diagonal wiring of battery bank

jimskau
jimskau Registered Users Posts: 21 ✭✭✭
:oops: I hate to admit it, but reading through some of the excellent correspondence on battery stuff I stumpled upon the actually obvious tip of wiring the inverter and chargecontroller diagonally to reduce voltage difference in the cells. Well the embarrassing part is that I have my 4 x 250 ah battery bank wired at the nearest end to my units, ignorantly thinking I was doing the best.
Now I would like to make sure that I do everything correct before I submerge myself into the sulphuric clouds of my battery store.
1) Do I have to keep the cables from both inverter and mppt controller at the same length for positive and negative terminals? I was considering extending the positive inverter lead to reach the far end of the pack with a fuse box plus more cable of the same gage, but keeping the negative intact in its original length.
2) If the fusebox method is approved, what size fuse should I put in, and do you have any hints about where to get these on the net?

Data is as follows: 1000 ah monobloc battery store, 1500W pure sine inverter, 450W monocrystaline panels wired in 3 units to 36V, one mppt max 500W mp pt.

3) if anybody could direct me to a website to buy propper DC overcurrent protection breakers for an affordable price it would make me very happy. I am located in southern Spain, and there don´t seem to be a lot around these parts.

Yours sincerely, Jim.
8 x 250Wp Suntech (wife insisted on a dish washer), Outback VFX 3024, Midnite Classic 150, 4 Rolls 6CS25p. Solar installer in Southern Spain. 

Comments

  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: diagonal wiring of battery bank

    i'll take a stab at answering your questions here.
    1> no they do not have to be the same length as the + and - leads are seen additively as a whole circuit with the load inbetween them. if you are lengthening the wire you should recheck if the larger voltage drops it creates are acceptable. odds are you will need thicker wires, but if it's a matter of using up wire on hand you could doubleup on the wire you have, but that complicates the connections.
    2&3> a fusebox is always a good idea, but locate it somewhat away from the batteries to avoid corrosion and keeping it more readilly accessible. i'll not comment on the fuse size as that is usually specified in the manual for each inverter and this can vary widely depending upon the voltage rating of the inverter. to start your search, try here: http://store.solar-electric.com/electrical.html also note that fuses and breakers used in high powered audio equipment for cars can sometimes be used depending on voltage and current ratings and i doubt you'll find something acceptable for a 1500w inverter from audio stuff, but nowadays you never know.
  • System2
    System2 Posts: 6,290 admin
    Re: diagonal wiring of battery bank

    are you sure you can 'double up' on wire as you suggest? wouldn't wiring (DC included) just pick the least resistive path and completely ignore any parallel circuits you make?

    aka (ASCII art to follow, you've been warned)


    [battery]=========[battery]

    each
    represents one pair of wires.

    It's just going to use the lowest resistance pair... no?
  • crewzer
    crewzer Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
    Re: diagonal wiring of battery bank

    No. Take a look at a practical example of two light bulbs, one 50 W and the other 100 W, wired in parallel. Do they both light up, or just the 100 W bulb (lower resistance)?

    HTH,
    Jim / crewzer
  • System2
    System2 Posts: 6,290 admin
    Re: diagonal wiring of battery bank

    I did that with my bank of 6, L-16's early this Summer. As it was, everything was OK, except when the 1/2 HP submersable pump, out in the lake would run, then I would drop a couple of volts and both the inter-connection cables and battery terminals would heat. Since I doubled up on ALL the battery cables and located them one on each side of each battery terminal for twice the contact area, I now have no noticeable voltage drop and no heating! Well worth the effort.
    You can also think of it as water flowing through two pipes of equal length and size, side by side under a road. The flow of water splits, with each pipe taking half the flow.
    Cheers
    Wayne
  • System2
    System2 Posts: 6,290 admin
    Re: diagonal wiring of battery bank
    crewzer wrote:
    No. Take a look at a practical example of two light bulbs, one 50 W and the other 100 W, wired in parallel. Do they both light up, or just the 100 W bulb (lower resistance)?

    HTH,
    Jim / crewzer

    That's not the same situation, IMHO. Take two identical/parallel power runs to your 50W and 100W light bulbs, do they share power on the power lines or do they just pick one path?
  • System2
    System2 Posts: 6,290 admin
    Re: diagonal wiring of battery bank

    :-o YES! The two identical/parallel power runs definitely DO share the load! 50/50 !!! Basic electricity. Ohm's Law.
    This horse is dead.
    Wayne
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: diagonal wiring of battery bank

    "are you sure you can 'double up' on wire as you suggest? wouldn't wiring (DC included) just pick the least resistive path and completely ignore any parallel circuits you make?" by joe.

    first question's answer is yes you can double up on wire.
    no not in doubling, it won't ignore it as it will just cut the resistance in half. this is very evident in wires that are comprised of many smaller wires twisted about each other to form one larger wire. the resistance gets lowered by having multiple paths for current to flow that individually tend to be small, but twisted together or paralleled will show the current additive.