Battery-less off-grid? Panel -> controller -> inverter -> load

[Sunking]

Now, why is overcharging a problem?

Battery Internal Resistance. Flooded lead acid batteries on average can only take a maximum C/8 charge current where C = th ebattery capacity in Amp Hours and 8 is the hour charge rate of 8 hours. So a 10 AH batery should be charged with no more than 10 AH / 8 Hours = 1.25 amps. A 700 watt panel would push 59 amps causing the electrolyte to boil violently and spew everywhere.

Don't today's smart chargers trickle batteries when they're at capacity? Where does the excess current go if not through the inverter to the load?

Thus i swhat you do not understand. Everything has to be made to work with each other. What you are thinking of is putting a Saturn V rocket to power a child's wagon. Batteries have a minimum and maximum charge/discharge rate. Fo rFLA batteries the minimum is C/12 and max is C/8 with C/10 being perfect. If you have a 12 volt 100 AH battery needs at least 8.3 amp charge current and a maximum of 12.5 amps. On the discharge side no more than C/8 and on the same 12 volt 100 AH battery the maximum inverter size is 12.5 amps x 12 volts = 150 watts.

So for your theory Inverter of 1500 watts would require a 12 volt 1000 AH battery or 100 times larger than a 12 volt 10 AH battery. Th minimum panel wattage required to charge a 12 volt 1000 AH battery is 1000 watts which will produce 80 amps of charge current.

FWIW a Trickle charger is one that charges at C/20 or less. They are not capable of recharging a battery, only maintaining a fully charged battery.

[cdevidal]

Let's please take a step back and look at the big picture. I am asking what happens to excess current if only the barest minimum spec of battery is used. Whether the battery is 10AH or 100AH or 10,000,000AH is beside the point; What matters is that it's the minimum required, whatever that is. I only threw out some numbers because I thought it might illustrate my question, but I see it's only clouded the issue. So let's forget the numbers.

I'm only asking about PV theory at this point. There are some facts of PV theory I think I understand, but want confirmation on.

When I see the schematic in the OP, the inverter is directly in line with the array, so I'd suppose that what happens is that when the sun is shining, the battery charges and excess power is sent directly to the inverter. I wouldn't suppose that when the sun is shining, the freezer only runs as long as the battery has capacity.

Is my general understanding of PV theory correct?

If there are any doubts, please have a closer look at the stated goals.

[SunEagle]

Thank you, I think you're understanding what I'm asking, whereas the others seem to be tripping over imaginary numbers.

Now, why is overcharging a problem? Don't today's smart chargers trickle batteries when they're at capacity? Where does the excess current go if not through the inverter to the load?

I believe Sunking provided the answer.

Most expensive battery chargers can provide the correct amount of charging amps to a battery but Solar Charge Controllers do not necessarily have the smarts to do this and if there is a very high input (wattage or amps) then it gets passed onto the battery even if the battery can't handle it. The battery fluid then boils away and it has a early death.

Now a trickle charger is like an IV. Just a little amount of current gets to the battery to keep it happy and stable but not to charge it from a discharged condition or to over charge it.

As much as I would like to say that building a solar battery charging system is easy you have to remember that Physical Science and Math keeps getting in the way.

[SunEagle]

Let's please take a step back and look at the big picture. I am asking what happens to excess current if only the barest minimum spec of battery is used. Whether the battery is 10AH or 100AH or 10,000,000AH is beside the point; What matters is that it's the minimum required, whatever that is. I only threw out some numbers because I thought it might illustrate my question, but I see it's only clouded the issue. So let's forget the numbers.

I'm only asking about PV theory at this point. There are some facts of PV theory I think I understand, but want confirmation on.

When I see the schematic in the OP, the inverter is directly in line with the array, so I'd suppose that what happens is that when the sun is shining, the battery charges and excess power is sent directly to the inverter. I wouldn't suppose that when the sun is shining, the freezer only runs as long as the battery has capacity.

Is my general understanding of PV theory correct?

If there are any doubts, please have a closer look at the stated goals.

The problem is that electricity takes the path of least resistance so without a way to throttle how much goes to the battery and how much goes to the load the chances are one is starved and the other is over stuffed. Batteries get over stuffed if they are too small and have low resistance. You need some type of control device in the middle or at least have all of the components the right size to handle what is thrown at them.

[cdevidal]

The problem is that electricity takes the path of least resistance so without a way to throttle how much goes to the battery and how much goes to the load the chances are one is starved and the other is over stuffed. Batteries get over stuffed if they are too small and have low resistance. You need some type of control device in the middle or at least have all of the components the right size to handle what is thrown at them.

But my question isn't about too small batteries.

[SunEagle]

But my question isn't about too small batteries.

Well your example of a 10Ah battery to a 700 watt panel would fall into the "small" category. But again I know you were just pulling numbers out of the air for the example.

Like I said there is a way to do what you want depending on the type of load (lights, computer, refrigerator) which will determine the type and size of the inverter.

The problem is the ratio of panel wattage to battery Ah (and CC type). Under normal designs it should be 1 : 1 but (and I am guessing) maybe it could be smaller like 1 : 1/2 but may never get below 1 : 1/3 or 1/4. I don't really know. I do know that the smaller the battery Ah is compared to the panel wattage the chances of over charging the battery is much higher regardless of the load. Also having a properly sized battery will help guarantee running the load continuous regardless of the changing sunlight.

[cdevidal]

The problem is the ratio of panel wattage to battery Ah (and CC type). Under normal designs it should be 1 : 1 but (and I am guessing) maybe it could be smaller like 1 : 1/2 but may never get below 1 : 1/3 or 1/4. I don't really know. I do know that the smaller the battery Ah is compared to the panel wattage the chances of over charging the battery is much higher regardless of the load. Also having a properly sized battery will help guarantee running the load continuous regardless of the changing sunlight.

Ok, so I've properly sized the battery. When the sun is shining the battery charges. During charging and afterward, excess power is sent through the inverter to the load. Do I understand this correctly?

[Sunking]

In a properly sized system your theory works in application. When the battery is fully charged the charge controller turns off. If a load device demands power intially the batteries provide it until it voltage falls below the charge controller set point, and then the controller turns back on taking power from the panels to supply the load and recharge the battery. Point here is it had better be properly sized or your theory blows up in your face.

[cdevidal]

If a load device demands power intially the batteries provide it until it voltage falls below the charge controller set point, and then the controller turns back on taking power from the panels to supply the load and recharge the battery.

BINGO!! That's the information I didn't have.

[cdevidal]

Is there a separate class of devices that can power a freezer without requiring the whole charger --> battery --> inverter setup? Maybe an inverter, but a different kind?

[Sunking]

Is there a separate class of devices that can power a freezer without requiring the whole charger --> battery --> inverter setup? Maybe an inverter, but a different kind?

Yes a 12/24/48 volt DC powered fridge like Danfrost or Sun Dancer. Very expensive. Not sure how it would handle the extreme voltage swings of a panel without a battery to regulate voltage, and a controller cannot work without a battery. Keep in mind solar panels are current sources, not voltage sources.

[cdevidal]

Yes a 12/24/48 volt DC powered fridge like Danfrost or Sun Dancer. Very expensive. Not sure how it would handle the extreme voltage swings of a panel without a battery to regulate voltage, and a controller cannot work without a battery. Keep in mind solar panels are current sources, not voltage sources.

But not a standard AC-powered chest freezer, with maybe a voltage regulator?

And please explain, "panels are current sources not voltage sources."

[Wy_White_Wolf]

Voltage source is a circuit element that maintains a prescribed
voltage across its terminals regardless of the current flowing in those
terminals.

Current source is a circuit element that maintains a
prescribed current through its terminals regardless of the voltage across
those terminals.

Or the long version

http://www.facstaff.bucknell.edu/mastascu/eLessonsHTML/Source/Source1.html

WWW

[cdevidal]

Voltage source is a circuit element that maintains a prescribed
voltage across its terminals regardless of the current flowing in those
terminals.

Current source is a circuit element that maintains a
prescribed current through its terminals regardless of the voltage across
those terminals.

WWW

Ok. An AC-powered freezer on a voltage regulator is a no-go?

Edit: No-go. It's DC power. Needs a sine wave, duh.

[paulcheung]

Is there a separate class of devices that can power a freezer without requiring the whole charger --> battery --> inverter setup? Maybe an inverter, but a different kind?

Yes they call it grid tie inverter. you have to have the grid for it to work.