Wire size and battery charging question

jthale · July 2012

Hi, I need some questions answered. Thanks for your time. I have a simple off grid 12v system and recently added 5-230w. panels. The specs are 30.3 vpp, 7.6 amp at peak, open circuit voltage 37, short circuit current 8.24. I wired them in parallel. I ran alum. 100 amp entrance cable(2-#2) app. 90 ft. I also have a outback flexmax 80 installed. From the readout on the controller, on a rather hot day, I'm seeing 20-23 volts, 36 amps input and 13.3v, 57 amps output at that certain time. I'm connected to 2 strings of 6 volt energizer golf cart bats. #EGC2, res. cap. 105 min. My question is am I ok with that size wire? I'm not certain if the batteries are getting the most from the system. They don't seem to ever reach float. They are app. 4-5 years old and were hooked to windmills for that amount of time.Sometimes setting awhile under voltage. I've seen 70+ amps output at times, is that telling me that the panels and wire are doing their jobs? Also, I'm using a 5000 watt mod. sine inverter. When at night when we aren't using power is the inverter using that much from the batteries that I should consider turning it off? It seems I should be getting more stored power than I'm getting. Any info would be appreciated. Thanks, Jim

Thom · July 2012

Re: Wire size and battery charging question

5000 w inverter at 12v ! Wow

Cariboocoot · July 2012

Re: Wire size and battery charging question

Let's look at this back-to-front and see what you should have and how that compares with what you do have.

Your battery bank is approximately 440 Amp hours @ 12 Volts, for which you'd want to try for 44 Amps peak current. That would need about 685 Watts of array. You've got 1150 Watts which should be capable of 73 Amps and apparently you occasionally get near that. In other words that array should be more than sufficient to charge those batteries.

Now, can those batteries be charged? They could be toast. First, check the charge settings on the FM80. Absorb Voltage should be 14.4, Absorb time limit should be up to 4 hours. Float Voltage should be 13.8.

The big question is: what sort of specific gravity readings do you get from the cells? You need to check the SG on each and every cell. Chances are there's more than one that's quite different from the others and they are probably all low as these batteries have seen use and, frankly, abuse.

So when you buy your new batteries you might want to re-wire your array. Either buy an additional matching panel or remove one of the five. Yes, you really only need four 230 Watt panels for 440 Amp hours @ 12 Volts. You should be able to get about 60 Amps from 920 Watts. Wire the panels in two parallel strings of two in series. This will raise the array Voltage to around 60 Volts and allow you to use 8 AWG copper wire instead of that aluminium stuff (which has connection problems at both ends).

You may also want to go up to a 24 Volt system. Your 12 Volt 5kW MSW inverter isn't the greatest design in the world. It won't use a lot of power at idle, but don't try to run it full out!

Pretty sure those batteries are going to turn out to be toasted.

niel · July 2012

Re: Wire size and battery charging question

aluminum wire may also present a bigger v drop than one may have thought. without looking it up specifically i'll say that it is most likely close to being like #4 copper wire. i will recommend you figure the v drop for your wire with a v drop calculator of which there are a few around. the one in my sigline does not do aluminum.

in any case you could measure the voltage at the controller's pv input and at the pvs themselves. the voltage at the pvs - the input voltage to the cc equals the total v drop and divide that by the actual voltage at the pvs for the %. if you can't measure the voltage at the pvs then use the vmp instead. actually measuring voltage drops is often better to do as sometimes there are bad connections or other unaccounted for resistances.

this can and should be done for all wire segments to be sure there isn't a problem with v drops and the total should never be above 5%, but we strive for 2% or 3% tops.

Cariboocoot · July 2012

Re: Wire size and battery charging question

Acting on Niel's suspicions I ran the numbers for 2 AWG aluminium, 90 feet one way, 38 Amps peak @ 24 Volts:
Nearly 9% V-drop. Pretty horrible. In between copper #4 & #6.

Drop one panel, reconfigure the array for higher Voltage. With the #2 aluminium the V-drop will go down to 1.75%

inetdog · July 2012

Re: Wire size and battery charging question

Cariboocoot wrote: »

Acting on Niel's suspicions I ran the numbers for 2 AWG aluminium, 90 feet one way, 38 Amps peak @ 24 Volts:
Nearly 9% V-drop. Pretty horrible. In between copper #4 & #6.

You absolutely have to use a larger gauge wire for Aluminum to get the same current carrying capacity. But when you do, you find that even with buying the wire a couple of sizes larger, you end up paying only about 1/3 the cost of copper for the same voltage drop.
You do have to take some care to use terminations rated for Aluminum and to attach them properly with anti-oxide paste, etc.
Some people recommend using a butt splice connector designed for Cu to Al to give you a copper pigtail so that you can then use a regular Cu termination to connect to the batteries, etc. That also makes it easier to bend and get into tight spaces right at the ends.

Here is a calculator that gives you the option of aluminum: http://www.electrician2.com/vd_calculator.htm

mike95490 · July 2012

Re: Wire size and battery charging question

When installing the 988' of #6 alum wire for my pump, I used the "increase 2 wire gauges" factor. So #8 copper would be #6 aluminum.
(240V pump was at the 300' mark, plus the 100' #10 submarine cable for it)

I also used breakers rated for aluminum, anti-ox paste, and the right torque for the set screws.

Where I had to splice or tap, I used these:
Electrical Splice Blocks suitable for Alum wire
Attachment not found.Attachment not found.

They are not cheap, but the cost savings in copper, makes it work.

yes, that was 3, 1,000 foot spools of stranded #6 !
Attachment not found.
Attachment not found.
Cables were run via a pulley, down into the conduit. Photo album.

icarus · July 2012

Re: Wire size and battery charging question

Why the 5 kw MSW inverter? What possible load can you use on such a small system that needs that big an inverter, especially a MSW? The inverter loses alone, as a percentage are huge, especially when powering alow loads, or no loads.

Consider a couple of inverters if you really need such a big one once in a while. Size the inverters to the average load, so that y can use them most efficiently.

Tony

Cariboocoot · July 2012

Re: Wire size and battery charging question

I'd guess it was a really inexpensive 5kW inverter. Probably one of those Awful Inverters that are Made Stupidly. :roll:

Wire size and battery charging question

Comments

Categories