Battery interconnect cable sizing

salexsalex Registered Users Posts: 2
Greetings and thanks in advance for your time.

I put together a little DC only solar system to power 12-15 Watts of LED light.  I used a calculator to determine the panel size and battery capacity, and the little system works pretty well.  I need to replace one of the batteries (have 3 different systems) and found 32 AH AGM batteries for a great price used.  I'll need to run them in parallel to get the AH I need.  What AWG cables will I need to connect two of these 32 AH batteries in parallel?


  • BB.BB. Super Moderators, Administrators Posts: 32,230 admin
    You have two needs to satisfy...

    First is that the leads are heavy enough to carry the needed discharge and charging current. The NEC (National Electric Code) is relatively conservative (I like to derate current ratings by 1.25 -- i.e., 10 amps load * 1.25 = 12.5 amp rated circuit+fuse+breaker ratings).

    For paralleling batteries--In theory, each battery provides only 1/3rd of the operating current... However, batteries and connections fail, so I suggest that for 1/2/3 parallel battery connections, that the wiring be rated for full load, not 1/3 or 1/2 of load. Also, how you connect batteries in parallel does matter:

    And then there is sizing the wiring for maximum voltage drop. Ideally, you want a maximum drop of 0.05 to 0.10 volts for 12 volt batteries when charging and a maximum of 0.5 volt drop when discharging.

    Say you have a 10 amp load and want to send the voltage 15 feet, using a generic voltage drop calculator and trying different size wiring, you would find:

    12 AWG wiring, 10 amps and 15 feet One way run, gives you:
    Voltage drop: 0.48
    Voltage drop percentage: 4.00%
    Voltage at the end: 11.52

    You can see, if that was 10 amps of charging current and you wanted a max of 0.05 to 0.10 volts of drop, then the wiring would have to be much heavier--Or the better solution is to keep the charge controller to battery wiring short and heavy... Say 3 feet away:

    A 12 AWG cable 3 feet away gives you:
    Voltage drop: 0.095
    Voltage drop percentage: 0.79%
    Voltage at the end: 11.905

    And then there is safety... Even a relatively small lead acid battery bank like yours can output 100's of amps into a short circuit. You need to ensure the wiring (and battery tops) cannot get shorted, and any wiring that leaves the battery be rated for load and protected by a fuse or circuit breaker (10 amp load, 12 AWG wire--10a*1.25=12.5 amp rounded up to 15 amp minimum fuse... 12 AWG would be a maximum of 20 amp fuse per NEC).

    Also, fuses vs breakers... Breakers are nice because you can turn off load with breaker (when needed). Don't use inexpensive automotive fuse holders (plastic fuses with two tabs out bottom) for any heavy loads (over ~5 amps)--The holders and fuses are not very reliable and have been known to melt at even light currents).

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • jonrjonr Solar Expert Posts: 1,386 ✭✭✭✭
    edited February 2018 #3
    The smartgauge recommendations are a good starting point, but to get accurate parallel balancing you need to measure current flows (with a clamp ammeter).

    I am available for custom hardware/firmware development

  • salexsalex Registered Users Posts: 2
    Thanks for the links and expertise!  So using one of those calculators, based on the load of 15W/12V=1.25A and distance (10ft) I went with 12 AWG stranded wire in a protected sheath.  As I mentioned, the system is up and running and working well.  The daily load is being replenished by the 30W panel, even on cloudy days, in less than 3 hours.

    Sounds like I should put in a breaker on the load cable, even with the small 1.25 A load.  Maybe a 5 Amp fuse?

    So to determine the AWG for the battery interconnect cables run in parallel, I would use the input Amps from the panel 30W/12V=2.5A*1.25=3.125A?  The batteries will be side-by-side in a cooler for protection, maybe a distance of 8".  I found a deal on 4AWG cables; would this be a big enough AWG?
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