Battery Bank Cable Lengths

CanemanCaneman Solar Expert Posts: 71 ✭✭✭✭
Hello NAWSF... I am getting ready to make cables for my battery bank... the BB is 24v made from 4 6vdc Sam's Club golf cart batteries in series... questions that I have:

1) is it important to make the series connect cables all the same length?

2) can the cables on each end of the BB that connect to the main bus bar be different lengths than the series cables?

any other advice would be appreciated... I am using 2/0 welding cable and copper lugs, will make the connection by crimping and soldering... thanks!


  • westbranchwestbranch Solar Expert Posts: 5,183 ✭✭✭✭
    Re: Battery Bank Cable Lengths

    simply 1. yes
    2. No, see 1....

    there is a good response by johnp about inter battery connections, includes cable vs solid connections. sorry cant find it right now.

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  • BB.BB. Super Moderators, Administrators Posts: 32,005 admin
    Re: Battery Bank Cable Lengths

    Read the webpage at SmartGauge about the "correct" way to parallel battery strings.

    In summary, lead acid batteries have very low internal resistance and by the time your cables get over a few feet in length, typically, the cables have enough resistance that they can "steer" current to the batteries with the shortest/lowest resistance and away from strings with higher resistance.

    Personally, I like to recommend a single series string... There are now 4 volt and even 2 volt cells--These allow you to have very high AH capacity cells with a maximum weight of ~150-300 lbs (about the largest cell one or two people can move around without a crane/forklift) and still wire up a single series string.

    If you still want/need to make parallel strings, then each string should have its own fusing/breaker, and I would recommend 2-3 parallel connected strings maximum. There are lots of issues with parallel connected batteries--Which we can discuss more if you wish (however, there are many people that that are happy with the 4+ parallel strings too).

    Besides the usually battery tools (like a good quality hydrometer--for flooded/wet cell batteries), I would suggest getting a DC Current Clamp Meter (here is a "good enough" $60 meter which includes standard DMM type functions). You can clip on a (single) cable and measure current flow during charging/discharging and make sure your batteries are properly sharing the loads.

    Some fine testing from John about 1/2 and 3/4 copper water pipe used as bus/jumpers for battery banks:
    john p wrote: »
    IM not wanting to get into another discussion on bus bars again but just reposting the post from a long time ago and the results for FARMERJOHNAZ to see if this answers his question..
    It shows the very tiny losses you get from using cheap copper tube available from any hardware store.. you just hammer or put in vice the ends of the short lengths of tubing to flatten them then just drill a suitable sized hole each end..

    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..

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • CanemanCaneman Solar Expert Posts: 71 ✭✭✭✭
    Re: Battery Bank Cable Lengths

    BB, thanks for those tips I am going to do that with the meter and copper pipe... bought the Sears clamp on meter last weekend and can't wait to start playing with it...

    the batter bank is series, is there a problem with making all the interconnects 18"? this would include the end cables that attach to the bus bar, making all cable attached to the battery bank the same length...
  • CanemanCaneman Solar Expert Posts: 71 ✭✭✭✭
    Re: Battery Bank Cable Lengths

    looks like the Coot already answered this here:

    In a single series of batteries the wire lengths between them do not have to be equal nor do the positive and negative cables to the inverter or controller. But it is always best to do as vtMaps says and keep them as short and fat as possible to reduce the over-all resistance in the circuit.
  • BB.BB. Super Moderators, Administrators Posts: 32,005 admin
    Re: Battery Bank Cable Lengths

    It is always relative to current flow... 2 awg may be overkill for a small bank/load, and undersized for a large bank with 6kW 12 volt inverters (not that I would recommend that).

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • mike95490mike95490 Solar Expert Posts: 9,358 ✭✭✭✭✭
    Re: Battery Bank Cable Lengths

    Use a stout hydraulic crimper, it can cold weld the wires to the lug.
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
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  • 2manytoyz2manytoyz Solar Expert Posts: 373 ✭✭✭
    Re: Battery Bank Cable Lengths

    The largest cable I've been completely successful in soldering is 6 ga. While larger cables "look" properly soldered, the inner strands don't soak up solder. Cut one apart with a bandsaw and you'll likely discover what I'm referring to.

    The other issue with soldered connections is they are less than ideal for high current applications. A poor connection on a battery terminal can cause a lug to get hot, the solder then melts, and the cable could pull free of the lug. Bad things could happen with a stray cable...

    I have a hydraulic crimper from Harbor Freight. Rated at 8 tons of compression:

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    It works well, and is simple to use.

    Another crimper that I've used for years is an anvil type. These are available online, or in the welding section of Ace Hardware:

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    And if you want simple, these clamp type, made by Thomas & Betts, work great. I bought these at Home Depot. These have been in use on my battery bank since 2007.

    Attachment not found.

    FWIW, I have a 12V battery bank consisting of 8 6V Golf Cart batteries. Each two are in series to make 12V, then connected to a bus bar. All four pairs are in parallel. All the battery cables are the same length.

    The copper used for the bus bars was calculated based on the maximum current my system would ever see, then I selected an even larger size. They were custom ordered (width, thickness, length) from here:
  • 2manytoyz2manytoyz Solar Expert Posts: 373 ✭✭✭
    Re: Battery Bank Cable Lengths

    This forum only allows 5 pics per post, so this is part two...


    Rather than tap the copper, I drilled holes completely through the copper, and used 1/4-20 stainless hardware. The insulators were purchased online from Storm Copper. P/N 1100-A1, $1.05 each.

    The copper buss bars were purchased from Online Metals. 2 pieces @ $15.88/piece
    Copper 110 H04, Rectangle, 0.25" x 1.25", Cut to: 12".

    The bars were mounted to a piece of plywood, with a center divider. The divider prevents a dropped (conductive) tool bridging both sides of the bus bars. A Lexan cover also protects against accidental shorting, while still providing the ability to visually inspect the connections.


    Attachment not found.

    I've been using this setup for the past 5 years. My oldest batteries in this setup were purchased in Jan 2007. The SG says these are still about 85-90% SOT. Not bad for $71 big box store batteries.

    And to verify the pairs of batteries were sharing the load, I put a heavy load on the inverter, and used an Extech clamp-on DC Ammeter on each string.

    Edit: Wanted to mention I did use a copper polish, Nevr-Dull. It removes the surface oxidation, and prevents it from coming back. After it was shiny, a soft cloth wiped off the polish residue. Now several years later, the copper is still shiny. I checked with a meter to make sure there was no measurable resistance with the polish applied. This product has been used by the Military for years for cleaning copper connections. Often called "wad pull". Available at Ace Hardware.
  • CanemanCaneman Solar Expert Posts: 71 ✭✭✭✭
    Re: Battery Bank Cable Lengths

    Robert, thanks for the info!

    I stumbled across your webpages a few months ago and just finished a bus bar based on your design, it worked out really well. :D
  • DillDill Solar Expert Posts: 170 ✭✭
    Re: Battery Bank Cable Lengths

    I've been fighting this issue with my 4 golf cart battery bank for the last few weeks. I reworked all the cables but I'm still drawing (and thus charging) one set of batteries more than the other. They are different brands of battery, purchased within a week of each other, so maybe I'm just chasing my tail. I'm going to EQ both pairs separately to see if that helps at all. I have a similar setup as Robert, with both strings tied into a bus bar with 2 gauge cable. The positive cable is relatively short, maybe 12 inches, but the negative cable is probably 30 inches to reach the back negative post. Both sets of cables are the same length within a quarter inch or so.

    My inverter has been acting up lately, my alarm clock in the house has been slow by 10 minutes the past week, and I've noticed a couple lamps with CFL's flickering. Maybe I'll just man up and buy a 24V inverter and convert my bank to 24V.
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Battery Bank Cable Lengths

    batteries of different brands may be your problem. they may be similar, but they aren't the same and that's probably enough to throw it off.

    great job on those buses and you've done it almost exactly how i have said for others to do it in the past. oversizing is key because when one drills into the bar it sets up an increased series resistance at that point and given a string of them spaced out could throw it out of balance if not oversized to minimize these resistive bumps in the road.
  • DillDill Solar Expert Posts: 170 ✭✭
    Re: Battery Bank Cable Lengths
    niel wrote: »
    batteries of different brands may be your problem. they may be similar, but they aren't the same and that's probably enough to throw it off.

    good thought, I might grab the cheaper pair to use at my cabin and get another set of the 'better' batteries for at home.
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