Solar Charger and Panels

[Gina D.]

I use a 10g 20ft pigtail.

The solar panels should be spitting money out at me...

[Roy in TO]

Well, yes and no. ...
At 45 watts under full sun ...

Why does this matter? Since most charge controllers limit the charging voltage sent to the battery to 14.5 volts or less, any volts over 14.5v are lost anyway, so buying heavier, more expensive wire won't improve your solar system performance.

Is there any difference when one is not under full sun? Not being too knowlegeable in this area, I would think that in less than perfect conditions, one would want to minimize voltage drop to keep the voltage heading into the controller up to 14.5 to facilitate charging.

Roy

[peterh]

Is there any difference when one is not under full sun? Not being too knowlegeable in this area, I would think that in less than perfect conditions, one would want to minimize voltage drop to keep the voltage heading into the controller up to 14.5 to facilitate charging.

Roy

You're confusing the number of volts the panel puts out with the amount of power the panel produces. That's not surprising: the relationship between volts, amps, and watts are kind of complex and non-intuitive.

The voltage from a solar panel and the amount of power it puts out are two different things. Once a solar panel starts getting enough light to produce power it'll generally register close to or the same voltage whether it's in the shade or under full sun. What changes as the sun gets brighter is the number of amps the panel produces at that voltage; the actual power output of the panel, measured in watts, is equal to volts the panel produces times the number of amps produced. So Volts * Amps = Watts.

Using the panel on my trailer's roof as an example, my panel pumps out 17 volts whether it's in full sun or in the shade. Under full sun it pumps out 3 amps for a total of 51 watts of power (17 volts * 3 amps = 51 watts); when it's sitting under the shade tree in my front yard it'll still put out 17 volts, but might well only manage 1 amp for a total output of 17 watts of power.

The sad news is, because pumping 3 amps at 17 volts into my 12 volt battery isn't good for it, my charge controller "down regulates" the voltage it sends the battery to a maximum of 14.5 volts. Doing the math, that means my battery only sees 44.5 of the 51 watts of power my panel pumps out under full sun.

--Peter

[Brian B-P]

Why does this matter? Since most charge controllers limit the charging voltage sent to the battery to 14.5 volts or less, any volts over 14.5v are lost anyway, so buying heavier, more expensive wire won't improve your solar system performance.

Peter, you're assuming that the panel can sustain enough output voltage to get 14.5V to the controller. If not, then the loss in the cabling does matter; in 8 of the 12 example calculations, the loss would matter.

...The sad news is, because pumping 3 amps at 17 volts into my 12 volt battery isn't good for it, my charge controller "down regulates" the voltage it sends the battery to a maximum of 14.5 volts. Doing the math, that means my battery only sees 44.5 of the 51 watts of power my panel pumps out under full sun.

This sounds like a PWM (pulse-width modulated) controller. It is the most common type of advanced controller, but not the only one. The panel is not likely producing 51 W, since it is not putting out 3A at 17V... it's pulsing at that rate, with zero current in the off times between the pulses. It's only putting out 44.5 W, plus some lost in the controller. If the 17V and 3A are average readings, then it's really pulsing at more than 3A, and the difference between 44.5W and 51W is all controller loss.

My simple controller is just on-off: it's a switch which allows the panel output to connect directly to the battery, or just turns it off. The switching off occurs at something like the 14.5 V level, and it comes back on when the battery drops to some lower level. Again in this case, if the panel can't maintain more than the switch-off voltage, less cable resistance will mean more charging current. Since it doesn't frantically switch on and off, I know the 30W panel is not maintaining anything like 17V of output while connected to the battery... it's not even maintaining 14.5V after a couple feet of wire.

The best controllers are MPPT - Maximum Power Point Tracking. They work as voltage converters, shifting the voltage up or down (and thus the current down or up) to maximize the panel output. An MPPT controller in Peter's 17V/3A situation would convert the panel output voltage down to meet the battery needs, and thus pump more than 3A into the battery, getting the full 51W from the panel... if the panel can really produce that much. I've only read about these... it would cost more than my little 30W panel is worth.

[Frederick L. Simson]

Just got my Harbor Freight mailer today ("Postmaster: please deliver between 9-4-07 & 9-6-07")
Hurry! Sale ends September 24, 2007



45 watt Solar Panel Kit, LOT # 90599
$199.99 regular price: $249.99
plus, there is a coupon to take an additional 15% off on the front page

[Roy in TO]

You're confusing the number of volts the panel puts out with the amount of power the panel produces. That's not surprising: the relationship between volts, amps, and watts are kind of complex and non-intuitive.

The voltage from a solar panel and the amount of power it puts out are two different things. Once a solar panel starts getting enough light to produce power it'll generally register close to or the same voltage whether it's in the shade or under full sun. What changes as the sun gets brighter is the number of amps the panel produces at that voltage; the actual power output of the panel, measured in watts, is equal to volts the panel produces times the number of amps produced. So Volts * Amps = Watts.

Thanks for the explanation Peter, I know have a better understanding how the solar panels work.
Although I consider Volts * Amps = Watts to be a simple mathmatical relationship.

Frederick L. Simson Posted Sep 5 2007, 09:41 PM
plus, there is a coupon to take an additional 15% off on the front page

Thanks Frederick!

How can we get a coupon if we are not on the mailing list?
Are copies available in store or on line?
Seems worth the trip to cross the border ... might even get to check out the Niagara meet at the same time!

Unless of course someone here is crossing the border into Canada ... (hint hint)

[Frederick L. Simson]

How can we get a coupon if we are not on the mailing list?
Are copies available in store or on line?

I have seen copies of the ad available in the store, and at one time there were coupons available online, but not at the Harbor Freight site.

[Bigfoot Mike]

Harbor Freight 45 Watt Solar Panel Kit
http://www.harborfreight.com/cpi/ctaf/disp...temnumber=90599

[peterh]

Peter, you're assuming that the panel can sustain enough output voltage to get 14.5V to the controller. If not, then the loss in the cabling does matter; in 8 of the 12 example calculations, the loss would matter.

My solar panel may differ from other panels, but my understanding is that the amps any solar panel produces will drop off more rapidly than the voltage does when the sun goes down. Curious about that, I tested my panel when it arrived. (This was to both confirm my "new" used panel was in working condition and operating as advertised, as well as to satisfy my curiosity about the functioning of a solar panel.) I measure my panel's output by hooking both leads from the panel to the 10 Amp leads on my multitester and reading the value -- in other words, I shorted the panel out to measure the amps it produces. In full summer sun my panel pumps out 2.99 amps at 17.5 volts (so my panel's output is actually 52.3 watts, right in the middle of the 50-55 watts range Seimens advertises.); under my shade tree it pumps just under an amp, and still registers just over 17 volts. When I flip the panel over at a 30 degree angle over my cement driveway it registers just over 7 volts and virtually no amps at all. This performance mirrors what I have read about solar panel current production (amps) falling off much more rapidly than its voltage. (If you test your panel by this method make sure you connect your multimeter leads to the 10 amp plugs and set the multimeter to the 10 amp test setting BEFORE you touch the probes to the panel leads, otherwise you'll blow your multimeter's internal fuse and it won't measure amps anymore until you replace the fuse. Trust me on this.)

This sounds like a PWM (pulse-width modulated) controller. It is the most common type of advanced controller, but not the only one.

You've pegged it. Mine is a Sunsei 10 amp (PWM) controller that lets on-off-on-off pulses of electricity through so the voltage value on the wires going to the battery average out to a maximum of 14.5 volts when charging at its highest rate.

My simple controller is just on-off: it's a switch which allows the panel output to connect directly to the battery, or just turns it off. The switching off occurs at something like the 14.5 V level, and it comes back on when the battery drops to some lower level. Again in this case, if the panel can't maintain more than the switch-off voltage, less cable resistance will mean more charging current. Since it doesn't frantically switch on and off, I know the 30W panel is not maintaining anything like 17V of output while connected to the battery... it's not even maintaining 14.5V after a couple feet of wire.

The simple on/off charge controllers I looked at when I was building my system turned on at 13.0 volts and off at 14.2 volts, but remember that that voltage level doesn't represent the voltage of the panel, it represents the voltage of the panel + battery system, which will be lower. As for voltage loss due to wiring, that's easy to calculate if you know the resistance of the wire using Ohms Law, volts = amps * resistance. I used the resistance values and calculator on this web page to come up with my figures.

--Peter

[Brian B-P]

...I measure my panel's output by hooking both leads from the panel to the 10 Amp leads on my multitester and reading the value -- in other words, I shorted the panel out to measure the amps it produces. In full summer sun my panel pumps out 2.99 amps at 17.5 volts (so my panel's output is actually 52.3 watts, right in the middle of the 50-55 watts range Seimens advertises.); under my shade tree it pumps just under an amp, and still registers just over 17 volts.

Peter, maybe I'm just misreading your description, but it sounds like you're combining the open-circuit voltage (nothing but the voltmeter connected to the panel output; no current flow) with the short-circuit current (ammeter across the panel output with no other resistance). If you hook up both ammeter and voltmeter at the same time (call your friends and get your spare meter back...), I think you'll see that while delivering 2.99 amps, it is delivering much less than 17.5 volts. The two separate readings can't be multiplied to get power, because they're not happening at the same time, or under the same conditions.

For the mechanically inclined, you can think of open-circuit voltage like peak engine torque, and short-circuit current like redline speed: you can't multiply the two together to get engine power available. The peak torque (voltage) is only available at a lower speed (current), and at the maximum speed (current) much less torque (voltage) is available.

I agree that open-circuit panel voltage does not vary strongly with sun intensity, but the ability to deliver current does.

[Brian B-P]

..The simple on/off charge controllers I looked at when I was building my system turned on at 13.0 volts and off at 14.2 volts...

Thanks for the reminder of these cutoff levels, Peter; I think those are roughly the same limits as my controller, and simple automatic battery chargers.

...but remember that that voltage level doesn't represent the voltage of the panel, it represents the voltage of the panel + battery system, which will be lower.

But it is the panel voltage under the operating condition of being attached to the battery. The panel's voltage depends on the current passing through the panel, due to internal resistance, just as a battery's output voltage drops while it is delivering high current, and comes up to a higher level instantly when the current stops. So my "30W" panel puts out close to 20V in full sun when open-circuited, but when I connect it to a battery to be charged it delivers a couple of amps at a lower voltage determined by the battery's state of charge and internal resistance; the intervening controller sees that voltage is less than 14.2V, and stays on. Eventually, the battery's internal voltage comes up with charging, the resulting voltage at the terminals rises as the panel puts out more voltage and less current, and the controller shuts off at 14.2V.


Anyway, in a real panel-and-load combination, I would expect the operating voltage to be easily less than the controller's cutoff or controlling level, and so it seems that wire resistance both limits charge rate and dissipates energy.

[Brian B-P]

...I measure my panel's output by hooking both leads from the panel to the 10 Amp leads on my multitester and reading the value -- in other words, I shorted the panel out to measure the amps it produces.

A meter on the high-current setting should have extremely low resistance. An ideal ammeter has no resistance at all, but realistically it needs enough to produce an accurately measurable voltage internally... I just checked two of my (cheap digital) meters against each other; I measured the resistance with one (set to various ohms ranges) across the terminals of the other (set to various amps ranges). Meter resistance (of the meter set to a current range) in each case varied from about 2 ohms (on the highest low-range current setting of 200mA) to about 1000 ohms (on the lowest current setting of 200 microamps), and was too low to read (less than or about equal to 0.1 ohms) on the 10A setting (and using the 10A terminals).

To sustain 17.5 V across the ammeter at a current of 3 A, the meter would need to have about 6 ohms of resistance in 10A mode - I certainly hope it doesn't have that much, which would cause major interference with the operation of the circuit being measured.

I'm not questioning Peter's observed current - that's interesting information. It's just that the characteristics of the meter ensure that this is nearly a short-circuit current, and does not occur in combination with the open-circuit voltage.

[Greg A]

Ok, Brian - Peter, great info but you guys lost me about 4 threads up.

In laymans terms if I can find the 15% off coupon and go get the Harbor Freight Panels for $169(pretty screaming deal) will they work well with my Group 27 deep cycle wally special battery right out of the box or will I need something else in addition to what is included?

[peterh]

Ok, Brian - Peter, great info but you guys lost me about 4 threads up.

In laymans terms if I can find the 15% off coupon and go get the Harbor Freight Panels for $169(pretty screaming deal) will they work well with my Group 27 deep cycle wally special battery right out of the box or will I need something else in addition to what is included?

The Harbor Freight system will charge your batteries just fine, though you'll need to make some adjustments to what lights and appliances you run when you boondock. (Things like using LED lights instead of the 12v incandescent bulbs most RVer use.) Gina's article on going solar has lots of good advice & ideas. The only think I'm not sure about is the charge controller that comes with the panels; the reviews I've read suggest you should buy a different charge controller.

Brian and I are being nerds; he's giving me something to think about and some more experiments with electricity to perform. With a little luck my valued mentor will teach me something.

--P

[Roy in TO]

In laymans terms ... will they work well with my Group 27 deep cycle wally special battery right out of the box or will I need something else in addition to what is included?

Greg,

It should work OK right out of the box, start back at the beginning of this thread to see what others have done.

I don't know how your system is wired, but you might want to check the diagram in this thread and see where a fuse is placed on the +ve line between your battery and the rest of your wiring in the trailer.

[Roy in TO]

it sounds like you're combining the open-circuit voltage (nothing but the voltmeter connected to the panel output; no current flow) with the short-circuit current (ammeter across the panel output with no other resistance). If you hook up both ammeter and voltmeter at the same time (call you friends and get you spare meter back...), I think you'll see that while delivering 2.99 amps, it is delivering much less than 17.5 volts. The two separate readings can't be multiplied to get power, because they're not happening at the same time, or under the same conditions.

Considering this thread started off discussing the Harbour Freight Panels, the manual is online which gives the specs below.

[Brian B-P]

Excellent find, Roy!
Such a radical idea... reading the manual

I downloaded the whole manual (over 8 minutes of dial-up ). Although Roy has already extracted the only real technical content for the panels, this setup does have an interesting controller/meter/adapter box, and I think this is a good opportunity for anyone considering buying the setup to have a look to see if it's what they want.

The Harbor Freight specs include:

• Peak Voltage - 23.57 OCV (Open Circuit Volts)
• Panel - 15 watts per panel, 14.5 volts output (working)
• Nominal Voltage - 14.5 volts battery output

In other words, placed in full sun with no load (open circuit, zero current) you should measure 23.57 volts from the panel, but the 15 watts is produced at 14.5 volts, and just over one amp, which is the condition normally encountered near the end of charging a battery.

My guess is that Peter is seeing a lower open-circuit voltage on his panel because he's not sitting in an equatorial desert at high noon on a perfectly clear day...

[Roy in TO]

Excellent find, Roy!
Such a radical idea... reading the manual

Thanks Brian,

I got one better for you, and it is a real big download (1.2 M. Charlie C posted yesterday about the Coleman solar panel deal at Costco. That includes the Phocos charge regulator (manual onlline here), they just announced a new remote LCD display that works with this controller which displays panel current, load current and battery voltage.

I've got some questions that I'll be asking you and Peter about this which really should be in topic on its own in another area since this discussion has strayed Frederick's original topic.

And from what I see on the bottom of page 4 of the HF maual, Peter's readings might also be affected by the fact that PV panels can experience a 20% degradation within the 1st six months.

[Brian B-P]

...I've got some questions that I'll be asking you and Peter about this which really should be in topic on its own in another area since this discussion has strayed Frederick's original topic...

A separate thread sounds like a good idea. The Moderators could even move many of the recent posts in this thread to a new one, for that reason... maybe called something like "Solar Panel Performance and Wiring".

[Brian B-P]

...the Phocos charge regulator... just announced a new remote LCD display that works with this controller which displays panel current, load current and battery voltage.

That dislay looks like a nicely packaged unit. The termination block can mount on a DIN rail, which will be familiar to many industrial electronics users. The controller can be mounted to a DIN rail, as well.

Hey, Per, you've got enough stuff in your trailer (and change it enough) that serious equipment management options might be of interest. Ready for DIN rail mounting?

The meter shows panel current and "load" current separately, because the associated controllers are intended to have the loads (your trailer's lights, fans, pumps, etc) connected through the controller, not directly to the battery.

It is not, however, a serious battery monitor: it doesn't keep track of charge into and out of the battery, so it can't act as a battery "fuel gauge".