System Compatibility???

Re: System Compatibility???

You really do not need a fuse for a single or two parallel strings.

The 15 amp rating is based on the ability of the solar panel wiring (and internal construction) to handle a 15 amp maximum current without overheating the wiring and causing a fire.

If there is more than 15 amps (per string--power coming from other paralleled solar panels), then the 15 amp fuses protect the "shorted string" from being over powered by the other parallel connected panels (if that run-on sentence makes sense:roll:).

The assumption for the panels is that they are the power source for short circuits--Not the charge controller.

NEC is designed around a single fault and subsequent resultant failures.

In other words, the only way that you could have a solar panel wiring failure in your setup would be if the array got a short AND the solar charge controller would also have to have an internal fault (battery to solar panel input short)--then a fuse in the solar panel wiring could protect against excess current.

But in normal day to day usage/design, that (double fault failures) is not a problem that is addressed by code.

If you have the fuse holder--put a 15 amp fuse in it. Why not.

20 amp may OK but does not meet the specifications.

10 amp fuse is getting close to where you may have a false blow years in the future--plus smaller fuses have higher resistance (fuses are small heating elements) and there is no reason to loose the extra power by having a smaller than needed fuse.

Fuses and circuit breakers are designed to be "unreliable"--so that the will eventually trip at 100% load and not trip at all at 80% load.

If you don't need the fuse--it is just one extra thing that could fail in the future.

-Bill

Re: System Compatibility???

SolarSteve wrote: »

I have TWO solar panels in series. In series the current stays the same, but the voltage doubles. (Today it hit 40 V. on the controller). But if EACH of these panels is putting out roughly 7+ Amps, why would a 15A fuse be big enough??? On a perfect day, wouldn't I be taxing that fuse with almost or even slightly over the full 15A????

Sharp ND-123 ujf 123 W
Series Fuse Rating 15A

Solar panels are not really "solar batteries" even though everyone sort of treats them that way...

A battery is a "voltage source" and can output between 0 and 100's of amps at 12 VDC (depending on size / specifications of battery)...

A solar panel is actually a current source that reaches near Voc (Voltage Open Circuit) as soon as there is weak sunlight (morning or evening) direct sunlight on the panel...

At that point, more or less, the panel will output X amps of current--no matter if the load is Vmp (voltage maximum power) or near zero volts (into a dead short). So, solar panels naturally limit their output current to the amount of sunlight hitting the panel.

The amount of output current from the panel is pretty much proportional to the amount of sunlight hitting the panel... Near zero watts per square meter of sun light, then it will output near zero current.

When the full noon time sun hits the panel at ~1,000 watts per square meter--The panel will output around rated current. If you have a big mirror putting 2x1,000 w/sqmeter on the panel, you will get 2x rated current (assuming panel and wiring does not overheat).

So, Isc is basically the maximum current the panel will output under "full sun" at standard test conditions. Using a fuse that is around 2x Isc is picking a value that the panel will never reach in normal operation--so it should never blow in normal use.

And the copper wiring/insulation/solar panels construction is designed to handle a maximum of 15 amp of current safely (no overheating).

If the panel gets more than 15 amps (short circuit, miss-wired connections, etc.)--the Fuse will pop and (hopefully) will greatly reduce the risk of fire/electrical failure for the panel.

In your case, the two panels are in series--so the current from one panel flows through the second panel... The Varray output voltage adds because you placed the two panels in series--but the output current is still ~8 amps from each panel maximum (current does not add with series connections).

If you paralleled the connections, the voltage would not add, but the current would double--but you would place a 15 amp fuse in series with each panel--so that no failures would "back feed" a shorted panel and possibly cause it to overheat from a short circuit.

Does that make sense to you? (too many words--I know:roll:).

-Bill

Re: System Compatibility???

SolarSteve wrote: »

1. You can have more amps or you can have more volts. But not more of both, one or the other. It's your choice.

Yep.

Two "batteries" in series--the voltage goes up.

Two "batteries" in parallel--the current goes up.

Of course, you can do combinations (series/parallel)--but they still follow the same basic rules.

2. One gives you more quantity of electricity (amps) but the push (volts) through the wire is less, so in the end you don't get anything extra, no free lunch.

Yes again.

The basic governing equation in this situation is:

• Power = Voltage * Current

With the same two "batteries"--you either double the voltage, or double the current (or something in between with series/parallel connections if you have 4 batteries to play with, etc.).

But, as you can see, nothing magic with the basic math.

3. If you have more volts, you don't need as many amps because what there is, is coming in under greater pressure (volts).

Pretty much... See the above equation.

The MPPT charge controllers (typically switch mode buck type power supplies with computer control) are almost like the transmission on your car...

In low gear, the engine revs at high RPM (but low torque of the motor) to low RPM / High torque of the tires...

Or in high gear, the wheels turn faster (because you are now going 70 MPH), but the torque to the tires is less (you have to down shift to go up hill).

Same amount of Horse Power from the engine--just adjusting the RPM/Torque to the requirements of the load (speed and drag).

If you are familiar with Variacs (variable/adjustible AC transformers)--the MPPT controllers are, sort of, the DC version of the AC transformer.

They both can take high voltage/low current and "down convert" to low voltage/high current... Still the same amount of power (100 watts in, roughly 100 watts out).

4. SUMMATION: You can have more amps or more volts by simply wiring it different, BUT IN THE END YOU WILL HAVE THE SAME --WATTAGE-- which is what really does the work.

Yes--But remember that high voltage / lower current means (usually) you can use smaller wire gauge (diameter) because of the lower current (and voltage drop).

It is very difficult to send significant amounts of energy at 12 volts any more than 10-20 feet or so.

Your house wiring is 120/240 volts and you can send the same power (1/10 or 1/20th the current) hundreds of feet.

Or your electrical company will have 12,000-24,000 volts on the poles in your neighborhood so they can send the power 1,000's of feet.

etc...

5. And in my situation, I have these 2 panels in series because I am gaining a slight advantage over parallel wiring because I have a MPPT controller. Correct?

Several advantages:

1. High array voltage allows you to use smaller wire and/or send your power longer distances (array to controller/battery bank) more efficiently and without spending a ton of $$$ for heavy copper cables.
2. A second effect is that solar panels in hot weather actually have their output voltage depressed (maybe 20% or so)--Very hot batteries and very cool batteries, and your panels may not output full current to your batteries. Your setup with the high Varray voltage completely bypasses that issue.
3. Lastly, you will see claims that MPPT controllers can give you 25-30% more power--Sort of yes--If your in a very cold (way below freezing) environment. Cold solar panel output voltage rises and the MPPT controller can convert that extra higher voltage into more current for charging your battery bank..

In reality, on average, your power increase may be more on the 10% or so improvement in cooler weather... Not bad, but probably not worth paying 2-3x more for a charge controller for this privilege.

However, the first two reasons are more than enough reason (usually) to justify the extra costs when building out a larger system.

Also, MPPT charge controllers tend to be more expensive (and newer designs), so you will see more bells and whistles (networking, data logging, programmable charging options, etc.). Sometimes very nice options to have.

Under 200 watts, think about PWM controllers (less expensive and may be more efficient).

Over 400-800 watts, probably MPPT will be the better choice.

In between--"it depends" on your needs and wallet... :roll:

-Bill

Re: System Compatibility???

SolarSteve wrote: »

Excellent Bill! Thank you.

You are very welcome Steve.

By asking the questions and documenting your system--You also help the next person too.

So... if I understand you right, I am currently -not- maxing out my Morningstar SS-MPPT-15L charge controller (15A limit) because I have my 2 panels in -series-, so therefore I could actually run/add ANOTHER set of the same panels again in series, where only then would I be up to the 15A limit????

With most MPPT controllers--there is not really a hard power input limit.

It is Vmax/Vmin and issues around input current (too much, need an input fuse; too little and you waste power/money).

Voc (voltage open circuit) or the maximum input voltage from a cold solar array becomes the issue with placing panels in series...

The MS 15 amp MPPT controller has a Voc limit of 75 volts... Assuming you are in Florida and your cold days are not very low... For your panels (using the Xantrex Calculator) for a 20 F day, the maximum series Voc for 3x panels is 73 volts.

Getting pretty close to the 75 volts maximum. It is not a very expensive charge controller if you ever had a cold snap and fried the controller--MorningStar does not replace over voltaged controllers. Perhaps you should call them and confirm that 3x panels in series for your area is a good idea or not.

You are now looking at ~369 watts on a 15 amp MPPT controller charging a 12 volt battery bank (correct?). You are about 20-30% larger than the 15 amp controller can manage--so it will spend significant amount of time with more power than it knows what to do with.

Personally, I would think it is not a good use of your money... A second controller or a higher rated controller might be a better choice.

Part of the decision will be your eventual plans (never more than 3 panels, etc.).

From my prior posts you'll remember I had asked how to wire the 2 panels in series, confused by all the connections on each panel. Tell me... there must be some practical use for all the other connections that go to all the potted diodes, etc. Each panel has SIX connections. Now we're just using the plain end terminals that don't attach to any diodes. I've tried to study a bit, and some may be bypass diodes that would help if any part of one of the panels got shaded. There's lots of trees up on my property, and I know early morning and late afternoon will unfortunately have abundant shading. Shouldn't we be taking advantage of some or all of those diodes???

The bypass diodes are there to protect the shaded panel from getting over voltaged by the other 2 or more series connected panels (remember solar cells are really just really big silicon diodes--and if you apply too much reverse voltage, you will damage the diode junction).

The fact that the string will still pass some useful current if one panel is partially shaded is an advantage for two panels in series vs two panels in parallel.

For your needs, these extra connections are not going to do a thing for you.

Some panels, this allows a person to replace the bypass diodes if there is damage--but hopefully you never need to do this.

-Bill

Re: System Compatibility???

There is no really right or wrong answer (at least as defined by me)--Just what makes best sense towards meeting your needs.

What voltage to turn off the inverter--Actually that is not an easy answer. Battery voltage as a measurement of state of charge is only accurate after the battery has sat for a few hours (no charging or discharging current).

A battery monitor would be a better tool... But if you are limited to what you have on hand, you can start with 11.5 volts or so and see how that works for you.

You can estimate the power and simply time your TV time... For example, say you have a 300 watt 120 VAC load, on a 85% efficient inverter, with a 225 AH 12 volt battery bank, and you want to run it to ~50% of rated AH capacity (good cycle life for battery)... The amount of time to run the system would be:

• 225 AH * 12 volts * 0.50 max discharge / (300 watts * 1/0.85 inverter eff) = 4 hours of run time

So, assuming everything is equal (~77F battery bank) and every thing is working to specifications--When the system stop supporting 4 hours of run time (from a 100% full battery), your battery is worn out/needs replacing.

It is probably going to take your solar panels ~2 days to fully recharge the battery bank back to 100% after that 4 hours of use in average sunny weather...

-Bill