Measuring the voltage of a solar panel, while it still connected to the charging circut

[LBNote]

I'd like some ideas please on the best way to determine the output voltage
of a solar panel while it is still connected to the battery / charging
circuit. What I plan to do is log the voltages of both the solar panel and
the battery while it charges over the period of a few days. I have the
circuit for the voltage measuring just not sure how to get an accurate
reading from the solar panel without disconnecting it from the charger.

Thanks.

 

[Rod Speed]

I'd like some ideas please on the best way to determine the output voltage
of a solar panel while it is still connected to the battery / charging
circuit. What I plan to do is log the voltages of both the solar panel and
the battery while it charges over the period of a few days. I have the
circuit for the voltage measuring just not sure how to get an accurate
reading from the solar panel without disconnecting it from the charger.

Depends on what you mean by 'accurate'

Clearly if you mean the voltage it delivers to the charging
system, all you need to do is measure it accurately.

The open circuit voltage is rather irrelevant, but you could measure
that by having a relay open the circuit for the measurement.

 

[happyhobit]

I'd like some ideas please on the best way to determine the output voltage
of a solar panel while it is still connected to the battery / charging
circuit. What I plan to do is log the voltages of both the solar panel and
the battery while it charges over the period of a few days. I have the
circuit for the voltage measuring just not sure how to get an accurate
reading from the solar panel without disconnecting it from the charger.

Thanks.

Pardon me for saying this but voltage is only half the picture. You might do
better if you measure the Solar Cell power output and the battery voltage.
Power is the product of the voltage and the current.

As a battery charges, the load drops, the charging voltage increase and the
current drops. The're all related. You really can't separate the panel
voltage and the battery voltage without disconnecting it.

Jay

 

[LBNote]

From what you have said then I think I do mean the voltage delivered to the
charging circuit,
But if the charging circuit has a load connected to it wont that mean the
true voltage into the charging circuit will be skewed.

Let me put it another way what I want to do is find out how effective the
positioning of my solar panels are over the course of a day by measuring the
output voltage from them while they are still connected to a charging
circuit ?

 

[Ralph]

Pretty hard to "find out how effective the positioning of my solar panels
are over the course of a day" by measuring the voltage.
How do you allow for clouds in your calculations?
If you want to measure the voltage, I would remove the battery/charging
circuit and replace them with a constant load so that you have removed
another variable. ie the state of the battery.

 

[LBNote]

OK, OK

Sorry if it was a stupid newbie question.

Just after an explanation in the interests of knowing for next time

thats all

 

[Rod Speed]

From what you have said then I think I do mean
the voltage delivered to the charging circuit,

But if the charging circuit has a load connected to it wont that
mean the true voltage into the charging circuit will be skewed.

Nope, the only thing you care about is the voltage it
delivers to the load presented by the charging system
and maybe the current it delivers at the same time.

Thats all that matters when it comes to the crunch,
the watts it is delivering to the charging system.

Let me put it another way what I want to do is find out how effective the
positioning of my solar panels are over the course of a day by measuring the
output voltage from them while they are still connected to a charging circuit ?

Thats more complicated and much harder to log.

Basically because the voltage will vary with basics
like cloud and the state of charge of the batterys.

You'd really have to switch in a constant load
and measure what the panel can deliver to that, to
eliminate the different charge state of the batterys.

And that will only really tell you much about the clouds seen.

I guess you could get some useful info about how well the
panel is positioned by using the highest voltage seen with
a constant load averaged over multiple days. Basically
looking for values with no cloud. In some locations that
isnt going to be trivial to achieve and logging will mostly
just provide info on the cloud question.

 

[LBNote]

Thanks for you answer,

I think I'm starting to build a picture now. I am fairly new to this so I
appreciate your verbose answers:

If I simplify my problem even further by just wanting to know if there is
enough sun around to warrant the panel being connecting to the charging
circuit ?, given that I do know the specs on the battery being charged

Lorne...

 

[mikegw]

From what you have said then I think I do mean the voltage delivered to the
charging circuit,
But if the charging circuit has a load connected to it wont that mean the
true voltage into the charging circuit will be skewed.

Let me put it another way what I want to do is find out how effective the
positioning of my solar panels are over the course of a day by measuring the
output voltage from them while they are still connected to a charging
circuit ?

A few quick questions.....

What type of installation are we talking here? Do you plan on moving the
panels for different times in the year Summer/Winter? Is your load going to
vary across the year/day? Where are your batteries? What is your load?
What type of charging circuit do you have? Where are you (latts and longs
would be best)?

The easiest way to position a panel ( not the best just the easiest) is to
tilt it in such a way that its shadow is largest at midday when cast onto a
level surface. Of course this depends on the seasonal changes in the sun
height.

Mike

 

[LBNote]

It's only a small hobby installation, and I'm only fairly new to this
application person

With a 20 Watt Panel, Old Car Battery

With the load being a couple of UHF Radios

I don't plan on moving the panels at all, just want to find the best setup

I want to build my own charging circuit, with low power cut off, safe and
intelligent charging controlled by a pic chip

Regards

 

[Rod Speed]

It's only a small hobby installation, and I'm only fairly new to this
application person

With a 20 Watt Panel, Old Car Battery

With the load being a couple of UHF Radios

I don't plan on moving the panels at all, just want to find the best setup

In that case you dont really need to measure anything,
just calculate the best angles for the panel.

Thats not as easy as it first looks tho because you may want to
optimise the angle elevation particularly for winter unless the use
of the radios means that they dont get used as much in winter etc.

I want to build my own charging circuit, with low power cut
off, safe and intelligent charging controlled by a pic chip

Sure but thats a separate issue to positioning.

Positioning can be calculated, no need to
measure. For some strange reason the
position of the sun is completely calculatable.

 

[Eugene Rosenzweig]

If all you want to do is to monitor the light conditions by voltage output
of the light panel, it is relatively straightforward. You want the O/C
voltage of the light panel so you need to disconnect the light panel from
the charger. Since you are using a pic, you can have the pic disconnect the
panel from the charger using a relay, measure the O/C voltage, reconnect the
panel. This would take some milliseconds (as long as needed to have the
relay respond properly). This could be repeated once a
minute/half-hour/hour. Pretty good idea, I think. What method would you be
using to measure the voltage with a pic?

However if you want a low-power cutoff, as the others have mentioned, a
little more work is needed.

Eugene.

 

[Brian Goldsmith]

Pardon me for saying this but voltage is only half the picture. You might do
better if you measure the Solar Cell power output and the battery voltage.
Power is the product of the voltage and the current.

As a battery charges, the load drops, the charging voltage increase and the
current drops. The're all related. You really can't separate the panel
voltage and the battery voltage without disconnecting it.

Jay

I'd say that current is more than half the picture. The illumination of the
panel is proportional to the current.



**** Huh?????I would have thought that the current available is,amongst
other parameters, proportional to the illumination!!!!!!

Brian
Goldsmith.

 

[Phil Allison]

might
I'd say that current is more than half the picture. The illumination of the
panel is proportional to the current.



**** Huh?????I would have thought that the current available is,amongst
other parameters, proportional to the illumination!!!!!!

Brian
Goldsmith.

** Some engineurs have trouble picking a chicken from an eggs.



............. Phil

 

[mikegw]

It's only a small hobby installation, and I'm only fairly new to this
application person

With a 20 Watt Panel, Old Car Battery

Buy a hydgrometer to test the state of charge in your battery. This is
probably the best indicator of *SYSTEM* preformance.

You will get far better results if you get a better battery. I would
recommend a deep cycle marine battery. The marine bit is not important,
that just what they are called. However, the deep cycle bit is. A deep
cycle battery is designed to be discharged to a low state of charge. A car
battery is just for starting the motor so is not so great for this
application. But if this isn't a mission critical thing begining with a car
battery is a good enough option. For best results keep the battery off the
ground. Resting on a few bits of wood will help.

With the load being a couple of UHF Radios

Radio's I guess will be a hard load to workout they use different levels of
power in transmit or recieve.

I don't plan on moving the panels at all, just want to find the best setup

Good place to start is get your lattitude and add 23degrees. Make this your
angle of tilt relative to the ground. Face the module north (I'm assuming
southern hemisphere) . This way at midwinter noon your module will face the
sun directly.

Take a look at http://solar.anu.edu.au/Sun/help/PVguide.html and the link at
the bottom of that page http://solar.anu.edu.au/Sun/PVPanel/PVPanel.html

I want to build my own charging circuit, with low power cut off, safe and
intelligent charging controlled by a pic chip

The adavtage of wet lead acid batteries is you can treat them fairly rough.
You probably could get away without a charging circuit all together just a
series diode to make sure the battery dosen't discharge through the module
overnight( especially if you are using an old car battery). Although a
high/low voltage disconnect will be all you will need as there is no chance
a 20W module could overcurrent a car battery

Mike

 

[LBNote]

Hi,

My plan is to use a divide by 4 voltage divider and one channel of the ADC
converter of a pic 12F675.
This measuring part I have working now with acceptable results being
displayed on an LCD.

From the outset (gauged by the answers I've received here ) I can see I have
taken too lightly the science involved in what I am trying to achieve here.
Thinking that I could just measure the voltage on the solar panel and make a
decision on weather they were still required to be connected to the charger.
Oh well I had to ask and find a reason why not. Thanks everyone by they way.

With that in mind what other factors are involved in the low voltage cut out
section of my project ?

 

[Adrian Jansen]

No it wasnt a stupid question, even if the answer(s) wasnt what you wanted.

To give you some more background, a solar panel behaves roughly like a
constant voltage source in series with a variable resistor, whose resistance
is controlled by the illumination. So any light at all produces a voltage,
depending on the number of cells in the panel, sufficient to supply some
current to the load ( your battery ). The more light, the higher the
current, not because the voltage goes up, but because the internal
resistance of the panel goes down.

So measuring the voltage of the panel, connected to the battery or not, is
of little use, but measuring the current delivered to the battery is the way
to determine the system performance.

--
Regards,

Adrian Jansen
J & K MicroSystems
Microcomputer solutions for industrial control

 

[Alan Peake]

It's only a small hobby installation, and I'm only fairly new to this

application person

With a 20 Watt Panel, Old Car Battery

With the load being a couple of UHF Radios

I don't plan on moving the panels at all, just want to find the best setup

I want to build my own charging circuit, with low power cut off, safe and
intelligent charging controlled by a pic chip

Regards

A fairly simple way is to put a shunt resistor in series with the panel and
just log the current going in to the battery. what you need going into the
battery is Amp-Hours. If you don't run the radios too often, a 20W panel might
be OK - I have a 53W panel for the same purpose and use the CB for an hour
or so per day as well as about 4-5 hours of running a 15W CFL light.
Alan