Cable sizing Question

FarmerGreen

I'm helping a friend set up his solar system. He's bought most of the components on advice from a salesman, but with no instructions. He has 2 arrays of 12 BP SX-195B panels (total 24 195w 24.4v panels), 2 Outback FM80 controllers, 1 Outback GS8048 Inverter, and an 8 layer stack of Absolyte 3-100A-31 batteries. I believe the plan is to run each array of 12 panels to an FM80 controller and then to the batteries. The farthest panel is a little less than 75 feet from the controllers. What gauge cable should be run from the panels to the CC? From what I've read, the amperage would be about 8 amps.

vtmaps

Re: Cable sizing Question

FarmerGreen wrote: »

I believe the plan is to run each array of 12 panels to an FM80 controller and then to the batteries. The farthest panel is a little less than 75 feet from the controllers. What gauge cable should be run from the panels to the CC? From what I've read, the amperage would be about 8 amps.

If all 12 panels were in series the current would be about 8 amps. But they are NOT all in series (Voc would be too high for the FM80), so therefore the current is greater than 8 amps. How much higher??

In order to answer your question, I need the array configuration (how many in series to make a string, and how many strings in parallel), and I need to know the Vmp and Imp of the panels.

--vtMaps

westbranch

Re: Cable sizing Question

that sounds like it would be about 8 amps and 293Volts...?
We need more details on those panels.

BB.

Re: Cable sizing Question

The panels appear to be rated:

Maximum power : 195 Watts
Voltage at Pmax (Vmp): 24.4 Volts
Current at Pmax (Imp): 7.96 amps
Warranted minimum Pmax: 177.5 watts
Short-circuit current (Isc): 8.6 Amps
Open-circuit voltage (Voc): 30.7 Volts
Temperature coefficient of Isc: (0.065±0.015)%/°C
Temperature coefficient of Voc: -(111±10)mV/°C
Temperature coefficient of power: -(0.5±0.05)%/°C
NOCT (Air 20°C, Sun 0.8 kW/m²): 47±2°C
Maximum series fuse rating: 15 amps
Maximum system voltage: 600 volts (US NEC rating)

On a 48 volt battery bank... Nominally, the Vmp-array should be in the range of 70-100 VDC (need to double check against min/max temperatures in your area).

That would be around 3 panels in series by 4 parallel strings OR 4 panels in series and 3 stings in parallel (very close to Voc-cold-max -- Depending on your local weather condition sand the FM 80 specifications.

The farther the array is from the charge controller, you want Vmp-array to be higher so you can use smaller diameter copper wire from the array to the charge controller.

However Voc-array-cold rises the colder the panels net--You don't want too many panels in parallel or you run the risk of "over voltaging" the charge controller.

The maximum "typical" controller output would be:

• 12 * 195 watt panels * 0.77 panel+controller derating * 1/59 volts charging = 30.5 amps per array

So, it looks like the 24 panel array can run on one FM 80 controller pretty easily.

The batteries (you need to confirm) look like:

• 3-100A31 6 volt 1,700 AC (20 hour rate) x 8 packs for 48 @ 1,700 AH batter bank (very pricy???)

We recommend 5% to 13% rate of charge (solar array) for a lead acid battery bank--Or:

• 1,700 AH * 59 volts charging * 1/0.77 panel+controller derating * 0.05 rate of charge = 6,513 Watt array minimum
• 1,700 AH * 59 volts charging * 1/0.77 panel+controller derating * 0.05 rate of charge = 13,026 Watt array nominal
• 1,700 AH * 59 volts charging * 1/0.77 panel+controller derating * 0.05 rate of charge = 16,934 Watt array "cost effective" maximum

And you currently have a 24*195watt = 4,680 Watt array...

That is a pretty small array if I have the right batteries identified.

And, we have not even discussed how much power he is planning on using... A typical 2 day of storage and 50% maximum discharge:

• 1,700 AH * 48 volt battery * 0.85 AC inverter eff * 0.25 discharge = 17,340 WH per day = 17.34 kWH per day of AC power

That is not a small system... Needs a large solar array and a good sized battery charger + genset for backup power.

Also--If the batteries are already purchased/on site--You need to check the resting voltage and make sure that they are still properly charged. AGMs should usually be recharged every 3-6 months, or they will begin to sulfate and die an early life.

Of course, if I do not have the correct batteries identified--Then all of the above is not very useful--So I will stop typing here.

-Bill

FarmerGreen

Re: Cable sizing Question

Yes, those are the correct batteries and are onsite, but not hooked up yet. How should the batteries be configured? 48v? There are 144 2v cells.
My area is East TN, average winter low is 25F, record low is -24F. We got down to 0 this past week.
The solar panels are also there, but not hooked up yet. He also has a couple of combiner boxes, but I don't recall which ones.

BB.

Re: Cable sizing Question

Here is the Outback solar array sizing tool. You fill in the questions and it should give you the results. You may be left with a maximum of 3 panels in series to stay below the maximum controller input voltage:

http://www.outbackpower.com/outback-resources/string-sizing-tool

Regarding the batteries... Even just 24 cells, you need more than your 24 panels already if the system is cycling even lightly (5% minimum solar array is highly recommended).

To just float the batteries, you should be looking at a minimum of 1% charging current. That set of 144x 1,700 AH @ 2 volt cells would need (roughly) a minimum of 7.5 kW array to just properly float them (my guess). If the batteries are already nearly fully charged, and they do not have much self discharge, as small as a 1,000 Watt array may keep them trickle charged (0.1% rate of charge) for awhile.

In any case--Do you now how much daily power usage/size of solar array/size of AC inverter/etc. your friend wants for the final system? Large battery banks are a real pain. They need ongoing charging/monitoring to make sure they do not slowly self discharge and start quickly sulfating.

At this point, he can only keep ~24 cells (for 48 volt bank @ 1,700 AH) happy at this point if he plans on cycling the bank. He should start sorting cells (and a few spares) that he plans on keeping, and trying to sell/give away the rest. Or dramatically increasing his solar array/backup charging capabilities. Per 24 cell string at 10% nominal rate of charge, he is looking at:

• 1,700 AH * 56 volts AGM charging * 1/0.77 panel+controller losses * 0.10 rate of charge = 12,364 Watt array "nominal"
• 1,700 AH * 0.10 rate of charge = 170 Amp @ 48 volt nominal AC Battery charger
• 1,700 AH * 56 volts AGM charging * 1/0.80 charger eff * 1/0.67 Power Factor * 1/0.80 genset loading * 0.10 rate of charge = 22,201 Watt/VA rated genset "nominal" charging with typical AC charger
• 1,700 AH * 56 volts AGM charging * 1/0.90 charger eff * 1/0.95 Power Factor * 1/1.00 genset loading * 0.10 rate of charge = 11,136 Watt/VA rated genset "nominal" charging with "best in class" AC charger and "prime mover" rated genset

This will not be a trivial system to build out if he wants to use some/all of those cells... If he does not want such a large system, he may need to look at a 24 volt bank--That is sort of reasonable with the amount of solar panels+FM 80's he has now. Would need a 24 volt inverter plus backup AC charger(s).

-Bill

FarmerGreen

Re: Cable sizing Question

At this point, he doesn't know how much power usage he will need. I've told him he needs to do a survey of what he expects to operate. He hasn't done that so far.

Are you saying that the 24 panels won't charge these batteries? Once they are charged, don't you just have to replace the power that you use that day? These batteries have a very small self-discharge. <1% a month IIRC.

I am a complete newbie at this and I'm not getting all of what you are explaining. But I certainly appreciate your trying to educate me. :-)

westbranch

Re: Cable sizing Question

for any battery you need to look at the Ahr rating to determine the recharge needs, as a rule of thumb, take a battery of 100Ahr, it will need between 5% amd 10% of that 100 = 5 to 10 Amps to be recharged. So those 1700Ahr batteries will need 170 Amps per battery at that batteries voltage, 12 , 24, 48 or ? Do you see the problem he has? too many batteries and too few solar panels...

FarmerGreen

Re: Cable sizing Question

So these panels won't maintain a charge on those batteries?

BB.

Re: Cable sizing Question

If the question is maintain a charge (nominal age batteries, nominal temperatures, nominal weather, etc.)--That is different than cycling.

I could not find any information on the Absolyte 3 100A 31 batteries... I did find a bit on the G31 versions.

Anyway, the GNB.Com site shows a 10% "assumed" rate of charge--I could not find any details on min/max charging (i.e., one was charge at a.bc votls per cell with "unlimited" current).

One "G" data sheet shows a typical range of self discharge to 1% per week at ~70F and recharging stored batteries every 6 months. Using a common engineering rule of thumb, you would probably need to recharge every 3 months if the batteries were at ~88F (2x more self discharge for every 10C increase in temperature).

So, looking a 144 fully charged 1,700 AH 2.0 volt cells. With 1% self discharge per week. You would need:

• 1% * 1,700 AH per week * 1/7 days per week = 2.43 AH per day self discharge per cell
• 144 cells * 2.43 AH per day float self discharge * 2.33 Vpc float * 1/0.77 panel+controller losses * 1/4 hours per day of sun = 254 Watt array minimum to float all batteries at ~70F for 9 months of the year

So--Yes, if all your cells are "typical", your temperatures are 70F or less, and you get >4 hours per day (on average) for 9+ months of the year, you could probably float that large bank of batteries.

You will have to monitor the bank--If there are a few "bad cells" that have higher self discharge rates (~2% per day is a good "end of life" rate)--You could end up with a few cells killing the entire bank if not checked every few days/once a week for "weak or shorted cells".

That is entirely different than sizing a system to actually properly cycle a deep cycle battery bank with ~20-50% per day depth of cycle.

If you have a 5% rate of charge and 4 hours of sun per day (typical for deep fall/early spring, and sometimes not even that during winter)--A 50% discharge will take you 2.5-3 days to recharge with Zero Loads (5% recharge per hour, 4 hours of sun per day, ~20% per day recharging). Leaving batteries set for days/weeks at partial charge (typically less than 75% state of charge) is usually not healthy for a battery bank (sulfating).

Another issue is when a bank gets old--You can get upwards of 1% self discharge per day (and 2% self discharge current is a cell that is in danger of self destructing)--You need enough solar array to keep up with both the loads and the self discharge.

A 1% discharge per day, similar to above formulas:

• 1% * 1,700 AH per day = 17 AH per day self discharge per cell
• 144 cells * 17 AH per day float self discharge * 2.33 Vpc float * 1/0.77 panel+controller losses * 1/4 hours per day of sun = 1 W,852 Watt array minimum to float "old batteries" at ~70F for 9 months of the year (1% per day self discharge rate)

What will happen to these batteries? I really do not know. But as they age, you can see that their self discharge will become a significant amount of the solar power needed to just keep the batteries happy.

And many vendors will have a (generic) recomendation of 10% rate of charge minimum for good battery life. And some specialized AGM batteriers require 30-40% rate of charge minimum (those "Mil-spec" AGM batteries) or they will have very poor cycle life.

Lead acid storage batteries generally do not do well in "float service" and do not (usually) like too low (<5%) or too high rate (>13-25%) of charge.

AGMs/sealed/VRLA batteries are different... And in some ways, they can be the "very close" to ideal Lead Acid Storage battery--But you really need to find the specification for those batteries.

You can find telecom/standby batteries that will cycle hundreds to thousands of time with 20-50% deep cycles--But will not last too many years in Float. And you can find Float type lead acid batteries that will last 20-40 years in float, but if you discharge them more than 15-25%, they will not last very long.

Sorry I do not have any great answers. You may need to contact GNB or Exide and see if you can find out more about those particular batteries and how best to take care of them.

We use rules of thumbs around here a lot--Because, in general, it works really well in most situations. But, sometimes, you need the exact data sheets to figure out what is needed to take care of specialized/industrial batteries of XYZ model/brand.

-Bill