Solar panel question

Dave E

Hello all,
This may not be an actual Basic Stamp question, but I have seen a lot of info about solar panels on this forum so...
I have a question about solar panels that I hope the Parallax Forum users can answer.

I have made a solar battery charger with a Basic Stamp that modulates the charging current to the batteries. The transister is full on until the battery voltage reaches 14.4, then·the stamp·modulates the transistor to hold the battery voltage at 14.4.

I was wondering if holding the panel voltage at maximum power point (17.3 V for my panel) would give me more charging current. I assume that more current going to the batteries is THE indication of more charging. I programmed the stamp to do this but I got less current going to the batteries at max power point. What am I missing here?

Any info would be appreciated.

Dave E

Beau Schwabe

How much current are you measuring, and what current are your panels rated at?

Are you taking into consideration that the difference in the battery voltage and the difference in the solar voltage may be close to one another therefore causing the current to be lower?



Also... check your battery chemistry requirements... It may not be good to keep the float charge level at 14.4V once the battery has reached that level.

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Beau Schwabe

IC Layout Engineer
Parallax, Inc.

Dave E

Thanks for the reply, Beau.

I have an 80 watt panel that will supply a bit over 3 amps. The issue is that at this time of year I only get about 2 hours of full sun light so I leave the "charge to" voltage at 14.4 which my deep cycle battery manufacturer states is the maximum voltage to apply. Also according to the specs, the 14.4 should not be applied for more than 8 hours so with the limited sunlight, it is rare to get the batteries up to this voltage at all. This is why I assume it is ok to leave them at this charge level. During the summer, I set them to·charge to 13.6 volts.

I have the 2 different charging algorithms in 2 different slots of my 2px. This way I can swap between them with a flip of a switch. When I do this, there is a noticable difference in the current between the 2 different charging routines.

When I run the original program, the panel voltage is within a few tenths of a volt of the battery voltage depending on the current because of the small resistance in the wires and such. When I flip the switch to hold the panel voltage at 17.3, the current drops. I assumed that I would get an increase in current. The wattage may in fact be higher, but again, I assume that current charges the batteries, not wattage.

Right now, the panel is·giving me 240 mA at 12.5 V· or· 190 mA at 17.3 V (the sun is still in the trees). Running at 17.3 does give me a higher wattage but will it charge the batteries faster than the lower differential voltage with higher current? Because the charger manufacturers are selling MPPT controllers, I thought that I could get more charging capacity if I ran the panel at MPP.

Thanks again for your input.

I may be out in la la land with my understanding of solar charging so don't worry about offinding me with direct comments. I am a firm believer in asking the perverbial dumb questions.

Dave E

Beau Schwabe

Are your Panels 12V or 24V... the 80Watt 3 amps makes it sound like a 24V panel.
·
What is the measured open circuit voltage of your panels in full sunlight?
...and What is the measured voltage or your panels under a load? <--Measured resistance of the load also?
·
"the panel voltage is within a few tenths of a volt of the battery voltage depending on the current because of the small resistance in the wires and such." - I have found that IR drop plays a very small part (contrary to some of my previous posts) compared to the effective internal resistance of the panels themselves.· What you will probably notice is that the panel voltage will track the battery voltage very close, and that small voltage difference that you see is mostly a combination of the MOSFET and protective diode in the charger and/or the panels.
·
The internal panel resistance can be measured and characterized with the open circuit and load condition reading from above, and from there I can give you a little better idea of what is going on.· If you can take a few different load voltage readings that would be better.

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Beau Schwabe

IC Layout Engineer
Parallax, Inc.

Post Edited (Beau Schwabe (Parallax)) : 11/2/2008 3:20:37 PM GMT

Dave E

The panel is a Sharp NE-80EJEA. 12 volt nominal. 21.6 OC.

I will take a few measurements when the sun is full on the panel.

Thanks

Dave E

Dave E

Right now I get 14.08 panel voltage, 13.60 battery volts and 3.14 amps.

A flip of the switch.....

At MPP, 17.3 panel volts, 13.47 battery at 2.26 amps.

OC in full sun is 21.2.



As far as load resistance: Can I just disconnect the batteries and measure the resistance of the batteries with my meter?

Thanks for your help, Beau.

Dave E

Tracy Allen

My guess is that your circuit puts resistance in series with the panel in order to limit the current enough to bring the panel up to 17.3 volts. (mosfet?)

Why do you say 17.3 volts is the MPP? It is not, by your measurements...
17.3 volts * 2.26 amps = 39 watts
whereas
14.08 volts * 3.14 amps = 44 watts.

In any case, 17.3 volts sounds high to me as an MPP, which should be usually closer to the rated voltage of the 12 V panel.

Realize that the MPP changes with the amount of sunlight and also with the panel temperature and it changes gradually as the panel ages. A MPP controller in its most complex form may have to monitor and account for those changes. A true MPP controller will, I think, will have to have an inductor or transformer to make a constant power transformation. That is, the inductor (or transformer) will take power from the panel at one voltage and current, and deliver it to a load (the battery) at a different voltage and current, with minimal loss of power. And the controller of course also has to avoid overcharging the battery and divert or cut off the panel power.

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Tracy Allen
www.emesystems.com

Post Edited (Tracy Allen) : 11/2/2008 5:05:29 PM GMT

Dave E

Thanks everyone. That makes more sence.

The MPP voltage is stated as 17.3 volts in the spec sheet at "rated" maximum sunlight of 1000 W/m^2. Even at 600W/m^2 it looks to be about 16 volts. Not sure what the actual sun power is here in north Georgia.

According to the specs I may have more power at 17.3 volts but like Tracy said I must be wasting it by increasing the in-line resistance with my mosfet. I'll take a few measurements to verify that but it does make sence.

Thanks again for clearing that up.

Dave E


·

Beau Schwabe

Dave E,

"As far as load resistance: Can I just disconnect the batteries and measure the resistance of the batteries with my meter?" - It doesn't quite work that way.· The internal resistance effect is dynamic and proportional to the load put on the cells.

I took some measurements earlier today around 4:00pm with one of the 15Watt panels I have.

     Open Voltage     Closed Voltage     Resistance     Current     Watts
1)   20.05V           18.78V             100 Ohms       188mA       3.53 Watts
2)   20.03V           17.46V             50 Ohms        349mA       6.09 Watts
3)   20.05V           16.10V             33.3 Ohms      483mA       7.77 Watts
4)   20.03V           14.54V             25 Ohms        581mA       8.45 Watts
5)   20.00V           12.94V             20 Ohms        647mA       8.37 Watts
6)   20.00V           11.47V             16.7 Ohms      690mA       7.91 Watts
7)   19.99V           10.12V             14.28 Ohms     708mA       7.16 Watts
 

Just to be consistent, I measured the open circuit (without resistor)·every time before the closed circuit (with resistor)... For each test I added a 100 Ohm resistor to place in parallel gradually increasing the load.· You will notice a peak Wattage near 14.54V· ... I could fine tune this further and find the exact peak value but·for this explanation, the ball park figure is adequate (The·peak power for the panels is rated at about 15V anyway, so this is close enough)

To determine the internal resistance, I use the peak power value I found....

                       ?R      R    
Solar Panel (+) >-----/\/\----/\/\-----< (-) Solar Panel

If we know R, in this case 25 Ohms for the peak power, we can solve for the internal·resistance of the panel....

                       ?R    25 Ohm    
Solar Panel (+) >-----/\/\----/\/\-----< (-) Solar Panel
                      5.49V   14.54V

I = V / R

I = 14.54V / 25 Ohm
I = 581mA

Now assuming that the Solar panels are still actually producing 20.03 Volts, we can determine that 5.49V are being dropped across the internal resistance (20.03V - 14.54V = 5.49V) , and since we know the current, we can determine what the internal resistance is.

?R = V / I

?R = 5.49V / 581mA
?R = 9.45 Ohms

So the internal resistance of my solar cell under·peak power is about 9.45 Ohms.· Now, as I said, this value changes depending on the load and consider also, that I took these measurements late in the day when the sun was pretty low so maybe I was getting 9 Watts out of my 15 Watt panels.


To further complicate things, the·batteries that you are charging ALSO have an internal resistance which by the way can be·measured in a similar manor as described above for the Solar panels.
·


▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Beau Schwabe

IC Layout Engineer
Parallax, Inc.

Post Edited (Beau Schwabe (Parallax)) : 11/3/2008 12:07:32 AM GMT

Dave E

Thanks again Beau,

I did not know that the internal resistance would change in the panel.

I will give this a bit more thought.



Dave E

sam_sam_sam

Beau Schwabe

·Thank You for sharing this info on internal resistance effect is dynamic and proportional to the load put on the cells

I had not though of this I just bought a 15 watt solar panel my self from Northern Tools

Are all Solar Panel Controlers the same or is there a real good one that you can buy that will not cost an arm and a leg

Please take a look a this charger and tell me what you think of it

here is the link http://www.atlantasolar.com/product_info.php/cPath/70_190/products_id/1379

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
··Thanks for any··that you may have and all of your time finding them

·
·
·
·
Sam

Post Edited (sam_sam_sam) : 11/3/2008 1:07:04 AM GMT

Beau Schwabe

sam_sam_sam,
·
I'm not familiar with the charger you mention.· The charger(s) that I have been using (actually three of them) can be located in the link below.· These accommodate a total of·24 x15Watt panels (360 Watts).· Each charge controller·is connected to 8 panels·...
·
http://cgi.ebay.com/Solar-Panel-Regulator-Charge-Controller-12V-10A-120W-ca_W0QQitemZ220303013821QQcmdZViewItem?hash=item220303013821&_trkparms=72%3A1205%7C39%3A1%7C66%3A2%7C65%3A12%7C240%3A1318&_trksid=p3286.c0.m14
·
...I'm only using these for charge regulation purposes and they seem to be very robust.· After opening one up to "peek under the hood" they appear to be very well designed.· The direct output off of the batteries however I am handling in a different way through a custom controller of my own design.


▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Beau Schwabe

IC Layout Engineer
Parallax, Inc.

sam_sam_sam

Beau Schwabe

Thank You for the info i will order one soon

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
··Thanks for any··that you may have and all of your time finding them

·
·
·
·
Sam