What wire diameter (4 guage?) to parallel two 100 a/h batteries?

aussie_paul · Guru

What wire diameter (4 guage?) to parallel two 100 a/h batteries?

Phil C · Guru

Hi Aussie Paul

The maximum current you expect at any given time will determine the cable diameter as stated in AS3000 Australian Wiring Rules "maximum demand"

OK that said means we have to work smarter not harder. As an ex sparky I have installed 2 x 10mm square cables (welders cable) of very short length on both pos and neg terminals of my parallel battery setup, I have worked out that that size should accommodate the demands placed by charging and discharging of the battery. The trick is to avoid voltage drop over the cable as much as possible. Keep the cables short and fat with good solid crimped connectors at each end. Whatever you do dont tie pos and neg cables together, they set up magnetic fields in opposition and can cause some issues, especially with higher currents.

Now I am monstrously over cautious and will probably draw some flak from some "experts", however safety is my priority.

Safe travels

briche · Guru

Hi Phil C

From ex sparky to another. Having worked with battery banks over many years in the aviation field you can never be over safe. The system you describe for parraelling batteries is spot on. I have seen the results of incorrectly connected battery's and it's not nice. The comment about cable size is also correct. I have seen some wiring systems in caravans that just shocks me that someone would do such a thing and unfortunately some of those examples have been in fairly new vans.

thanks for your comments, helps to put the truth out there.

regards

Brian

-- Edited by briche on Thursday 28th of August 2014 11:01:02 AM

-- Edited by briche on Thursday 28th of August 2014 11:01:39 AM

aussie_paul · Guru

Phil C wrote:
Hi Aussie Paul

The maximum current you expect at any given time will determine the cable diameter as stated in AS3000 Australian Wiring Rules "maximum demand"

OK that said means we have to work smarter not harder. As an ex sparky I have installed 2 x 10mm square cables (welders cable) of very short length on both pos and neg terminals of my parallel battery setup, I have worked out that that size should accommodate the demands placed by charging and discharging of the battery. The trick is to avoid voltage drop over the cable as much as possible. Keep the cables short and fat with good solid crimped connectors at each end. Whatever you do dont tie pos and neg cables together, they set up magnetic fields in opposition and can cause some issues, especially with higher currents.

Now I am monstrously over cautious and will probably draw some flak from some "experts", however safety is my priority.

Safe travels

Trying to get my head around this. What is the approx diameter of a 10mm square cable?

Aussie Paul. smile

PeterD · Guru

Paul, look at it the other way round $ B&S = 21.15 mm2 (square mm.) The attached file shows the dimensions of a few different gauges of wire. The diameters refer to solid wire, stranded cable will have a larger diameter.

-- Edited by PeterD on Friday 29th of August 2014 10:58:48 PM

oldbobsbus · Guru

You need a piece of cable about as thick as your little finger and as short as practical and like has been said with HD terminals crimped..

If you goto your closest Auto Elec he should be able to make you up a cable that will do the job..

aussie_paul · Guru

oldbobsbus wrote:
You need a piece of cable about as thick as your little finger and as short as practical and like has been said with HD terminals crimped..
If you goto your closest Auto Elec he should be able to make you up a cable that will do the job..

Thanks Bob, is soldering acceptable? I need 400mm in length to maximise the space.

Aussie Paul. smile

Phil C · Guru

Thanks PeterD, forgot some people not into mm square when describing cables.

Cheers

oldbobsbus · Guru

aussie_paul wrote:
oldbobsbus wrote:
You need a piece of cable about as thick as your little finger and as short as practical and like has been said with HD terminals crimped..
If you goto your closest Auto Elec he should be able to make you up a cable that will do the job..
Thanks Bob, is soldering acceptable? I need 400mm in length to maximise the space.
Aussie Paul.

I used to think soldering was better than Crimping but I must admit that with the really good crimping tools the pros now have they do a much better job of making a perfect contact than I could ever hope to make with soldering..

Reason:- unless you have a full hot dip tank and can achieve the perfect solder job the good crimp tools do a much better job..

My Auto Elec usually crimps the cables for me for free when I buy them from him..

aussie_paul · Guru

This is how I read the chart. Is the red 10.02 the size you were referring to Phil? It does not gel with me as little finger thickness!!!!

Aussie Paul. smile

Diameter Diameter Cross Inch

Mm Inches Sectional Fraction Drill

Area mm2 BWG

3.57 0.1406 10.02 9 9/64

Phil C · Guru

aussie_paul wrote:
This is how I read the chart. Is the red 10.02 the size you were referring to Phil? It does not gel with me as little finger thickness!!!!
Aussie Paul.

Diameter       Diameter             Cross                                                   Inch
Mm                Inches                   Sectional                                     Fraction Drill
                                                     Area mm2                   BWG

3.57                0.1406                  10.02                         9                          9/64

Yep, thats it. The cross sectional area is the key. 2 cables this size on each terminal will do the trick. I am against soldering and very pro crimping large current terminals because hi current equals heat and maybe enough to weaken a solder joint... NOTE I didnt say melt.

Safe travels

oldbobsbus · Guru

I have a heap of these cables.. they are 520mm overall.. They are a bit thicker than you need but they are much better than using ones that are too small..

I you would like a couple you are welcome to them...you just need to PM me your address...

aussie_paul · Guru

oldbobsbus wrote:
I have a heap of these cables.. they are 520mm overall.. They are a bit thicker than you need but they are much better than using ones that are too small..
I you would like a couple you are welcome to them...you just need to PM me your address...

Much appreciated Bob. Sent a pm.

Aussie Paul. smile

PeterD · Guru

aussie_paul wrote:
This is how I read the chart. Is the red 10.02 the size you were referring to Phil? It does not gel with me as little finger thickness!!!!
Aussie Paul.

Diameter       Diameter             Cross                                                   Inch
Mm                Inches                   Sectional                                     Fraction Drill
                                                     Area mm2                   BWG

3.57                0.1406                  10.02                         9                          9/64

Paul, you need to use the use the column labelled AWG or B&S not BWG. That is gauge that the manufacturers use.

The following contains a lot of off topic information but it will be interesting/useful to some of you.

I don't know why we still use the old gauges and not the recommended metric sizes. You can read about the history of wire gauges here. They lead to confusion (such as people confusing the gauges.) The old 240 V power wiring was based on the diameter and number of strands (eg 23/.0076 for flexible and 7/.029 for fixed wire.) All 240 V wiring is now measured in square mm (mm2) so you know how much coper is in the cable.

Another abomination is the auto cable sizes. The size refers the the size of hole needed to pass the wire through. If you go to this link you will see where the author has measured several samples of auto cable. He measured the cross section of copper in the range of 1.25 to 2.0 mm2 and he even found some Italian cable with 4 mm2 of copper in it.

Another problem is the labelling of cable with its maximum current capability. This is simply the amount of current you can draw without melting the cables insulation. It may be OK to refer to this specification for very short lengths. However I can guarantee you that if you use this specification (or half its value) when wiring in caravans then the voltage drop in your cable runs will be excessive.

One of the advantages of using the international standards for wires and cables is you can easily calculate the voltage drop in your circuit. From the above link we get the formula:

Voltage drop equals [cable length (in metres) X current (in amps) X 0.017] divided by cable cross-section in mm2.

When using an earth return cable the cable length is twice the length of cable run. If you are using a chassis return the cable length in the formula can be simply the length of the cable run. For design work in caravans a maximum voltage drop of 3% is recommended.

For more info on caravan mains power leads I suggest reading this link. I believe the bit "pre-2008 restrictions on cable length have been eased" is the relaxation of the maximum voltage drop from 3% to 5%. This has resulted in us being allowed to have longer cable lengths for each size cable. You may note that the cable sizes are measured in mm2.

PeterD · Guru

ps The links in the previous message were well researched articles mostly written for inclusion in the CMCA magazine Wanderer. There a lot more available on his web site. A lot of them have been revised to reflect new changed information. These are included in this link. The others are in this link. Happy reading.

aussie_paul · Guru

PeterD wrote:
ps The links in the previous message were well researched articles mostly written for inclusion in the CMCA magazine Wanderer. There a lot more available on his web site. A lot of them have been revised to reflect new changed information. These are included in this link. The others are in this link. Happy reading.

Thanks Peter, much appreciated.

Aussie Paul. smile

aussie_paul · Guru

Finally finished setting up the second battery and case. Leads courtesy of oldbobsbus, thanks Bob.

Aussie Paul. smile

2nd battery set up. (2).jpg 2nd battery set up.jpg

dorian · Guru

On the subject of crimping versus soldering, I try to do both, depending on the application and the type of joint. First I crimp the wire and then flow solder into the crimp. Then I let the joint cool before crimping the insulation.

AIUI, tinning the ends of the wires before crimping them violates some electrical standards. In fact my TAFE lecturer was responsible for an amendment to the SAA Wiring Rules in this regard. Apparently the solder softens over time and the connection becomes loose.