Basic charge controller advise needed

hollisterbulldawg

I have 4 solar panels (Vmax= 36, imax=9A) two in series and two in parallel to give me 72V, 18A output. I have a 48V battery bank (6V, 420Ahr each). From there I will use an inverter to support a 4080W/day load. Which MPPT charge controller would be best suited for this system? Thank you very much.

Vic

Hi Hollister..,  Welcome to the Forum.

Even if the PV modules have a Vmp of 36 V (and not the max,  which is the Voc value),  it is quite probable that strings of two PVs in series will not have a sufficiently high string Vmp to allow most battery types to be fully charged on many days of the year,  for most MPPT CCs and most battery types.

What are the specs or the manufacturer and model number for the PVs?
What is manufacturer and model number of the battery that you intend to use.
What is the planned total rated PV power for your system?

For a 48 V Flooded battery,  one would customarily design for a minimum STC String Vmp of about 85 - 95 volts,  when using an MPPT CC,   and Flooded batteries.

Thanks,   Vic

Photowhit

Are you located in a very sunny area? Looks like a 1300 watt array. While I suspect a Schneider C40 or Morningstar 45 would be fine and cheapest/most reasonable charge controller, I suspect you will need a larger array to support your system. I have read that Battery temperature sensors are no longer available for the C40 that would push me toward the Morningstar.

https://www.solar-electric.com/c40.html

https://www.solar-electric.com/trts12vochco.html

Locating your battery bank near the charge controller and array will help with voltage drop.

You have a 48 volt 420 amp hour battery bank, and with a daily load of 4 Kwh, you would want to be able to reach a 10% charge on the battery bank or 42 amps. Solar panels typically produce about 75% of their panel rating, so a 1300 watt array would produce about 1000 watts, the drop is mostly in voltage, but a small reduction in amperage through a PWM type controller. still even if producing at full amperage, you only have 18 amps charging available.

Most of the US has about 4 hours per day charging on average. You can check solar isolation tables and check your area. You might be fine if this is for a nursery or something seasonable, just during the spring and summer, but the shorter days of fall and winter will be very challenging.

hollisterbulldawg

So my budget system does not cut it. Thank you for the great information. It's obvious that I will have to double the battery current with another string. So far I have discovered batteries are the number one cost in an off-the-grid system. I am going back to the drawing board but here are the specfics of what I am noodling with:

Location: Central California, 50 miles inland from the coast. 4 hours sunshine in winter, 3 days in a row with no sun. (I am ignoring this year as the weather is abnormal.) Typical winter temp is in the 50s. Summer temp in the 80s.

Plan is to run an AC standalone freezer and a standalone refrigerator. I have had a Kill-A-Watt meter on them for a week and they are consuming together 136Whr. Multiplied by circuit inefficiency of 1.25 and 24 hours/day of use, I get 4080Whrs/day as the demand.

I like the mono panels from Canadian Solar at 345W, Vmp=38.1, imp=9.06, $269 each. I'll need 4 panels, 2 in series, 2 in parallel to support  4080Whrs/day (72V and 14.17A). These will be ground mounted about 10 feet from the battery bank.

For batteries, I'm going with 6V FLA for the longevity. Looks like I will change the series to get 24V with two parallel strings. I need to find a 550Ahr model. I was looking at Surette based on the reputation. I am assuming a 50% DOD.

Batteries, charge controller, inverter will be inside a building within 10 feet of the load.

More critiques and comments are desired. Thank you.

Estragon

Do/will you have a generator available?

hollisterbulldawg

Estragon, yes, I have an Onan 7500W gas powered generator as backup now for my fridge and freezer. Too hard for my wife to manually crank it so I am looking for an alternative. The generator is only used if the grid goes down (twice in the last 35 years because of quakes, for three days).

Estragon

Okay, that kinda makes sense now. If I understand what you want to do is use the solar/battery system to run reefers during a power outage.

The batteries would likely keep you going for a
number of days if sunny, maybe a couple if poor sun. If not doing it all the time, you can get away with a deeper drawdown (to around 25%SOC) than normal off-grid use. If you had a really bad run of weather (which might be why the power went out in the first place), you can run the generator to charge and prevent battery damage.

The panels as planned would be sbout enough to supply the load, but not likely enough to do that and also catch up charging deficit.

One problem with reefers is they have big starting currents. At 24v you should be able to find an inverter with enough surge capacity to start one at a time, but if both happened to start up at once, it might overload and require a manual inverter reset.

The chances of this happening are probably small enough that taking measures to prevent it shouldn't be needed, assuming someone will be around to reset.

hollisterbulldawg

I would like to recharge the batteries (50% DOD max) during the day and discharge them at night. Apparently I am using a calculator that only barely achieves this. I can go up to Surette 6V, 770Ah batteries (maybe 36V bank, with two banks in parallel to get 1540Ah) or I can add more solar panels (preferred option). Do you think 6 panels, 2 in series in each bank and 3 parallel banks (72V, 27A) would do it? My calculator says that is overkill, but I cannot see the algorithm behind the numbers. Thanks.

BB.

Is there a reason you don't want to go with a genset + some sort of stored fuel (or even natural gas, if you have it)?

For emergency backup power, gensets tend to be more cost effective and less maintenance (genset off--no fuel used, no aging other than a small starter battery, if not pull/crank start). I recycle the fuel back into may car+pickup truck every year, and store gasoline with a fuel stabilizer. If you choose the right size genset and keep your emergency loads "small"--You can store a week or two of fuel pretty easily (i.e., Honda eu2000i 1.1 gallons of gas for 4-9 hours per day, depending on loads--20 gallons ~4 days of usage to keep a freezer+freezer cold--run at night to run some lights too). Good for days to ~ a couple of weeks to get you buy (natural gas, much longer, as long as you have NG).

Don't get too wrapped up in figuring out battery bank configurations just yet. I suggest that if your battery bank is over ~800 AH, you look at going the next step in voltage (i.e., 12 volts @ 900 AH is the same stored energy as 24 volts @ 450 AH). This keeps the wiring "reasonable" gauge (diameter), and you don't have two or more ~60-90 amp solar charge controllers in parallel (10% rate of charge of 800 amps is 80 amp charging current--10% is good minimum for full time off grid, 5% can work for weekend/emergency off grid).

Battery banks are not great for backup power unless you need "uninterrupted" power (such as running computers). A fridge+lights and even a laptop can wait until you get a genset cranked up.

Also--When you start looking for batteries+hardware--In general, 12/24/48 volts are the standard voltages for off grid home power systems. 36 volts and >48 volts are getting rare/specialized.

You have several options... Run a "small" solar + inverter system (2-4 6 volt @ 200 amp*hour "golf cart" batteries + a 300 Watt AC inverter) to run lights, an RV pump for water (if needed), and a few other loads (LED TV, laptop computer, cell phone charging). That would give you ~1,000 Watt*Hours (or ~100 AH @ 12 volts) per day (2 days storage, 50% maximum discharge). And call it ~376 to 753 Watt solar array (depending on size of battery bank, how much sun you get for your location, etc.).

If you want to run your home off grid for months--Then look at a 3.3 kWH per day system (3,300 Watt)--That will run a very energy efficient home for 9+ months of the year without even needing a genset. A 24 volt @ 647 AH battery bank (~24x 6 volt golf cart batteries), 1,218 Watt solar array minimum, and a ~1,200 to 1,500 Watt AC inverter will keep you nice and cozy (maybe ~2 kWatt inverter for fridge+freezer). All for ~$10,000 or less in costs (DIY). Golf cart batteries are not ideal--But would keep your system running for 3-5 years to learn the system and figure out if it will work for you. And if you make any mistakes, replacing the battery bank will not kill your bank account. There are larger batteries what would be less maintenance than 24x golf cart batteries--But that is always a trade-off between costs and "better" components.

Of course, you can build a larger system, but then you are looking at multiple $10,000 of thousands of dollars for such a system.

If you are around Hollister California, a solar array near San Jose will see:
http://www.solarelectricityhandbook.com/solar-irradiance.html

San Jose
Average Solar Insolation figures

Measured in kWh/m2/day onto a solar panel set at a 53° angle from vertical:
(For best year-round performance)

Jan	Feb	Mar	Apr	May	Jun
3.81	4.54	5.69	6.53	6.70	6.80
Jul	Aug	Sep	Oct	Nov	Dec
6.60	6.54	6.29	5.72	4.40	3.57

A 3.3 kWH per day system, say 4 hour a day of sun minimum:

• 3,300 WH * 1/0.52 off grid system eff * 1/4.0 hours of sun minimum = 1,587 Watt array "break even" at 4 hour of sun
  

Would a "new genset" be easier for your wife to start? Running a large solar system for emergency power is really hard to justify the costs (I live near San Francisco--The last multi-day outage we had was ~55 years ago on the coast--And the bigger issue was concerns that we would run out of city water due to no pumping to tanks). Most are minutes to a couple of hours long--I would like a off grid solar backup system--But a small genset+fuel is really all I can justify cost wise).

-Bill

Estragon

If you just want to power overnight, you don't need to upsize bank. You could get by with 2 strings of golf carts for around 440ah@24v for occasional use. I would use 6 panels to be sure batteries get charged as well as powering loads. You only need to use more battery if you want to last multiple days with lousy weather, and then you would need more panel to charge the bigger bank. Batteries also die of old age even if rarely used, and will need periodic replacement. Bigger bank = higher replacement cost.

Bill's right about using battery solar for occasional outages. A new generator might be a cheaper/better solution. I don't think the eu2000i will run your reefers, but there are nice larger ones with electric start. You're probably in the $7000 (diy) range for the 400ah 6 panel setup above, and $1000 every 5 years or so to replace batteries. You could buy a nice generator and a lot of gas for that.

hollisterbulldawg

Good advice from both of you. The more I design what I thought was a small system for the fridge and freezer, the more I realize that I am looking at close to $10K for everything. Major cost driven by the batteries. Only redeeming thing is I have a master electrician who would do the wiring for free. Thanks to all of you.

Raj174

If a backup system is the desired goal, and you want to spend less money, then there is another option. Install a hybrid inverter charger, with a battery bank that will give you 2 or 3 days power in the event of an outage. If the outage lasts longer, then run the generator to charge the batteries and supply the loads for a few hours and your good to go for another 2 or 3 days. This installation does not require solar panels or a charge controller. Of course they could be added at anytime if desired.

 Hybrid inverter chargers are connected to the grid and and have an automatic transfer switch built in. The load is transferred to the batteries in milliseconds so there is no down time. A sub panel would have to be installed and wired with the appliances and outlets that you would need in an emergency. The hybrid inverter would power this emergency panel from the batteries during a grid failure. After power is restored the batteries are fully charged from the grid. During normal times, the inverter charger would be maintaining the batteries with a float charge, ready for emergency use.  

Outback makes hybrid inverter chargers in various sizes at 24 or 48 volts that will supply 120 or 240 volts AC depending on load requirements. There are other brands also, but I'm not that familiar with them.
  
Rick      

Dave Angelini

The previous response is a good one. If you are just looking for back-up power to the grid a cheaper solution is just a generator that your wife can start (electric start).

 Schneider Electric would also be a good choice for a hybrid inverter along with Outback.