Voltage Drop???

pabloesguapo · December 2011

I am designing an off-grid system. I am wondering about panel vs. battery placement.

The major components of the system are:

24 panels @ 210 Watts 18.70 Vmp
6 Volt deep cycle, 225 amp/hour wet cell batteries wired in series.
Xantrex Solar Charge Controller 60A MPPT-60
Xantrex Grid-Tie/Off-Grid Inverter/Charger 4500 Watts 48 Volts XW4548

I'm planning/hoping to mount these panels on a shed I'd like to build (a 30 degree slanted roof, facing due south) instead of on the roof of my house. The pitch of the house's roof is rather extreme, both sides face due east and west, not south, and I don't like the idea of having to get way up there to clean/maintain the panels. I'd rather do that on a shed roof 10-12 ft in the air set on a less sever angle.

I'd like to place the batteries in the shed (to provide electricity for the power tools) until we complete construction on the house. Then I'd like to permanently relocate them to the basement.

I have several questions:

1) Is it OK to have these batteries in the shed for the summer? Will the summer heat "kill" them?

2) Is it necessary to move them out of the shed and to the basement at all? Will the winter cold "kill" them?

3) Regardless of the location of my batteries (or charge controller or inverter, for that matter) what is the best way to avoid voltage drop? The shed will be located up to 50 ft. away from the house. (We plan on planting a few deciduous shade trees in between house and shed to aid in summertime cooling)

mike95490 · December 2011

Re: Voltage Drop???

pabloesguapo wrote: »

1) Is it OK to have these batteries in the shed for the summer? Will the summer heat "kill" them?

Keep the shed ventilated, and below 100F if possible

2) Is it necessary to move them out of the shed and to the basement at all? Will the winter cold "kill" them?

When charged, you have to go -40F to freeze a battery. discharged, only a couple degrees of frost before it ices up

3) Regardless of the location of my batteries (or charge controller or inverter, for that matter) what is the best way to avoid voltage drop? The shed will be located up to 50 ft. away from the house. (We plan on planting a few deciduous shade trees in between house and shed to aid in summertime cooling)

Run the panels as high of voltage (5 in series?) to reduce the amperage in the wires. Use the Xantrex string calculator, and your area record low temp, to insure you do not over-voltage the charge controller on a frosty morning.

pabloesguapo · December 2011

Re: Voltage Drop???

So, it seems that keeping the batteries, controller and inverter inside the house will be a wiser choice in the long run. Better safe than sorry. I'll still mount the panels on the shed, but run cable to the batteries in the basement in between foundation completion and the beginning of sub floor install...

But what about the string calculator? Where do I find?

TheBackRoads · December 2011

Re: Voltage Drop???

http://www.se-renbu.com/support/xwsizing/disclaimer_SE.aspx

pabloesguapo · December 2011

Re: Voltage Drop???

Called the company I''m planning on purchasing from, www sunelec.com. Anyone have any opinions or history with these guys? Their prices are really good...

I plan on buying this system.

They told me that for my long run, I should probably go with a #6 or a #4 cable between the combiner box (in the shed) and the voltage regulator (in the house). The length could possibly be between 50 and 70 feet. I used the following calculator to gauge voltage drop. http://www.stealth316.com/2-wire-resistance.htm Also, I found a good source for wire and cable. I can get #4 gauge wire for $1.61 a foot.

So, correct me if I'm wrong, please. The amperage of my panels (link), if wired in series, will be the same as the amp rating of one of the panels? So, that would be 11.23 amps. If run in series, the voltage of my panels will be 18.7 Vmp each, times 5 in a series, times 5 series? (Maximum power, I know...) That's 18.7Vmp x 5 x 5= 467.5 Volts? According to the aforementioned calculator, my voltage drop with 70 ft of #4 wire would be 0.1988 volts?

Their web site says the batteries would be run in series. (6v at 225 amp hours each). What would that get me in terms of voltage and amp hours?

BB. · December 2011

Re: Voltage Drop???

I am on my phone right now, but I think you are multiplying Vmp by one too many 5s.

Vmp x 5 for array voltage. And Imp x 5 if you have 5 parallel strings (for 25 series/parallel connected panels).

Now is the time to be very clear and exact about your system configuration.

-Bill

mike95490 · December 2011

Re: Voltage Drop???

here's the link to the voltage drop calculator listed in the FAQ, it's a spreadsheet,
with instruction notes at the bottom
http://www.solar-guppy.com/download/voltage_drop_calculator.zip

What you want to do, is get your array voltage as high as possible, for the long cable distance, and let the MPPT downconvert it to amps at the battery.

Seven · December 2011

Re: Voltage Drop???

Just a quick look at those panels shows them to be blems and have no UL or other certifications. Are you ok with that?

pabloesguapo · December 2011

Re: Voltage Drop???

SEven: OK, try these:

http://www.sunelec.com/canadian-solar-panel-230-watts-2960-vmp-p-1399.html

More bang for the buck...

B.B. : Yes, 25 panels, so times 5 times 5.

offgrid me · December 2011

Re: Voltage Drop???

pabloesguapo wrote: »

SEven: OK, try these:

http://www.sunelec.com/canadian-solar-panel-230-watts-2960-vmp-p-1399.html

More bang for the buck...

B.B. : Yes, 25 panels, so times 5 times 5.

No you will have 5 series strings which you will then parallel together in a combiner box. This will give you 5 times the vmp and 5 times the Isc. If you connected all the panels into one series string you would be well over the max voltage for almost any charge controller made. Be cautious with any package deal. They are rarelly all inclusive since every install has its own set of needs such as racking and combiner boxes with proper fusing. My limited experience with sun is that there prices are good but there customer service is horrible. Not a great place to shop if you are a beginner and dont know exactly what you need. Try our hosts NAWS at least as a starting point in designing your system.
Ned

pabloesguapo · December 2011

Re: Voltage Drop???

@Offgrid Me: OK, got it. Since I'm going into a combiner box, I only multiply by 5 once, nand not multiply again for all 5 strings. Right?

Also, thanks for the heads up on Sunelec. I have noticed that about them, great prices, but hard to get a hold of, hard to get good answers out of them. I have started calling their Miami offices to get questions answered instead of calling their Phoenix warehouse.

BUT... Being the raging cheapskate that I am, I am drawn to low prices (like a moth to a bug zapper) and am willing to put myself through a steep learning curve in order to save a few bucks. I can't stand to pay extra $$$ to have someone do something for me that I could easily do with a little bit of homework and a drop or two of my own elbow grease.

offgrid me · December 2011

Re: Voltage Drop???

Right. You will probably need to run the voltage drop calculator again with the ISC value of 56 amp. Dont forget to use the total run there and back (140') in the calculator. You may find that 6 awg is not large enough to maintain the recommended 2% max voltage drop for this long a run. I understand the allure of cheap prices but some bargains are not bargains in the long run. Do a lot of research before purchasing anything and run your design by the people on this site. They are not trying to sell you anything and will not steer you wrong.

BB. · December 2011

Re: Voltage Drop???

Some very rough numbers:

Vmp-array = Vmp * panels in series = 18.7 volts * 5 series panels = 93.5 volts Vmp-Array
Imp-array = Imp * panels or strings in parallel = 11.23 amps * 5 parallel strings = 56.15 amps Imp-array

Normally, we talk about 1% to 3% voltage drop in wiring (lower than 1% is expensive, over 3% is a lot of lost power):

93.5 volts * 1% drop = 0.935 volts drop (~cost effective minimum)
93.5 volts * 3% drop = 2.805 volts drop (~maximum recommended drop)

Using a generic voltage drop calculator, we can play with the numbers and match your configuration (note this drop calculator needs one way cable length and assumes return path is same cable/length):

70' foot run, 56.15 amps, 0.935 volt drop => 1/0 gauge wire for ~0.9 volt drop
70' foot run, 56.15 amps, 2.805 volt drop => 4 gauge wire for ~2.4 volt drop

6 awg would give you ~3.7 volt drop... Since power = Volts * amps:

56.15 amps * 3.7 volts = 208 watts of wasted power (as heat with 6 awg)
56.15 amps * 2.4 amps = 135 watts of wasted power (with 4 awg)
56.15 amps * 0.9 amps = 50.5 watts of wasted power (with 1/0 awg)

Not to say that 6 awg would not work--but a noon on a bright an clear day, you are basically "tossing" the power of one 210 watt panel away.

You have to decide if 210 watts of "wasted panel" vs the cost of 4 awg (or even 1/0 awg) cable is worth it to you or not.

Please note, you do not need to carry all of the decimal places--I just included them so you can follow (and hopefully repeat) my math.

-Bill

Voltage Drop???

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