Richa: Re-configuring/Adding to my off grid system--Questions

Richa · September 2019

I have 1760w of solar panels and 1260Ah of batteries(6 6V/420Ah/12v system) supplying a 3500w Aims Power inverter/charger. I intend to add another 140w panel to bring total up to 1900w. I will then have 2 ea. 6x100w strings and 1 5x140w string. According to Crown Battery info, I need 8-12A of deliverable current per 100Ah of battery bank capacity. So, 1260Ah/100Ah = 12.6 x 12 = 151.2A of deliverable current is needed to charge my bank in approx. 7.5 hrs. First, I need a reliable, properly-sized MPPT charge controller(s), i.e., 1 c.c. to receive total array V/A, or 3 c.c.s @ 1 per solar panel string. Second, what is the best solar panel wiring method, series, to provide higher voltage low current, or parallel, to provide higher current low voltage? Also, a correct wiring diagram would be greatly appreciated. My inverter is 12V, so I have to stick with that. Any replies are greatly appreciated.

Estragon · September 2019

A 3500w inverter is big for a 12v system. It would help to know more about your loads (max amps @120v, total amp-hours/day, etc).

What are the Vmp/Voc specs for the panels?

How far (wire run) are the panels from where the panels are to where the controllers will live?

BB. · September 2019

Welcome to the forum Richa,

You are sort of working the problem from the middle. And while we can answer the question you have asked, it may result in a less than optimum system for you.

The simple answer is... Depending on which brand/model of MPPT solar charge controller, the larger units are in the 60-80 Amp output range. Your 151.2 Amps would take 2-3 of these charge controllers connected to their own solar arrays (you cannot share panels between MPPT controllers) and paralleled to the battery bus. So, if you want 151.2 Amps charging, then the basic answer is:

151.2 amps * 14.5 volts charging * 1/0.77 solar panel+controller deratings = 2,847 Watt solar array

Note--Realistically, the "answers" for solar are accurate to about 10%... So you defined array--Anything within 10% of 2,847 Watts is an "exact answer" as far as solar is concerned (I used your digits so you can follow the math).

Next--You need to pick your solar panels--Or use the ones you have. With an MPPT solar charge controller, you want, at least, Vmp-array >~ 25 volts for proper MPPT controller operation (based on solar panel temperature deratings, charging a 12 volt battery bank @ ~15 volts, and such). And if you get higher end, moderate solar array maximum input voltage, your Vmp-array ~ 100 Volts maximum (assuming you are in a region with freezing/sub zero temperatures--Vs on a Caribbean Island).

Your solar panels are probably Vmp~17.5 volts, so the maximum series string would be:

100 VDC max array Vmp-std-temp / 17.5 volts = 5.71 ~ 5 panels in series

So your 6x 100 Watt strings are "on the edge" for exceeding maximum array input voltage for a "mid voltage" MPPT controller. If you are in a warmer climate, you could run a 6x100 string * 2 strings for 1,200 Watts:

1,200 Watt array * 0.77 solar panel derating * 1/14.5 volts charging nominal = 64 Amps

So, cost effective wise, if you run 6x100 Watt panels in series (2x parallel strings for 1,200 Watts), you would need a minimum MPPT controller rated for 64 Amps @ 12 volts (if cost is important, you can probably get away with 60 Amp controller and not lose much current).

The second 5-6x panel string 140 Watt panels, you probably again have the issue of 5 or 6 in series, and would get a second solar charge controller (6x140=840 Watt array)...

Obviously, if you want your 151.2 amps peak charging current, then you need more panels to get to ~2,847 Watt array. Adding more panels to the second controller, getting a third controller, etc...

The above sort of answers your question... I have some suggestions/concern about your configuration--A common issue trying to build a system with components you already have (In no particular order):

AIMs controller--Hopefully it is a TSW/PSW inverter (true/pure sine wave) and not a MSW (Modified Square Wave). MSW inverters tend to be hard on their loads (induction motors, electronic power supplies, etc.).
AIMs controller--Not the most reliable brand
3,500 Watt AC inverter--A very large Wattage AC inverter for a 12 volt battery bank. That needs very short cabling and very heavy/expensive copper wires and fuses/breakers. Normally, I would suggest a maximum 12 VDC AC inverter somewhere in the 1,200 to 1,800 Watt maximum "practicable" range.
With your 6x 6 volt @ 420 AH -- I would be suggesting a 24 volt battery bank (and inverter-charger/solar charging system). That gives you 1/2 the operational and charging current--However, 6x 6 volt batteries does not divide evenly into 24 volts--4x6 or 8x6 would be the nearest choices using your existing battery bank.
I highly suggest designing a system to support your loads. Loads=>battery bank. Battery bank=>solar panels, Hours of Sun per day (your location)=>solar panels. Loads => AC inverter specifications.
With many lower end AC inverters, they can have very high "tare losses" (energy required just to turn on the AC inverter).
If you run a 24 volt battery bank @ 630 Amps (different battery AH capacity--Such as 8x 6 volt @ 215 AH "golf cart" batteries, you could use a single MPPT controller rated >~ 75.5 amps (=151.2/2). There are several MPPT charge controllers that are 80+ amp output into 24 volts.

I could not find a 3,5 kWatt AIMS, but there is a 4,000 Watt AIMS 12 volt TSW inverter-charger:

https://www.aimscorp.net/4000-Watt-12-Volt-Pure-Sine-Inverter-Charger.html

Nominal Input Voltage: 12.0Vdc
Minimum Start Voltage: 10.0Vdc
Low Battery Alarm: 10.5Vdc-11.0Vdc
Low Battery Trip: 10.0Vdc-10.5Vdc
High Voltage Shutdown: 16.0Vdc
Low Battery Voltage Restart: 13.3Vdc
Idle Consumption: < 3.47 amps DC
Power Saver Mode Idle Consumption: <0.53 amps DC

Energy consumed by just the Inverter-charger

3.47 amps * 12 volts = 42 Watts just turned on
42 Watts * 24 hours per day = 1,008 WH per day
1,008 WH per day / 12 volts = 84 Amp*Hours per day

And if you assume that you use 25% of battery bank AH capacity per day and 50% of capacity:

1,260 AH * 12 volts * 0.85 AC inverter eff * 1/2 days storage * 0.50 max battery discharge = 3,213 WH per day "average" daily load suggested

And if your inverter consumes 1,008 WH per day if run 24 hours per day (your usage may be different):

3,213 WH daily load from bank - 1,008 WH tare losses = 2,205 WH per average day (2 days storage, 50% max discharge)

Almost 1/3 of your daily harvest would be used to run the 4 kWatt inverter 24 hours per day.

Running your inverter anywhere near 2,000 Watts would use your daily load in ~1 hour (or ~2 hours if you run one day, and take a couple days to recharge, or only run the inverter when the sun is up, etc.).

Anyway... Lots of issues and questions here. Your actual power needs are probably different than my (relatively) worst case usage assumptions.

Your questions/corrections to my stuff above?

-Bill

Richa · September 2019

Thanx for the welcome and the replies. I will try to fill in the blanks. 3500W is the surge rating on inverter. 3000W is actual power. It is a pure sine wave inverter. Yes, it is large for my current system. I was planning for possible greater power usage in future.

It is a cheaper make, dictated by wallet size at the time. My panel to C.C. wire run is about 40 ft. I did figure voltage drop.

100W panel: Vmp = 17.4V; Voc = 21.6V; Imp = 5.75A.

140W panel: Vmp = 17.8V; Voc = 21.7V; Imp = 7.87A.

I have an energy efficient 10 cu. ft. fridge-145W, an efficient 5 cu. ft. deep freeze-135W. I turn both off at 6p.m. summer and 5p.m. in winter. On at 7:30/8:00a.m. T.V. is 45W. DVR is 35W. Lighting is negligible, all LED-12W. Inverter loss is less than projected due to its Power Save feature, less than 25W usage at night. My calculations show 400-450 Ah.

Fridge and Freezer are rated 1/3 duty, although I have found they are a bit more. So, with a 1260 Ah battery system, I'm below 50% of capacity. I am going to stick with parallel wiring for panels & series/parallel for batteries with a 12V output. I plan to upgrade to a 24V system in a year or two.

Crown battery put out the info: 8-12A per 100Ah. If they are correct, at 8A, for 1260Ah, my bank would require 100.8A of charging current @ the low end of 8A per 100Ah. The 2x600W strings total Imp=69A. The 5x140W string=39.35A. Total available Imp=108.35A.

So, again I ask, do I need 3 C.C.s @ 1 C.C. per string? Or, should I use a 40-50A C.C. for the 700W [email protected]; and, say an 80A C.C. for the 2x600W string@69A? Currently, I am using 3 low end 40A MPPT C.C.s @ 1 per string. The 700W string @ 39.35A Imp is right on the edge of its 40A C.C.

Might you recommend a trustworthy make of C.C. to properly charge my L-16s given the parameters I have provided?

I do thank you for the lengthy dissertation already provided. All your help & advice is greatly appreciated, & I realize my system is small potatoes. It has been a work in progress since '02, when I started out with a 125W panel, 2 6V Trojan T-105 batteries, a 35A PWM C.C., and a 500W MSW inverter.

BB. · September 2019

No system is "small potatoes" here... It is what it has to do, and does what you need. Don't worry.

Putting timers on refrigerators and freezers... For a small system that you want to run those loads only during daylight hours. It does work, but the overall energy usage is about the same.

Say your fridge is 1/3 duty cycle 24 hours per day... Or it runs 8 hours a day (or 20 minutes every hour)... You change to 8 hour on (during sunlight)--And now the duty cycle probably approaches (or even meets) 100% on time during that 8 hours of sun. The pump still has to move (roughly) the same amount of heat that gets into the box over a 24 hour period, but you give the compressor only 8 hours to do it in.

For very small systems--This may be a "good way" to run larger loads... But once you get over a minimal size system (say over 3 kWH per day), doing these timer mediated run times--They frequently are not worth the effort (and if your freezer gets much above zero F, or the fridge approaches 40F+ "overnight", then the quality of your stored food may start to fall.

Also, when you have a wide swing in temperature, you can get drying of food and "freezer burn" (ice condenses on interior of plastic bag because of -10F in freezer, but food "warmed" to +10F--So you end up "Freeze drying" your food (somewhat).

-Bill

Estragon · September 2019

With a fairly short wire run of 40', I'd be inclined to use series strings of 2 panels for ~35Vmp. So 12 x 100w panels in 2 arrays. 600w each ÷ 11v is a 60a controller each. You probably won't ever see 60a, but conservative is better.

5 x 140w is more of a problem. 2 strings of 2 leaves an orphan panel if you have 5. You likely want another 60a mppt for 2 strings, and maybe a pwm for the orphan. There's really no good way to have a string of three like panels and a string of two on the same controller.

I'd consider pretty much any of the controllers sold by our hosts to be decent, with various features that may or may not be worthwhile in a specific application. There are some cheap "mppt" controllers out there that I wouldn't trust.

Richa · September 2019

Hello again. You are right about series wiring for the solar array. I can buy a 6th 140W panel to make a third string of 6x140W. 6x17.8Vmp=106.8V. Will this string work, wired in series, with a 60A MPPT C.C. with a 150VDC input and a 60A output? And I will also need 2 more C.C.s for my 2 6x100W strings(6x17.4Vmp=104.4V), wired in series, with a 150VDC input and a 60A output. Correct?

Richa · September 2019

Also: Will this total of 2,040W of solar array be enough to charge a 1290Ah 12V battery bank in 7.5 hours of New Mexico sun? And, in order to have 14.6V of bulk charging, I will have to limit the voltage drop to no more than 2.8V, i.e., 17.4Vmp-14.6V(bulk charge voltage). Correct?

BB. · September 2019

Generally, the recommended rate of charge for a flooded cell lead acid battery bank is 5% minimum (standby, weekend, summer system usage). And 10%-13%-20% for full time off grid. The math would look something like this:

1,290 AH * 14.5 volts charging * 1/0.77 panel+controller deratings * 0.05 rate of charge = 1,215 Watt array minimum
1,290 AH * 14.5 volts charging * 1/0.77 panel+controller deratings * 0.10 rate of charge = 2,429 Watt array nominal
1,290 AH * 14.5 volts charging * 1/0.77 panel+controller deratings * 0.13 rate of charge = 3,158 Watt array "typical cost effective maximum"
1,290 AH * 14.5 volts charging * 1/0.77 panel+controller deratings * 0.20 rate of charge = 4,858 Watt array "more or less max usable"

In general, with a fixed solar array--Discharging an FLA battery bank to 75% state of charge, will be rechargable to ~100% state of charge in one day (with enough sun).

So, the amount of power drawn from battery bank:

1,290 AH * 12 volts * 0.85 AC inverter * 0.25 discharge = 3,290 Watt*Hours (AC inverter 120 VAC output from battery bank)

Then fixed array facing south, and a random New Mexico city:
http://www.solarelectricityhandbook.com/solar-irradiance.html

Roswell
Average Solar Insolation figures

Measured in kWh/m2/day onto a solar panel set at a 57° angle from vertical:
(For best year-round performance)

Jan	Feb	Mar	Apr	May	Jun
4.80	5.28	6.32	6.57	6.43	6.12
Jul	Aug	Sep	Oct	Nov	Dec
5.92	5.74	5.82	5.74	5.14	4.54

This is a "good amount" of sun during winter... December 4.45 Hours:

3,290 WH per day * 1/0.52 off grid AC system eff * 1/4.45 Hours of sun per day (Dec Average) = 1,422 Watt array December "Break Even"

Note that solar power is, of course, variable.... For example, the 3,290 Watt array harvest in December... I would suggest that your "base load usage" (loads you must run like Lights, some water pumping, computer, cell charger, etc.) should be around 50-65% of "predicted solar harvest". And if you have a sunny/clear day (week ahead), then use your optional loads too (washer, irrigation watering, etc.).

And, the reason for 25% battery usage--There are usually enough hours of sun in a day to recharge the FLA battery bank. For 50% and deeper discharge, your battery charging time (something like 2-5 hours "bulk charging" and ~4-6 hours of "absorb charging"--The sun is not on the Array enough hours in a day (sunrise to sunset). Generally, a deeply discharged FLA battery is going to need at least 2 days to fully recharge.

-Bill

Richa · September 2019

Thank you for all your diligent work and attention to detail. This will take some digesting.
Regards, Richa

Richa · September 2019

To Estragon: Would you care to comment on my Post #7? I am wondering if I am correct on my assessment of your reply in Post #6. Basically, would adding another 140W panel to the string of 140W panels fix that string & would I be able manage it with a 60A C.C.? Thanks & Regards, Richa

Estragon · September 2019

Richa said:

Hello again. You are right about series wiring for the solar array. I can buy a 6th 140W panel to make a third string of 6x140W. 6x17.8Vmp=106.8V. Will this string work, wired in series, with a 60A MPPT C.C. with a 150VDC input and a 60A output? And I will also need 2 more C.C.s for my 2 6x100W strings(6x17.4Vmp=104.4V), wired in series, with a 150VDC input and a 60A output. Correct?

Use the open circuit string voltage (Voc, not Vmp) adjusted for the record low temp ever experienced in your location to make sure the voltage is under controller max. It should be under, but worth checking for a string of 6. With a 150v controller though, you could not add the 6th 140v panel, and just run a string of 5.

5 x 140w = 700w ÷11v = 63a. A 60a controller should be fine as you'll likely never get much more than ~500w in actual use.

6 x 100w = 600w ÷11v = 55a. Also fine on a 60a controller for each. On one controller, even using likely max of ~900w for 12 panels is too much IMHO. Even if the controller can limit, you probably want the second one so you get more of the potential output.

With 3 controllers, there's also the option to orient each array somewhat differently (eg 1 southeast, one south, and one southwest). Doing this reduces peak noon current, but produces more early morning and late afternoon current.

Richa · September 2019

Thank you sir. Regards, Richa

Richa: Re-configuring/Adding to my off grid system--Questions

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RoswellAverage Solar Insolation figures

Roswell
Average Solar Insolation figures