'm sure Gibbo would have thought of this, so one wonders why he hasn't published the results of such a test. 

[WotEver]

27 minutes ago, dor said:

I can't see how having different lengths of positive and negative connectors would be an issue

It wouldn't. That's already been noted in this thread. 

[Robbo]

3 minutes ago, WotEver said:

It wouldn't. That's already been noted in this thread. 

Although I would place the batteries to have a smaller positive cable/link, just to reduce the very tiny risk if they did come loose it less likely to touch something.