sizing battery interconnect cables

[defed]

what is the determining factor for wire gauge when connecting batteries in series?  in other words, how do i determine the amperage i need to wire for on a given series?  i would have (4) 6v 220ah.  i found a super deal on some 6ga wire w/ lugs...cheaper than i could even make them, but i think 6ga will be too small.

thanks.

[ghurd]

The charging amps or the load amps.

#6 is good for over 100A sustained in short runs like battery interconnects, which is more than the batteries are good for.

No need to worry too much about surge currents. 
The inverter has capacitors to help with short surges.
DC loads are usually OK if the surge has a bit of short duration voltage drop.

I think #6 would be fine for about anything suited to that size battery bank.
G-

[DanG]

If you have a serious jones for copper they can* be doubled up but 'as crimped' lengths have to match (ie: machine not hand formed) in each cable pair set.. .

[defed]

regarding cable lengths...do just 'same job' cables have to match?  for example, all of my series cables would be 8", my parallel cables would be be 12", my output cables would be xx"....the series and parallel cables don't have to be the same length, as long as their like use counterparts are equal?

thanks for all the help you guys have offered on this group.

[ghurd]

With paralleled batteries, it is best to have the same total combined length of wire from one buss to the other for each string.

Like if one bank has all 3 wires 12" long (3' total from buss to buss),
and the other bank has a 6", a 12" and an 18" (3' total from buss to buss),
then that would be OK.

If you plan on doubling the wires in parallel,
then the wire should match the length of the wire it is paralleled with.
Meaning paralleling a 6" wire with an 18" wire is not the best idea.  (at least on paper)

Its the same idea as your battery bank wiring sketch, and why you connected the green to one bank, and the red to the other.
Notice the wire length from buss to buss is the same for both banks.
Just keeps things a bit more balanced.

It's kind of hard to keep parallel strings balanced anywhere near perfect anyway, so every little bit helps I suppose.

That assumes all the cables are the same gauge.

Guessing you found a factory set of golf cart cables with all sorts of different lengths in the set.
G-

[defed]

i found some 8" 6ga cables w/ terminations that i think are for lawn mower starters...$2.60 for 8" and $3.40 for 12", just have to check shipping.  this seems cheap enough to not bother making them myself.  i did see some golf cart sets, but like you say, the wire size combos don't come out nice as if i just buy all 8" cables.  i planned to use 8" for series and 12" for parallels, so each bank would have 36" of cable.  just wanted to make sure the series connections didn't need to be 12" like the parallel.

[ghurd]

Be certain the cable terminals are compatible with the battery terminals and buss bar.

Drilling a 5/16" terminal out to 3/8" rarely works out well for me (like never).
G-

[ChrisOlson]

The charging amps or the load amps.

What ghurd said.

In series strings each battery in the string has to deliver the full amps delivered by the string.  In parallel strings each battery has to deliver the amps delivered by the string divided by the number of batteries in the string.

If you're connecting four 6 volt batteries in series for 24 volt, if the string delivers 100 amps to the inverter (2,400 watts of DC power), then every battery in the string has to deliver 100 amps.

If you're connecting two 6 volts in series for 12 volt, then paralleling the two 12 volt strings for 12 volt output to the inverter, then you're looking at 200 amps of output (2,400 watts of DC power).  Each series string will deliver 100 amps, and each battery in the bank is still delivering the same 100 amps.

With the 24 volt configuration, 6 AWG is OK for the interconnects but you'll want 4 AWG or bigger to the main DC bus or disconnect because the run will be longer.

With the 12 volt configuration, 6 AWG is OK for the series wires but I'd use dual 4 AWG (a positive and negative from each series string) to the main DC bus or disconnect up to 10 feet away.  I would not parallel the two strings, then pull off one end of the parallel wires.  Use the bus to do the paralleling of the strings.

With stranded battery cables, you can use the following guidelines for a run of 10 feet and a voltage drop of .5 volt.
6 AWG is good for 82 amps or 853 watts
4 AWG is good for 131 amps or 1,355 watts
2 AWG is good for 208 amps or 2,151 watts
1/0 is good for 330 amps or 3,414 watts of DC load

It's nice to have series and parallel wires all the same length, but if you're dealing with lengths under 1-2 feet it don't make much difference.

--
Chris

[defed]

thanks for all of the good info.

the max charging amps from the solar is 60 (controller max), and the 4' turbine is less than 10 on avg, tho i have seen 16 for brief moments.  the future 10' will obviously put out more than that.

at this time (and until at least next summer), the actual load on the bank will be quite small.  the bank will be (4) 6v in series and (2) sets parallel.  the main reason for wanting to use the 6 gauge now is the price.  for about $30 to my door, i can get all 8 cables (minus the bank to distribution block cable).  i'm sort of in a hurry to get this all hooked up before the snows hit...otherwise i would probably make/find some bigger cables since in the future i may have/need more power thru the system.  i always need small battery cables for my lawn mower projects, so when/if i upgrade, i should be able to find a use for the 6 gauge cables i would be using now.