Busbar as Battery Interconnects

Luminast · March 2012

I'm assembling my first off-grid solar system for remote cabin power. I'm new to this, but have read a lot, and am assembling the system from quality parts, and incorporating the best possible workmanship.

Can I get the gang here to tell me why copper busbar is almost never mentioned as an alternative for interconnecting batteries? I've done a fair bit of research on copper busbars. I know the cost, and I know where to source the proper copper bar at reasonable cost. After the research, the idea appeals to me as a way to avoid the potential problems that might come along with many crimped cable ends, and result in a neater looking battery bank.

Here is what my research reveals: The current carrying capacity of copper busbar is commonly figured as 1.25 amps per square mm of cross sectional area, which means that a 1/4-inch by 1-inch bar can carry 201 amps. The proper copper bar that size can be purchased for about $10 per foot, which is nearly the same cost as one foot of 2/0 copper wire plus two crimp terminal ends. Being conservative, I would increase the bar size another 50%. The rate of thermal expansion for copper is a little less than .001" per inch of length per 100 degrees (F) delta T, which seems easily acceptable for my sheltered battery location.

I know how to tin copper, and would tin the copper bars to limit oxidation of the copper. Battery interconnects for my batteries would each be about 7-inches long. I have a plan for insulating the bars as well.

Is anyone here using busbar to interconnect batteries? Or can you tell me why I should favor 2/0 copper wire? Or can you suggest any other things that should be considered in evaluating busbar interconnects? My installation is not subject to the satisfaction of an electrical inspector, so nuances of the NEC might not be of concern if they are not legitimate safety issues.

I suspect there is something I'm missing, or have not figured correctly.

KeithWHare · March 2012

Re: Busbar as Battery Interconnects

One advantage of heavy cable over buss bars is flexibility. With buss bars, the batteries bank will be fairly rigid. With cables, you have some flexibility to wiggle and move each battery.

Keith

vtmaps · March 2012

Re: Busbar as Battery Interconnects

I've seen it mentioned on this forum that bus bars can mechanically stress batteries... as some batteries age the posts may move, and this movement may be resisted by the bus bar. Other than that I can't think of any reason why its not a good idea. --vtMaps

BB. · March 2012

Re: Busbar as Battery Interconnects

Stresses from temperature cycling (and the expansion/contraction of metals/plastics/etc.) are actually a very common failure mode in engineering.

Typically, bus bars are used (at least around here) for common points in your wiring (the batteries terminate at the +/- bus bars, and then the wiring-to fuses-to loads from the bus bars).

-Bill

westbranch · March 2012

Re: Busbar as Battery Interconnects

If these assumptions are true , then why this type of commercial setup?

http://www.ieeco.net/Documents/Batteries/ExideAbsolyteGX.pdf

see page 1, all bolted down and interconnected

john p · March 2012

Re: Busbar as Battery Interconnects

I wrote an article a very long time ago on using copper pipe as interconnect and I think it disproved all the reasons that have ever been given as to why its not good to use it instead of the "usual copper wire and crimped connectors".

I also ask again same as another if solid buss bar is no good why do battery companies like Absolite use them???

I dont seem to be able to find that old article now on this forum. (Ican only find my posts back to early last year)

BB. · March 2012

Re: Busbar as Battery Interconnects

Hi John,

I can go back further if I type this into Google:

"john p" bus bar battery site:wind-sun.com

I am not sure exactly what post you are looking for.

I do like how clean a good bus bar setup can be... Perhaps I am too worried about thermal cycling with the large battery banks and usually shallow thermal cycling.

-Bill

dhsola · March 2012

Re: Busbar as Battery Interconnects

You can use a bicycle innertube to insulate the copper bar. Shrink it like shrink tubing. Or even use plastic automotive loom...

Silver flashed copper bar: http://store.electrical-insulators-and-copper-ground-bars.com/copper-barstock--250-thickness.html

Cripes, even amazon has copper bar now... http://www.amazon.com/Copper-Rectangular-Thick-Width-Length/dp/B000FN1AAG

Get some l-o-n-g bolts if you cross-tie

[and take out a loan too]

nsaspook · March 2012

Re: Busbar as Battery Interconnects

john p wrote: »

I wrote an article a very long time ago on using copper pipe as interconnect and I think it disproved all the reasons that have ever been given as to why its not good to use it instead of the "usual copper wire and crimped connectors".

I think this is it.
http://forum.solar-electric.com/showthread.php?7629-Battery-interconnections

mike95490 · March 2012

Re: Busbar as Battery Interconnects

Tin plating doers not slow down corrosive gasses very much.
Hard to attach the bar to battery terminal (I know, I had to make a couple dozen interconnects for my batteries - see the LUG link in my .sig) and hand made, repeatability is tough.
My batteries came with nickle plated interconnect links with "slots" instead of holes.

peakbagger · March 2012

Re: Busbar as Battery Interconnects

I have run into many inustrial battery banks used in power plants to supply emergency turbine lube oil cooling and other emergency loads with bus bars between the batteries. These battery rooms are quite impressive with numerous very large batteries hooked in series. My former employer also used bus extensively in off grid batter banks. The batteries in theory are going to expand and contract upwards and sidewards at equal rates. As long as space is left between the batteries there wont be a lot of horizontal stress imposed by the bus bar and any vertical displacement will displace the bus bars upwards into free air. There could be a issue with the transition from the bus bar to a firmly fixed charge controller but that could be easily dealt with.

john p · March 2012

Re: Busbar as Battery Interconnects

Thanks to those that found my original article.. I couldnt find it..
If anyone is interested and has never read it before its reproduced again. Hope it helps to understand about resistance in cable/lug interconnects and solid interconnects.

Here is the results of resistance testing and mechanical destruction testing of lugs on copper wires as used in battery interconnects and similar usage.

A total of 10 lengths of copper water pipe was purchased from hardware and plumbing supply stores. The 2 sizes purchased had inside diameters or 1/2 "(12.5mm) and 3/4" (19mm) thickness ranged from 1.25mm to 2mm. All pipe lengths were 10ft (3050mm)

Each size and thickness of pipe was purchased from a different supplier
All measurements were carried out on a calibrated desk multimeter.With a accuracy of+ or - .02%

1.1/2 pipe 1.25mm =.0012 ohms

2.1/2 pipe 1.25mm =.0013 ohms

3.1/2 pipe 2mm soft = .0008

4.1/2 pipe 2mm hard = .0009

5.3/4 pipe 1.5mm = .00065

6.3/4 pipe 1.5mm = .00061

7.3/4 pipe 2mm soft= .00042

8.3/4 pipe 2mm soft = .00041

9.3/4 pipe 2.2 hard = .00040

10.3/4 pipe 2.1 hard = .00039

As noted the resistance was given for 10ft lengths of pipe . If we are going to use any of those pipes as barttery interconnects etc obviously the length used would be about 8" to 10 ", so to get the resistance you would have to divide the above resistance figures by about 12.

To give worst case example the 1/2 pipe 1.25mm at .0013 ohms per 10ft divide by 12 =.0001ohms per foot
THe loss across that pipe used as a battery interconnect would be using 24v connected to a 100 amp load = 40ma
Now lets see how that compares to using a #2 cable and 2 lugs cable resistance = .00052 plus 2 crimped lugs at .00046 total resistance = 99ma loss

Any talk about wondering about losses for 1000 ft of ANY 1/2" copper pipe can clearly be shown to be a pointless exercise. and it way surpasses using #2 cable and the fact is most people would only be using #4 cable as interconnects.With far far greater losses.And as I said in a much earlier post use 3/4 " copper pipe.No matterif it has many impurities its still far far ahead of #0 cable for battery interconnects

Tests involved to measure lug on wire resistance and mechanical strength.
the tests involve #4 cable and closed ring closed tube copper lugs 2mm thick

1.Lug compressed with 500lb pressure on 3 serrated teeth jaws. resistance .00023 ohm.Lug then tested for breakaway

.seperated from cable at 223lbs pull

2,Lug compressed with 500lb pressure on 3 serrated teeth jaws then lug heated and filled with resin cored solder.

resistance .00015 seperated from cable ..failed as cable broke before cable seperated . test pull 325lbs

3. lug and cable resin cored soldered only. resistance .0008 seperated from cable at 127 lbs

As you can see solder only is not good. As a further test the joint melted when a 140a load was connected to the cable and a 12v battery to the lug. Obviously not good.

As you can see I didnt get much work done for employer the day I did all these tests, believe it or not it took 3 of us to do the tests .my work partner to verify the results ..As to do any destructive testing a workplace safety officer has to be present.
Hope some of this helps people understand a little more about cables lugs copper pipes..

mike95490 · March 2012

Re: Busbar as Battery Interconnects

john p wrote: »

.... As you can see solder only is not good. As a further test the joint melted when a 140a load was connected to the cable and a 12v battery to the lug. Obviously not good.....

Knowing that in advance, is why my 5 ton crimped lugs, are thru BOLTED to the 1" copper pipe adapter lug, the battery will melt before my terminals let loose.

http://tinyurl.com/LMR-BigLug

And solder makes a fragile wire/wire fragile. When you solder, it wicks along the inner strands, making them stiff. At the point the wicking stops, the wire changes from solid to stranded, and any flex or motion, is focused there, causing an eventual failure.

Luminast · March 2012

Re: Busbar as Battery Interconnects

dhsola wrote: »

You can use a bicycle innertube to insulate the copper bar. Shrink it like shrink tubing. Or even use plastic automotive loom...

Silver flashed copper bar: http://store.electrical-insulators-and-copper-ground-bars.com/copper-barstock--250-thickness.html

Wow. That vendor is expensive, even for the non-silvered bar.
Try this one, for non-silvered C110 bar, at about 1/4 the price:
http://www.onlinemetals.com/merchant.cfm?id=253&step=2&top_cat=87

Thanks for the hint about inner tube insulation. I had no idea it would shrink with heat.

Luminast · March 2012

Re: Busbar as Battery Interconnects

john p wrote: »

To give worst case example the 1/2 pipe 1.25mm at .0013 ohms per 10ft divide by 12 =.0001ohms per foot
THe loss across that pipe used as a battery interconnect would be using 24v connected to a 100 amp load = 40ma
Now lets see how that compares to using a #2 cable and 2 lugs cable resistance = .00052 plus 2 crimped lugs at .00046 total resistance = 99ma loss
..

I don't have any reason to doubt your conclusion about copper water pipe being a suitable substitute for copper busbar, but your calculations above are a little confusing. You probably know this, but the loss across the copper pipe is manifested as a drop in voltage between one end of the conductor and the other, or as watts dissipated as heat, but never as a loss in current. 100 amps pushed through any .0001 ohm resistance will result in a voltage drop of .01 volts (100 amps x .0001 ohms = .01 volts). The power loss in the conductor is (.01 volts) x (100 amps) = 1 watt.

Otherwise, your study is quite interesting. Thanks for posting the condensed version.

john p · March 2012

Re: Busbar as Battery Interconnects

Luminast you are correct and with hindsight it may have been better to have presented some of the data differently.
The reason it was given in MA simply was because that was the loss shown on the ampmeter with the load in series with the pipe/lug,wire. I didnt do any calculations ,

What I find interesting is that just about all the major battery manufacturers use and show always solid interconnects,and also major uses of these batteries,BUT everone else gives all the so called reasons why they are not the best thing to use..
solid interconnects =2 connections per battery
wire +lugs =4 connections per battery
And no matter how well done eventually there is going to be some increase in resistance between the lug and the wire.

waynefromnscanada · March 2012

Re: Busbar as Battery Interconnects

john p wrote: »

What I find interesting is that just about all the major battery manufacturers use and show always solid interconnects,and also major uses of these batteries,BUT everyone else gives all the so called reasons why they are not the best thing to use.

Hummm. You've got me thinking. Living in a colder climate, I like to snug my batteries up close and insulate them for Winter (they live in an outside shed), then remove the insulation and space them apart for airflow cooling in Summer. But, while I won't make any changes at this point in their 10 year old lives, when it come time to replace them - - - - - definitely something to think about.

dhsola · March 2012

Re: Busbar as Battery Interconnects

Luminast wrote: »

Silver flashed copper bar: http://store.electrical-insulators-and-copper-ground-bars.com/copper-barstock--250-thickness.html

Wow. That vendor is expensive, even for the non-silvered bar.
Try this one, for non-silvered C110 bar, at about 1/4 the price:
http://www.onlinemetals.com/merchant.cfm?id=253&step=2&top_cat=87

Thanks for the hint about inner tube insulation. I had no idea it would shrink with heat.

Thanks for the copper bar link. These days, every dollar counts

Luminast · March 2012

Re: Busbar as Battery Interconnects

Yesterday, I made a telephone inquiry with the Technical Service Department at Rolls-Surrette in Canada. The fellow that I spoke with was very helpful and very willing to patiently answer my every question. The batteries I have just recently purchased for my install are Rolls-Surrette AGM's, so I only asked about their AGM batteries. I did not ask about Rolls flooded lead acid batteries, although the engineer that I spoke with seemed to say to me that he thinks the higher cost of AGM's is only justified if a user really needs the beneficial features of AGM technology. (In my case, I'm willing to pay for the characteristic of AGM's that does not requre elborate ventilation, but I also like low maintenance, and low self-discharge rate).

Anyway, about busbar as battery interconnects, the Rolls engineer told me that busbars are perfectly fine, and that they are commonly used in many industrial applications. He informed me that some Rolls battery dealers would even supply me with appropriate busbars. He mentioned that Rolls-Surrette batteries are built to handle a very wide range of operating termperatures without damage, even if hard connected via busbars. Unfortunately, I did not think to specifically ask him if the busbar solution is just as acceptable with FLA batteries. I THINK that was his implication, but I could not attest to that. From my experence, I'm sure that Rolls would willingly elaborate on the answer that question for anyone who asked.

So, I want to thank everyone for contributing their thoughts to my question. My plan now is to order bare C110 copper bar, tin it myself, and fabricate identical busbars for my battery interconnections. This appeals to me as a way to shorten the interconnects, avoid many crimped cable ends, and acheive a nicer looking install.

john p · March 2012

Re: Busbar as Battery Interconnects

Luminist I agree with them and I think you are making the correct decission.
The following is a FACT .
solid interconnects =2 connections per battery
wire +lugs =4 connections per battery
And no matter how well done eventually there is going to be some increase in resistance between the lug and the wire.

Busbar as Battery Interconnects

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