Too Much power?

Belize1 · January 2020

I have two controllers side by side in an off grid residential application in Central America. Each controller handles eight PV panels split into two groups of four. Each panel is 250 watts with a max output of 8.3 amps ( in ideal conditions, as we all understand. The heat of Belize is not ideal for PV output)

With four panels in series they produce 130 volts output. But, what is their combined amp output? In series operating at 130 volts are the four panels producing 32 amps? Or 8 amps? Are the two sets of four (total eight) operating at 130 volts producing a combined output of 16 amps, or 64 amps?

Second question. The total array feeding two Classic 150 controllers is 16 panels for a total of 4kw watts.. Two groups of four on each controller.

Each 150 could be feeding as much as 2000 watts into the system. 2000 watts at a charge voltage of 27 volts would be 74 amps times two chargers for a total of 148 amps to the batteries???

The Batteries are in series at 24 volts. Eight batteries total ( 6 volt) for a total of 460 AH capacity. US Battery recommends charging at ~10% of C20. That would be a max of 43 amps to the batteries. The PV array is clearly capable of charging the batteries at 3 times the recommended rate.

a) Can the two Classic 150 controllers be set to limit their output to some predetermined max to avoid charging the batteries too fast?

b) What is reality on charge rates, i.e. what is happening to the batteries if they recharge at 20% of C20 (86 amps)?

BB. · January 2020

On the panels... When you put them in series, the voltage (Vmp/Voc) adds. When you put the panels in parallel, the current adds (Imp/Ioc/etc.).

You have 250 Watt panels with Imp=8.3 amps:

Vmp = Pmp/Imp = 250 Watts / 8.3 amps = 30.1 Volts Vmp-std conditions (sounds right)

So, if you have 4s panels by 2p strings:

4x 30.1 = 120.4 volts Vmp-std conditions (hot panels, hot climates, Vmp-hot falls).
1x string is Vmp~120.4 volts @ 8.3 Amps
2x parallel strings, Vmp~120.4 volts @ 16.6 Amps
Rated array 120.4 volts * 16.6 Amps = 1,999 Watts Pmp (2,000 Watts, roundoff error) for 4s x 2p array

The rules for Series/Parallel connections panel connections, work the same for battery connections:

4 x 6 volts @ 230 Amps = 24 volt @ 230 Amp*Hour battery bank
2x parallel strings = 24 volts @ 460 AH battery bank

For a general set of assumptions, assume that your solar array will, on the best days (cooler, windy, high noon, panels pointed at sun), your system will output somewhere around 75-77% of "rated" power output (mostly because Vmp falls when cells/panels are hot. Imp/Isc does right when panels get hot, but it only a slight amount and usually just ignored).

2,000 Watts * 77% panel+controller deratings = 1,540 Watts per "typical" solar array output available for battery/DC bus power
1,540 Watts / 56.0 Volts "nominal" battery bank voltage = 27.5 Amps from each controller
27.5 amps * 2 controllers/arrays = 55 Amps typical "best case" current from array

Now, there are a lot of variables here... As you can see, the actual battery charging current determines controller output current (batteries near full, less current, battery bank well discharged, higher current with same array wattage.

Also, solar charge controllers make their charging decisions (charging voltage) based on battery state of charge, temperature, other loads on the DC bus, charging state (bulk, absorb, float), if the panels are all pointing the same direction, if the chargers are "synced" (some charge controllers can be setup to all be in the same bulk/absorb/etc. modes, others will make their own decisions and not switch charge states in sync--Which is fine too.

-Bill

mike95490 · January 2020

Belize1 said:

...............

a) Can the two Classic 150 controllers be set to limit their output to some predetermined max to avoid charging the batteries too fast?

b) What is reality on charge rates, i.e. what is happening to the batteries if they recharge at 20% of C20 (86 amps)?

a) Yes, you can use the graphic panel or the Local App to set max amps output

b) the batteries heat faster at higher charge rates. This ages them faster.

In reality, the batteries self-regulate their current at any specific voltage. When the sun comes up and the batteries are really low, the solar charge rate slowly ramps up because the sun is at a poor angle (usually). By 10am, the batteries are partly charged and won't accept as much amps as they would have earlier in the day. The problem is a foggy or cloudy morning, and then at noon the sun clears and slams lots of amps into low batteries. If it happened every single day, that would be bad for the batteries, if it happens 2x a month, that's not very bad.

mcgivor · January 2020

Being at the same latitude as Belize the output of the panels will be less than rated due to high ambient temperatures @BB.is correct in his estimate the best I've observed from 6 × 250W panels is around 30A on a cool day, relitivly speaking which is 30°C.

The thing of most important factor, particularly with lead acid batteries is temperature, the life expectancy is dramatically reduced as the temperature is above 25°C, figure on 3 years give or take if 35°C is the normal ambient. Once the water consumption increases significantly it's a sign they are on the way out. Temperature compensation will help to some degree but it is a a reality that temperature is the enemy you are faced with.

How is Belize these days, spent some time there in the late 1990's doing small scale hydro mechanical systems for the Amish community with their woodworking shops. Miss the roadside Jerk chicken, spent most of my tine between Bellomopan and Placencia at a friends farm near Over The Top, it was a great time then but the locals were beggining to complain about the immigrants from Honduras taking the jobs they weren't willing to do in the first place.

Belize1 · January 2020

Thank you to everyone for the answers. You confirmed I was doing my math correctly. Lots of extra information in these posts is quite helpful.
Mcgivor, Belize is still wonderful. My house is at the very northern tip of the Placencia peninsula. The people continue to be unhurried and able to enjoy life. I don't get to stay in Belize all the time but love the time I get.

Too Much power?

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