Battery interconnections

sawmillsawmill Solar Expert Posts: 93 ✭✭✭
I have a 12 battery (t-105), three string 24 volt system. It is time to replace my trainer batteries (five years +). I want to change to copper bus bars for my battery interconnections and unsure how to compute the size of these bus bars.

I use a Outback 3524 and will retain my 4(O) cables for the inverter connection. Can someone point me to the correct formula to use for interconnects.

Thanks
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Comments

  • WindsunWindsun Solar Expert Posts: 1,164 ✭✭
    Re: Battery interconnections

    I am not a big fan of bus bars, but see this for some info on battery hookups

    http://www.sunxtender.com/pdfs/Sun_Xtender_Battery_Technical_Manual.pdf
  • sawmillsawmill Solar Expert Posts: 93 ✭✭✭
    Re: Battery interconnections

    Since all my equipment was purchased from Wind-Sun your expertise is highly valued. Could you elaborate on your opinion concerning bus bars and also crimpted lugs with and without solder?
  • audredgeraudredger Solar Expert Posts: 272 ✭✭
    Re: Battery interconnections

    I vote for figure 5-9 with one modification, add a fuse at each positive to bus connection.
  • john pjohn p Solar Expert Posts: 814 ✭✭✭
    Re: Battery interconnections

    being a cheapskate I have nearly always done the following .Go to hardware/store buy a length of coiled 3/4" copper pipe . cut into suitable lengths hammer or press flat the ends then drill a siutable bolt hole to go on battery lug. for bus bar do nearly the same hammer or press a suitable length of pipe then drill holes about 11/2 " apart ,use brass bolts to connect the lugs to the pipe.
    As for making lugs I have always crimped and soldered .A pair of vice grips makes a good crimper, I believe that soldering is best done using a small pinpoint gas burner, so as to only heat the lug mostly then get 1mm resincored solder and feed it down the space betwen the lug opening and the wire Hold the wire in air so the lug points downwards.After crimping of course. It very much helps to stop corossion of the wire ends. its even better if you put heatshrink over the lug and the cable.But many others are going to tell you not to solder as it does all sorts of bad things to the coppper wire, and it wont make a "good" electrical connection. Well mabe they right but its worked for me for over 40 yrs and that includes doing them for use in marine enviroments. NEVER A PROBLEM.
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Battery interconnections

    careful john as you don't know the ohms per 1000ft of the copper pipe you are using. you can't even go by its weight as different pipes can have different impurities and/or alloys in them. you just don't know if it is actually sufficient for all bus applications, but it can be physically configured with copper pipe with some degree of success.
  • john pjohn p Solar Expert Posts: 814 ✭✭✭
    Re: Battery interconnections

    You are correct Niel I have no idea of the ohms per 1000 ft of copper tubing,And to be honest I think even knowing it would be useless information.
    Let me ask this when you have ever used copper cable for any battery connection did you ever find out its resistance per 1000ft? Have you ever personally measured a cable or had someone use an extremely accurate meter to read resistance of a cable before you used it?? or do you just use the standard published figures for the average cable and hope the one you are going to use is going to have a resistance something like that of the published one.?
    I think its going to be difficult to get an accurate reading of a pipes resistance when only using about less than 1ft.
    But out of curiosity next week I will try to con the place where I work into buying a few random lengths of pipe 3/4 dia and about 10ft lengths.We have an very accurate device to measure resistance . I will post the results.
    I really believe 2 lugs on a 9" length of #2 copper cable would have a greater resistance than ANY 9" length of 3/4 copper pipe

    but it can be physically configured with copper pipe with some degree of successWell I must have been just always lucky as its always been 100% successful for me.
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Battery interconnections

    as far as my own wiring goes, yes i do look up the resistance as a factor as this involves voltage drop. wires have known resistances with each gauge number or in the case of many outside of the u s, the mm. bus bars are no different as they need to be able to meet certain resistance criteria or current capability. do you think it's ok for me to use a thin wire in my battery connections just because the resistance shows it to be low being only 1 ft long? if anything it should be even lower in resistance to insure equality between batteries and that higher currents will not be impeded and it is a part of a bigger picture involving voltage drop. using pipe introduces and unknown factor into the system.
    pipes not only, as i stated before, come with different alloys and impurities, but they come in different thicknesses too so it is important to get a handle on what it is you are using to at least have some idea on it.
  • BB.BB. Super Moderators, Administrators Posts: 32,999 admin
    Re: Battery interconnections

    Here is a site with Copper Bus Bar Tubing and their electrical properties... You can compare that with copper water pipe to make sure your measurements are in the same range.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • nsaspooknsaspook Solar Expert Posts: 396 ✭✭✭
    Re: Battery interconnections
    john p wrote: »
    You are correct Niel I have no idea of the ohms per 1000 ft of copper tubing,And to be honest I think even knowing it would be useless information.
    Let me ask this when you have ever used copper cable for any battery connection did you ever find out its resistance per 1000ft? Have you ever personally measured a cable or had someone use an extremely accurate meter to read resistance of a cable before you used it?? or do you just use the standard published figures for the average cable and hope the one you are going to use is going to have a resistance something like that of the published one.?
    I think its going to be difficult to get an accurate reading of a pipes resistance when only using about less than 1ft.
    But out of curiosity next week I will try to con the place where I work into buying a few random lengths of pipe 3/4 dia and about 10ft lengths.We have an very accurate device to measure resistance . I will post the results.
    I really believe 2 lugs on a 9" length of #2 copper cable would have a greater resistance than ANY 9" length of 3/4 copper pipe

    but it can be physically configured with copper pipe with some degree of successWell I must have been just always lucky as its always been 100% successful for me.

    Here are some specs for "electrical" copper tubing. Multiply resistance about X4 for unknown home-depot tubing type L.

    http://www.stormcopper.com/copper-bus-tubing-specs.htm
    http://www.engineeringtoolbox.com/astm-copper-tubes-d_779.html
  • john pjohn p Solar Expert Posts: 814 ✭✭✭
    Re: Battery interconnections

    Neil when I asked had you ever measured the resistance of wire for battery connections I was not saying to use "thin" wire or that you would use "thin" wire I ment what I consider nornal #4 wire and lugs between batteries and same size for say connection to a inverter, So I ask have you ever checked the resistance of the wire? then after you connected the crimped lugs on the ends ?
    BB your data supplied proves my point better than my txt for all doubters. it shows 1/2 pipe can handle if I read it correctly about 380 amps for a 30deg c temp rise. so then 3/4 " pipe can handle 540a way above what any device is ever going to connected at home to ANY battery system.
    lets have example 540a x 48v =about 26,000 watts. even if we say pipe from home hardware is only 1/4 as good electrically thats still 6480 watts .
    I think it safe to say for average home use any 3/4 copper pipe is way more than adequate and it solves the problem of the home person trying to get perfect crimps of the lugs on the #4 or bigger wires. And imperfect crimps are more likely to create problems than imperfect copper pipe is.
  • BB.BB. Super Moderators, Administrators Posts: 32,999 admin
    Re: Battery interconnections

    NSASpook is probably correct that copper water pipe is not alloyed for low resistance--so it will be interesting to see what resistance John,

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • john pjohn p Solar Expert Posts: 814 ✭✭✭
    Re: Battery interconnections

    BB thats is why I derated it by a very generous factor of 4 as was requested to do for unknown copper water pipe. But even after that it can carry more amps than any home user is ever going to use.
    Some how everyone has missed the point of my first post about battery interconnects. It was not aimed at the person charged with designing a 10GW solar plant ,It was ment for the home user that is likely to be doing something like this.. connecting 4x6v high amp batts in series.connecting 3or 4 12v batts in parallel or a combination of the above and using a inverter of up to 2kw.For that person to use 3/4 copper pipe is perfect for the interconnects and for the negative wire returns, it makes a great simple buss bar,
    A major problem for home handy person is making really good crimp joints on lugs at #4 wire size and above.I have seen some people try to hammer them flat .but because of springiness in the lug it makes for lousy joint .Although flat jawed vice grips can do a ok job on #4 cables.Where I work we use a 10 ton press with deep serated jaws then do the finger test ,, we connect the cable to a 100a load across a 12v batt for 5 mins and then touch the lug if its still cool its good.its then heated as little as possible and no lead solder is fed into the back of the lug until its full. then glue heat shrink is fitted.
  • nsaspooknsaspook Solar Expert Posts: 396 ✭✭✭
    Re: Battery interconnections
    john p wrote: »
    BB thats is why I derated it by a very generous factor of 4 as was requested to do for unknown copper water pipe. But even after that it can carry more amps than any home user is ever going to use.
    Some how everyone has missed the point of my first post about battery interconnects. It was not aimed at the person charged with designing a 10GW solar plant ,It was ment for the home user that is likely to be doing something like this.. connecting 4x6v high amp batts in series.connecting 3or 4 12v batts in parallel or a combination of the above and using a inverter of up to 2kw.For that person to use 3/4 copper pipe is perfect for the interconnects and for the negative wire returns, it makes a great simple buss bar,
    A major problem for home handy person is making really good crimp joints on lugs at #4 wire size and above.I have seen some people try to hammer them flat .but because of springiness in the lug it makes for lousy joint .Although flat jawed vice grips can do a ok job on #4 cables.Where I work we use a 10 ton press with deep serated jaws then do the finger test ,, we connect the cable to a 100a load across a 12v batt for 5 mins and then touch the lug if its still cool its good.its then heated as little as possible and no lead solder is fed into the back of the lug until its full. then glue heat shrink is fitted.

    A simple tool like this and a small sludge hammer makes nice wire crimps. One hit to set, two to make sure.

    http://crimpsupply.com/tools/crimping-tools/hammer-indent-crimper.html
  • WindsunWindsun Solar Expert Posts: 1,164 ✭✭
    Re: Battery interconnections
    nsaspook wrote: »
    A simple tool like this and a small sludge hammer makes nice wire crimps. One hit to set, two to make sure.

    http://crimpsupply.com/tools/crimping-tools/hammer-indent-crimper.html

    Which is basically the same thing we sell here http://store.solar-electric.com/hacrtoforlal.html

    As far as soldering, you do get a better electrical connection, but not always a better mechanical connection, and you can run into problems on high current systems. Soldering is an electrical connection only, not mechanical. We have seen cases where the terminal lug got hot and fell apart on solder-only connections.
  • john pjohn p Solar Expert Posts: 814 ✭✭✭
    Re: Battery interconnections

    Windsun that sems like an ok tool . never seen one here in Aus only the very expensive professional use lever type.
    You will notice that I tried to make it 100% clear that FIRST you need to make a good mechanical crimp BEFORE soldering. but it seems everyone missreads my saying that in every post by their replies as if im advocating solder only. IM NOT THAT IS HOPELESS AND FAILURE WILL MOST LIKELY RESULT UNDER HEAVRY LOAD
    I try one more time,
    MAKE A VERY GOOD MECHANICAL CRIMP FIRST THEN TEST THEN SOLDER LAST.:cool:
  • System2System2 Posts: 6,290 admin
    Re: Battery interconnections
    Windsun wrote: »
    Which is basically the same thing we sell here http://store.solar-electric.com/hacrtoforlal.html

    I drove up to Flagstaff a few weeks ago and picked up one of these and some 3/0 cable. This little crimper works great! I used a 2-1/2 or 3 lb mini sledge.
  • john pjohn p Solar Expert Posts: 814 ✭✭✭
    Re: Battery interconnections

    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..:cool:
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Battery interconnections

    :D

    That's all I'm saying.
  • audredgeraudredger Solar Expert Posts: 272 ✭✭
    Re: Battery interconnections
    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

    If you used #2 interconnects around here you would get beat at the least. 2/0 interconnects on my bank!
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Battery interconnections

    john,
    i applaud your testing of the pipe. you made it known as to how it stands versus wires and i suppose one may need to watch the connections to the bus as well and, although this is less likely to introduce a problem, it could still pose a problem just as wire connections do. as audredger pointed out it would not suffice as a bus in his battery bank and this was the reason i said to be careful as it was an unknown as to how well it could suffice as a bus in all cases. you have verified my thoughts that it does not give blanket coverage and peace of mind for all b bank interconnection needs.
    i have also always advocated the use of wires or buses that are 2-3x the equivalent gauge as the largest wires used anywhere in the system to the batteries. this serves to keep the equalization of the bank more equal allowing the bank to look and act more like 1 battery.
  • john pjohn p Solar Expert Posts: 814 ✭✭✭
    Re: Battery interconnections

    Niel .Audridgers reply is not correct. as if he used 8.3/4 pipe 2mm soft = .00041 (not the best one but average)it has a resistance on a 1ft length of .000034. that is still going to be a lot lower than if he used 2 parallel runs of #0 cable as the cable lug connections have a higher resistance than that even if he used a cable with almost zero resistance. And thats difficult to find even if you run the cable in liquid nitrogen.

    I think what the whole testing did was show the lug/cable connection is the weak link when you get to cables above #4. and it did prove adding solder into the lug does help both the mechanical and electrical connection.
  • audredgeraudredger Solar Expert Posts: 272 ✭✭
    Re: Battery interconnections

    My point was #2 wire is too small .... other point is; I live in the land of great shakes, cable has some give .... tube has none.
  • john pjohn p Solar Expert Posts: 814 ✭✭✭
    Re: Battery interconnections

    My point was and still is even 2 parallel runs of #0 cable or 4 runs of parallel #2 cable is going to have more resistance than 3/4x 2mm copper pipe.

    As for using cable to better absorb "earthquake" shakes I dont think you would find much in flex with 6" of #0 cable. You will notice all electric vehicles use solid connections between battery cells. why? only half the number of connections. Also military radiotrucks have all the FLA batteries with solid copper plate connections and they are designed to travel over very rough ground and keep working. So by comparison any vibrations in a house is not going to be a problem with solid battery connections, I would be thinking if the "house shaking" is that severe you more likely to be worried about the house collapsing rather than the batteries vibrating and loosening their connections.
  • john pjohn p Solar Expert Posts: 814 ✭✭✭
    Re: Battery interconnections

    Audredger I notice you using a 4000w inverter and 24 x6v batts. most likely connected as 6 strings of 4 batts per string??
    If surge rating of say 5000 watts is taken as max . and about 90% efficient that would be a max current draw of about 230a. Therefore if 6 string of batteries a bit under 40 a per string. A #2 cable has a 20deg c rating of 160 a. IF you were using #2 cable as interconnects you would only be still using avbout 1/4 of their capacity.
    Dont know what you are using between batteries and inverter but #0 cable only has a rating of 200a a little shy of what your max current draw could be. But if only about 2ft long its most likely ok.
  • audredgeraudredger Solar Expert Posts: 272 ✭✭
    Re: Battery interconnections

    Point taken. Think I'll keep the cables that I have though.
  • john pjohn p Solar Expert Posts: 814 ✭✭✭
    Re: Battery interconnections

    Audredger dont get me wrong im not criticizing your connections or any part of your system. its a good system and well connected
  • audredgeraudredger Solar Expert Posts: 272 ✭✭
    Re: Battery interconnections
    Audredger I notice you using a 4000w inverter and 24 x6v batts. most likely connected as 6 strings of 4 batts per string??
    If surge rating of say 5000 watts is taken as max . and about 90% efficient that would be a max current draw of about 230a. Therefore if 6 string of batteries a bit under 40 a per string. A #2 cable has a 20deg c rating of 160 a. IF you were using #2 cable as interconnects you would only be still using avbout 1/4 of their capacity.
    Dont know what you are using between batteries and inverter but #0 cable only has a rating of 200a a little shy of what your max current draw could be. But if only about 2ft long its most likely ok.

    John, yes 4 L16H 6 v tied together with 2/0 to make a 24 v bank; 100 amp fuse at each bank and #2 cable from fuse to 600 amp bus bar; 4/0 cable from bus bar to inverter breaker and then to inverter.

    BTW all my wire is oversize. I try to keep the line loss to <1%. Tough enough to generate it to loose it as line loss!
  • john pjohn p Solar Expert Posts: 814 ✭✭✭
    Re: Battery interconnections

    #4 wire is only rated at 160 amps really that is small for 4000 w as 4000 divide by 24 =166 amp not allowing for surge (probably little) or inverter loss . everything else you have is over kill but thats not a bad thing.
  • audredgeraudredger Solar Expert Posts: 272 ✭✭
    Re: Battery interconnections

    John, not #4 but 4/0 (0000); 11.68 mm not 5.18 mm ... 2/0 (00) is 9.2 mm. BIG welding cable
  • john pjohn p Solar Expert Posts: 814 ✭✭✭
    Re: Battery interconnections

    oh oh oh oh oh oh oh ok:D
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