Battery Cable length question

raydiasraydias Solar Expert Posts: 68 ✭✭✭✭
I have 4 50ah AGM batteries on one shelf connected to a buss bar. I wanted to add another 4 identical batteries to the same buss bar. The cable length for the current 4 batteries is 2ft the cable length for the second set would be 3 feet. The battery cable I am using is 2awg stranded copper cable. Would there be any issue with this setup or should I make all cables lengths 3 feet?

thanks

Comments

  • BB.BB. Super Moderators Posts: 26,872 admin
    Re: Battery Cable length question

    If you can, it would be better to make the runs equal length for equal parallel resistance. They do not all have to be 3' (2x3'=6')... They could be 2' on one and 4' on its mate, etc...

    Also, you might want to look an inexpensive DC current clamp meter (such as this one) so you can measure current during heavy charging/discharging to ensure that all strings are properly sharing current.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Battery Cable length question

    There's another potential issue: how old are the existing batteries? Too old, too "used" and they will not work well with brand new ones.
  • raydiasraydias Solar Expert Posts: 68 ✭✭✭✭
    Re: Battery Cable length question

    the batteries are within 6 months of each other and have yet to be used, kept in float so far.
  • raydiasraydias Solar Expert Posts: 68 ✭✭✭✭
    Re: Battery Cable length question

    So it would work if I make the - all 4ft while the + are 2ft. that would keep the resistance even across all 8 batteries
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Battery Cable length question
    raydias wrote: »
    So it would work if I make the - all 4ft while the + are 2ft. that would keep the resistance even across all 8 batteries

    Yes. You're good on battery age, equal length cables per side - all good to go. :D
  • raydiasraydias Solar Expert Posts: 68 ✭✭✭✭
    Re: Battery Cable length question

    Thank you. One last question

    If bank #1 has - cable at 2 feet and + at 4ft lengths can bank #2 have + at 2 ft and - at 4ft lengths or would that not be advisable.

    each bank will be on a separate shelf.

    thanks for the help
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Battery Cable length question
    raydias wrote: »
    Thank you. One last question

    If bank #1 has - cable at 2 feet and + at 4ft lengths can bank #2 have + at 2 ft and - at 4ft lengths or would that not be advisable.

    each bank will be on a separate shelf.

    thanks for the help

    You got it: what really matters is the over-all wire lengths that connect the battery to the circuit common points (bus bars). Length of positive cables plus length of negative cables need to be the same on each battery.
  • GreenPowerManiacGreenPowerManiac Solar Expert Posts: 447 ✭✭✭
    Re: Battery Cable length question

    More of a question than a response on this issue:

    I have a similar setup on my two banks.

    Bank #1 is SLA deep cycle.
    Bank #2 is GEL deep cycle.

    Both are non compatible under one charge controller and must be separate.
    ALL the wires -/+ are the same length/size. If the Amp Hours on any battery are more than 15% less than the others, don't use those batteries. Too small and may explode when overcharged or if the battery goes bad, same thing. The same length principle in wires allows the positive wire to break like a fuse just in case you miss a smaller battery in the loop. Each of my batts has 120 amp hours and 8 gauge wires.

    Now my question is this: Like in A/C, the hot wires are roughly a couple sizes bigger to accommodate the current and the neutrals are smaller in respect to not carrying any current after the power being used. A completed circuit. Can this be done with D/C wiring on the negative side ? A smaller wire to save resources and $$
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  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Battery Cable length question
    More of a question than a response on this issue:

    I have a similar setup on my two banks.

    Bank #1 is SLA deep cycle.
    Bank #2 is GEL deep cycle.

    Both are non compatible under one charge controller and must be separate.
    ALL the wires -/+ are the same length/size. If the Amp Hours on any battery are more than 15% less than the others, don't use those batteries. Too small and may explode when overcharged or if the battery goes bad, same thing. The same length principle in wires allows the positive wire to break like a fuse just in case you miss a smaller battery in the loop. Each of my batts has 120 amp hours and 8 gauge wires.

    Now my question is this: Like in A/C, the hot wires are roughly a couple sizes bigger to accommodate the current and the neutrals are smaller in respect to not carrying any current after the power being used. A completed circuit. Can this be done with D/C wiring on the negative side ? A smaller wire to save resources and $$

    No. Not with either AC or DC. It's a circuit: current flows through both sides, whether positive and negative on DC or Hot and Hot or Hot and Neutral on AC. The current expected on one "side" will be the same on the other, so wire gauge must be equal.

    Who wired your house with undersized neutral lines? Methinks someones been playing the "theoretically, if L1 feeds one half of the 120 duplex and L2 the other half and you have devices on each outlet of the same capacity then the current flow through the neutral line is zero" game. It misses out on the high improbability of there being two matched loads on the outlets running simultaneously, the fact that if only one side is used the current through the neutral is 100% of what flows through hot, the possibility of a neutral failure resulting in 240 VAC flowing through unbalanced loads and maybe setting the smaller one on fire, et cetera.

    Always think in terms of circuits; electricity is not a one-way street.
  • GreenPowerManiacGreenPowerManiac Solar Expert Posts: 447 ✭✭✭
    Re: Battery Cable length question

    Hmmm.... The power company required 3 lines: 2 - 4 AWT Hots and One 2 AWT Neutral to the meter. And the same to my main breaker. Are there different NEC codes in Canada ?
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  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Battery Cable length question

    Why yes, this is a different country and we do have different codes. But the laws of physics are the same so that's not the explanation here.

    What's different is the issue of wiring of a house and wiring to a house.

    Circuits you may have in your home include:

    "Straight" 240 VAC line such as would feed a baseboard heater or electric hot water tank; two "hot" lines of the same size and a ground wire. No neutral.

    "Straight" 120 VAC line such as would feed outlets or hard-wired lights; one "hot" line and one neutral of the same size and a ground wire.

    "Mixed" 240 VAC line such as would feed an electric stove; two "hot" lines of the same size and one neutral with a ground wire. This allows the stove to have 240 Volts for the elements and 120 Volts for lights, controls, and "appliance" outlets.

    The power feed to your house is most like the last. They known that much of the power will be balanced between the two hots, the neutral is (should be) tied to Earth ground at the box. Therefor they can skimp on the neutral feed line because it won't be carrying 100% of the "return" current - unless someone has done something horribly wrong.

    The lines that complete a circuit should be the same size "coming and going". In fact that's how GFCI works; by evaluating the current flow through the hot and neutral lines looking for a discrepancy. A difference outside of normal fluctuation tolerance indicates the current has found a different path to flow through (like your body in the bathtub) and causes the breaker to trip killing the circuit - hopefully before the circuit kills you.

    DC is like that: same current on the positive wire as on the negative. Not so within a circuit board, of course, but it comes out the same at the end. That's what the recurring issue of keeping parallel battery connections is all about.

    Clearer?
  • GreenPowerManiacGreenPowerManiac Solar Expert Posts: 447 ✭✭✭
    Re: Battery Cable length question

    Yes. Clearer. However still thought that when a circuit is used, the neutral side carries zero current yet the same voltage back to the load center, not requiring as large of wires. I see the house wiring sizes are identical except for hots/neutrals.

    Thought the same principles applied for D/C circuits too.

    Back to the question at hand.....

    Ideally, I'd make the longest cable the template length for the others. And would put smaller wires (8 gauge) on each battery to the buss bar. So when and if a battery goes bad, the positive wire breaks and takes it out of the circuit much less makes it easy to spot and replace. Fuse protecting them works good too, yet expensive with that sized wires. What happens is this: A battery that is bad will be charged by the other batteries in its circuit until it reaches a satisfactory charging state or try to overheat it, thus resulting in the breaking of the weakest link, the wire snaps.
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  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Battery Cable length question
    Yes. Clearer. However still thought that when a circuit is used, the neutral side carries zero current yet the same voltage back to the load center, not requiring as large of wires. I see the house wiring sizes are identical except for hots/neutrals.

    Thought the same principles applied for D/C circuits too.

    On a 120 VAC circuit in your house the neutral carries the same Voltage and current as the hot.
    In the case of the house feed the neutral only has to carry the current imbalance between the two hots.

    For all 240 VAC circuits, the two wires are the same size (hot to hot).
    For all 120 VAC circuits, the two wires are the same size (hot to neutral).
    For all DC circuits, the two wires are the same size (positive to negative).

    The current within a circuit is the same on one side as the other.

    Don't confuse "neutral" with "ground" either. Ground wires are often slightly smaller to save money on cable costs. This is because under normal circumstances the ground carries no current; it's there in case things go wrong. When that happens it may have to take the full load, but only momentarily until the fuse blows/breaker trips
  • raydiasraydias Solar Expert Posts: 68 ✭✭✭✭
    Follow up question: Battery Cable length question

    If i change my 12v bank to 24 does the length between the + on 12v battery #1 to - on 12v battery #2 need length consideration. From what i have seen in pictures and read you want it to be as short as possible and what matters is the +/- from the 24v combined battery to the charge controller. is that a correct assessment? And using a copper bar would be better than cable?
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Follow up question: Battery Cable length question
    raydias wrote: »
    If i change my 12v bank to 24 does the length between the + on 12v battery #1 to - on 12v battery #2 need length consideration. From what i have seen in pictures and read you want it to be as short as possible and what matters is the +/- from the 24v combined battery to the charge controller. is that a correct assessment? And using a copper bar would be better than cable?

    If you have just two batteries in series the consideration is the over-all length of the negative, positive, and connecting cable in respect to it all adding resistance to the circuit. The connecting cable should be the same size as the (+) and (-) and kept short to avoid adding resistance.

    If you have battery strings in parallel then the length of the connectors between batteries should be equal. This is particularly important on 3 or more in parallel where the (+) and (-) leads go to bus bars and need to be kept equal length.

    Yes you can use copper bars (or flattened copper pipe) for interconnecting. As long as it's large enough to handle the current it will be fine. John_p actually tested this and proved it. It's not better than cable in terms of Amp capacity, but it's not worse either. There's considerations for ease of installing too; sometimes the cable will be simpler to put in, but has connectors which can loosen, sometimes there could be issues with the bare length of copper.

    Not just cable length to the charge controller but to the inverter (if any) as well.
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