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Jeff Crystal

Voltaic - COO

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How Do Solar Chargers Work?

h5. Introduction

Solar chargers use solar energy to power my electronics. Neat! How do they work?

  • Short answer: Sunlight hits solar panels -> solar panels generate electricity -> electricity flows into a battery -> battery outputs clean power on demand to your device
  • Long answer: We’ve created a five-part tutorial to take you through every stage of the process. Solar is obviously much less predictable than plugging into the grid, so we’ll be focusing both on specifications and what to expect in the real world. Bring along a multimeter and some parts from Radio Shack and you can get a pretty good idea of exactly how solar charger work.

  • Tutorial 1: How do I measure Open Circuit Voltage and Short Circuit Current? (below)
  • Tutorial 2: How do I measure total power output?
  • Tutorial 3: How (and why) do I store power?
  • Tutorial 4: How do charge circuits protect the battery?
  • Tutorial 5: How do I use stored power to charge a device?
Tutorial 1: How do I measure Open Circuit Voltage and Short Circuit Current?

There are lots of great resources on how solar panels generate electricity, including Wikipedia. So we’re going to focus here on measuring the Open Circuit Voltage and Short Circuit Current of a solar panel in “perfect” and less-than-perfect conditions.

Every solar panel has a rated output that includes its Open Circuit Voltage (Voc), Peak Voltage (Vmp), Short Circuit Current (Isc), and Peak Current (Imp). The Peak Voltage and Short Circuit Current tell you the Voltage and Current of the panel before you connect it to anything (e.g., there is no load attached to the panel).

As a reminder, Voltage is represented by the symbol V for Volts and is a measure of the difference in electric potential energy between two points. Like air pressure, it flows from high to low. Current is a measure of the flow of charge through an area over time. We use the symbol I to stand for current and measure it in Amps, or simply A for short.

Let’s measure the output of a solar panel. You’ll need the following:

  • Multimeter
  • Solar Panel; we use our 2 Watt 6 Volt solar panel that uses Monocristalline cells, but you can use any panel you have lying around with any type of cells
  • Sunlight; alternatively, you could use a couple of high-powered incandescent bulbs, but then you don’t get to spend the afternoon outside

Measure Open Circuit Voltage

The black lead should be connected to COM and the red lead should be connected to V or VDC. Set the dial to 20, which means the Multimeter can measure up to 20 Volts.

Touch and hold the black lead to the “sleeve” of the solar panel connector or the black wire. Now touch and hold the red lead to the red wire or insert it into the “tip” of the solar panel connector.

You’ll notice that the Voltage moves around, but with the panel pointed at the sun, we saw between 6.89 and 6.98 Volts for Open Circuit Voltage. This is close to our specification of 7.0V Open Circuit Voltage on the 2 Watt panel.

2. Measure Short Circuit Current

The black lead should be connected to COM and the red lead should be connected to the mA. Set the dial to an amount greater than what you expect the current to be. In our case, we set it to 10.

We measured and got 0.33 Amps or 330 mAmps, which is close to our specification of 333mA.

3. Assess the impact of real-world conditions.

In the real world, it is not sunny all the time and our panels are not always pointed directly at the sun. So what happens when we move away from perfection?

Angle the panel so that it is facing the sun and record the voltage. Try slowly angling the panel away from the sun and note the changes in Voltage and current. Try shading parts of the panel and then the whole panel and note the changes in Voltage and current.

Here is what we recorded:

Thumb covering half a cell

Thumb covering half a cell

Thumb covering whole cell

Thumb covering whole cell

Solar panel with heavy shade

Solar panel with heavy shade

As you can see, minor changes in angle don’t have a very significant impact on Voltage or current. However, once you get to about 45 degrees away from the sun, current starts to drop very sharply, meaning total power will also drop.

Similarly, light shadows on the panels decrease current by about 25 percent, but a heavy shadow over all or part of the panel drop panel output by 90 percent.

Move on to Part 2 of our Tutorial—How do I measure total output? In this tutorial we start connecting solar panels to loads and measuring how much power it is capable of generating.

Part 2: How much power am I generating?

A solar panel does not do anything in isolation. It needs to be connected to something. That something is called a load. A load dissipates power and converts electrical energy to work.

We’re going to make a simple circuit where we connect a solar panel to a resistor. The resistance changes the behavior of the panel. The more resistance, the higher the Voltage but the lower the current. The lower the resistance, the lower the Voltage and higher the current. What we’re looking for is the point where the panel produces the most power.

In this tutorial, you’ll need the following:

  • 2 solar panels
  • multimeter
  • breadboard
  • a range of resistors
  • Voltaic circuit box (optional)

1. Measure Voltage and current through multiple resistors:

If you haven’t already, strip the leads on your panel and connect them directly into the breadboard as shown below. Re-measure both the open circuit Voltage and short circuit current.

Then, connect a resistor to create a simple circuit and measure both the Voltage across the resistor and current through the resistor.

Solar panel connected to breadboard

Solar panel connected to breadboard

Measure open circuit voltage

Measure open circuit voltage

Measure Voltage across resistor

Measure Voltage across resistor

Measure current through system

Measure current through system

2. Repeat using a variety of resistors; create a table like below (we used eight different resistors)

3. Graph it!

If you graph the Voltage and current on a scatter plot, it will look something like this:

The peak power output of the panel is at the “elbow” of the curve and looks to occur at a bit less than 5V and 0.31 Amps. At this point in time, we’re getting about 1.5 Watts out of our 2 Watt rated panel.

Wait, does that mean the specs are wrong? No, it just means that there are factors that decreased the output on this particular day. There were two major issues:

  1. It was a 90 degrees (Farenheit)—solar panels get less efficient at high temperatures. In Tutorial 3, we’ll show you what happens when you submerge panels in an ice bath before measuring power output
  2. It was a slightly hazy New York City day, meaning we weren’t getting all the sunpower that we would have received in a less humid environment.

4. Increase the total power

If you want more power, the way to do it with solar power is to increase the total area. You have two general options with solar panels, you can either put them in parallel or series. Voltaic makes a number of circuit boxes that simplify this process, but you can also do this by hand in the breadboard.

In our case, we’re going to use our 2 panel 6V/12V circuit box like this:

2 panel circuit box

2 panel circuit box

Measure the Voltage and current in both parallel and series.

2 panels in parallel - Voltage

2 panels in parallel – Voltage

2 Panels in Parallel - Current

2 Panels in Parallel – Current

2 panels in Series - Voltage

2 panels in Series – Voltage

2 panels in series - Current

2 panels in series – Current

You should see that when you put two panels in parallel, the Voltage stays the same and the current doubles. When you put the panels in series, the Voltage doubles and the current stays the same. Overall, you get twice the power.

Note: the one-half Volt drop in Voltage is from the diode in the circuit box. Extra credit if you spotted that.

Keep on going to part 3 of the tutorial, where we connect panels up to different batteries to measure power flow and estimate charge times.

Part 3: How (and why) do I store power?

Sunlight is inconsistent and often nonexistent (think nighttime), so storing power is important in many applications. In addition, many electronics are expecting a constant, specific Voltage with an ample power reserve. In this tutorial, we are going to show why you need power storage, how to measure power flow into two different types of batteries, and how to estimate how long it will take to charge a battery.

In the case of a solar charger, a battery stores power for when you need to charge your device. While it is possible to charge a phone directly from a solar panel, there isn’t always sun when your device runs out of power. However, the following tutorial and math applies whether you’re charging a device directly or an intermediate battery.

Here’s what you’ll need:

  • a solar panel—6 Volts or higher (we use our 2 Watt solar panel)
  • solder-less breadboard
  • a few different NiMH or other battery packs
  • diode (Schottky or other rectifier)
  • multimeter
    How long with it take to charge my batteries? It seems like a simple question, and we’ll show you how to answer it here. To do so we need to do the following:

a. measure the amount of power flowing into our batteries, and
b. calculate the power capacity of our battery pack.

1. Measure Power into NiMh batteries

Connect solar panel to pack of four NiMh AA2 and measure the Voltage and current at each step (where applicable). WARNING: DO NOT use Li-Ion batteries for this activity.

Measure the voltage of the battery pack

Measure the voltage of the battery pack

Connect a diode to the red wire of the solar panel in series. The diode will have a black marker (if glass) or a white marker (if plastic); this is the cathode or negative (-) terminal, and the other side is the anode or positive () terminal. The anode () should be connected to the positive wire of the solar panel. We use the diode to prevent the batteries from draining into the solar panels when there is not enough sunlight.

Note: Matching the voltage of the panel to the voltage of your battery pack is critical. If the voltage from the panel is between 2 and 3 volts more than the battery pack voltage, then the pack will charge. If the voltage is significantly higher, you may damage your batteries. If it is equal to or less than the batteries, they will not charge.

Connect the battery pack to the panel and diode. The negative wire from the battery pack connects to the negative wire of the solar panel and the positive wire of the battery connects to the diode (the end opposite the positive wire of the battery).

Measure the voltage of the panel with the battery load connected.

Measure the voltage drop across the diode: connect red lead to the cathode (+) and the black lead to the anode (-).

Measure the voltage of the battery pack with the solar panel and diode connected.

Measure the current flowing into the battery pack: adjust your multimeter to measure current (10A), and connect it in series with the battery pack (red lead connects to cathode (-) of the diode and the black lead connects to the positive (+) wire of the battery pack). This current measurement will be the same whether you measure it before or after the diode, solar panel, or battery pack. We will use this to determine how much power is being transferred to our battery, and how much is lost in the circuit through the diode.

That’s it! Now here are the results that we got:

2. Calculate Capacity

We just measured how much power is flowing from the solar panels to the battery. The next ingredient we need to determine charge time is capacity of the battery pack.

There are two types of capacity that we should be aware of. Batteries have a rating that tells us the charge capacity or how much electric charge they can store: the ampere-hour (Ah) or milliamp-hour (mAh) [note: 1000 mAh = 1 Ah]. However, it’s much easier to think about the power capacity or watt-hours. Power capacity can be calculated by multiplying the charge capacity of a cell by the voltage of the cell: amp-hours * volts = watt-hours or A * V = Wh (also: mA * V = mWh). We’re using 1.2V AA cells with a rate charge capacity of 2,700mAh. The power capacity for each of our 1.2V cells with 2,700mA would then be 3.24Wh (1.2V * 2,700mAh = 3,240mWh = 3.24Wh).

To find the total power capacity of our battery pack with four AA batteries, we simply multiply the watt-hour rating of one cell by the total number of cells. It doesn’t really matter whether we have all four in series, parallel, or two parallel sets of two in cells in series; our four AA batteries in series has a total power capacity of roughly 13Wh.

Now can we calculate how long it’ll take to charge? Yes!

Looking at our data we can finally estimate the time it will take to charge: divide the total power capacity by the amount of power flowing into the cells. But there’s a catch! The average efficiency of NiMH batteries is 65 percent, meaning 35 percent of the power put into them is lost as heat. We must multiply the power being put into the cells by .65 (efficiency coefficient) to get a “real world” estimate of charge time for our battery pack, which looks like this:

.99W from our panel is flowing into the battery pack (yes, this factors in a .1W loss from the diode!)

.64W (.99W * .65 efficiency coefficient) is being stored by the battery.

If our batteries were being charged from a completely discharged state, we have the whole 12Wh capacity to charge.

12Wh / .64W = 20.25 hours

Assuming that the power transferred into the batteries would double if we added another panel in parallel with the first (a total of 4W), we can get that charge time down to about 10 hours.

This scenario perfectly illustrates why it is necessary to get the most out of your panels. We can do this in at least two ways:

  1. using more efficient batteries (i.e., lithium-ion)
  2. using more efficient charge circuitry
  3. make the panels more efficient

Charge Smart Battery Packs

Our V11 USB Battery is a smart battery pack with electronics designed to optimize solar power to charge lithium-ion cells. This energy is provided at a regulated USB 5V standard up to 650mA. Additional electronics also offer protection features: thermal protection, short circuit protection, overcharge protection, and over discharge protection.

Connect the V11 to a 4W (2 × 2-Watt panel) and measure the voltage and current to calculate the power.

What?! Only 2.27 Watts out of 4 Watts of panels? It’s still a little better than the estimated power transfer of only 2 watts directly charging the NiMH battery pack. Those panels have been hard at work all afternoon; let’s cool them down with a nice ice bath and see if that helps.

With the panels cooled down, the output to the V11 increased to 2.73, a 20 percent boost! This is because solar cells are more efficient at a lower temperature and it was a very hot day out there.

Let’s see that in a table:

Calculate charge times: The V11 has a rated power capacity of 11Wh. 2.27W was being transferred from the panels to the V11, and assuming that 75 percent of the power from the panels (under normal conditions, not iced) is stored in the battery, we can safely assume that 1.7W is used. The charge time for a completely drained battery would then be 11Wh divided by 1.7W or about 6.5 hours. This is consistent with our field tests.

Coming up in part 4 is a look inside the circuitry that is designed to protect the battery.

Tags: solar, energy, electronics,

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