5
$\begingroup$

recently I bought 3 solar panels rated at 5V 200 mA each. I want to use them to charge a 5V battery bank to charge a phone. Thinking about the proper way to put them, I thought i can connect all in parallel to get maximum current, but realized that if the sun light was a little weak it will no generate full 5v thus preventing charging. So I decided to put 2 in parallel to give the equivalent of one 5V solar panel, connected in series with the 3rd panel to give the equivalent of 10v. sacrificing a little current to get higher voltage, to allow the charging to happen on a wider range of sun light power. The following picture shows the wiring and the schematic I intend to replicate.

enter image description here

enter image description here

Now that I have an equivalent of 10v, 400mA solar panel. I used a 7805 voltage regulator to cut down the excess to 5v.

enter image description here

Final step, I added a standard diode to prevent the panels from leaking the battery in the shade.

and now measuring:

enter image description here

Questions:

  1. I didn't think about this before putting the diode, but is it okay to put the blocking diode on the ground wire? because I know some applications do not use the ground except for safety (i.e. 3 phase system). it would be helpful also to avoid the 0.7v drop across the diode before the regulator.
  2. according to previous calculations, I'm supposed to get a maximum of 10v output before regulation, and considering that the sun was pretty shinny today, why was the reading I got not more then 6v? I have measured across each panel seperatly and got around 5.5V, are the connections right?
$\endgroup$

3 Answers 3

4
$\begingroup$

Putting a single panel in series with two other panels that are in parallel does not accomplish what you think it does. The overall current of such a setup is limited by the single panel to 200 mA, so the three panels will not produce any more power than you'd get by just putting two panels in series.

A single solar cell can be thought of as a current source in parallel with a silicon diode. The current source is driven by the incoming light. The diode "shorts out" the current source, which is why the voltage across a single cell can never be more than about 0.65 V, the forward drop of a silicon diode. A 5V panel is approximately 10 such cells connected in series. The current through all of the cells will be limited by the cell that is receiving the least amount of light.

Also, the blocking diode in your diagram is pointing the wrong way.

If your panels are rated at 5V, and your "battery bank" requires 5V to charge, then you don't need to do anything more than put all three panels in parallel and hook them directly to the battery. Forget about blocking diodes.

$\endgroup$
2
  • $\begingroup$ Very useful information. Now it makes sense why the circuit is so much power inefficient. I will take your advice to connect in parallel, however I want to keep the blocking diode to prevent the panels from draining the battery when the voltage drops, (say I forget the charger in the shade...). $\endgroup$
    – Nadim
    Jan 25, 2016 at 23:29
  • $\begingroup$ for the diode direction I thought the symbol (arrow) should be pointing along the flow of current, i. e. positive terminal of the source along negative terminal orientation. right? and finally, each panel is measures at approx 5.3v. what's the worst and best case scenarios if I keep the regulator? $\endgroup$
    – Nadim
    Jan 25, 2016 at 23:36
2
$\begingroup$

That's a mess, and your labels about what is + and what is - out of each panel seem inconsistant.

For the most effective use of the panels, wire them all in series. That will put out around 15 V under full sun. Now use a buck converter to make a regulated 5 V from that.

There are many buck converter chips available off the shelf at these low voltages. Finding one with a built-in switch and synchronous rectification shouldn't be too hard. You only have to supply the inductor, a few caps, and maybe a charge pump diode depending on the chip you chose.

The other advantage of a switching power supply is that you probably won't need heat sinking. The resulting overall circuit will be smaller, cheaper, and give better performance than just throwing a 7805 regulator at it. At 10 V in and 5 V 1 A out, the 7805 will dissipate 5 W. That's way beyond what it can do without a heat sink.

$\endgroup$
0
$\begingroup$

A panel "rated" at 5V 200mA will normally put out 5V 200mA at it's maximum power point, in full sunlight.

In lower light, it will put out less power.

But the voltage depends on the battery, not on the light. When you have more light, you have more current, and the battery charges faster.

When you have less light, you have less current, and the battery charges slower.

If the battery voltage is higher, the voltage on the panel is above the maximum power point, and current is lost inside the panel, and battery charging is slower.

If the battery voltage is lower, the voltage on the panel is below the maximum power point, and voltage is lost inside the panel, but there is no more charging current than at the maximum power point.

If the panel is matched to the battery, this is a "self regulating" system, as the battery charges, it's voltage goes up above the maximum power point, and charging slows.

When you are charging a lead-acid battery, a self-regulating system is OK. After you overcharge, you just add water. With other kinds of cell, going over-voltage will permanently damage the cell, and self-regulating panels are not used.

When there is low light, it does not mean that the panel voltage goes down. It means that the charging current goes down.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.