How (and why) do I store power?
- 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
b. calculate the power capacity of our battery pack
1. Measure Power into NiMh batteries – Connect solar panel to pack of 4 NiMh AA2 and measure the Voltage and current at each step (where applicable). WARNING: DO NOT use Li-Ion batteries for this activity.
| Description | Voltage (V) | Current (A) | Power (W) |
| At panel – Unconnected | 6.44 | – | – |
| At battery – Unconnected | 5.12 | – | – |
| At panel – Connected | 5.79 | .19 | 1.1 |
| At battery – Connected | 5.22 | .19 | .99 |
| Drop of diode | 0.53 | .19 | . |
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: charge capacity and power capacity. 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 4 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 4 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%, meaning 35% of the power put into them is lost as heat. We much 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.
- using more efficient batteries (i.e. lithium-ion)
- using more efficient charge circuitry
- make the panels more efficient
Charge Smart Battery Packs – Our V11 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 x 2-Watt panel) and measure the voltage and current to calculate the power.
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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…
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With the panels cooled down, the output to the V11 increased to 2.73W, a 20% 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:
| Description | Voltage(V) | Current(A) | Power(W) |
| 4W Panel Setup to V11 | 4.54 | .5 | 2.27 |
| 4W Panel Setup to V11 (after ice bath) | 4.71 | .58 | 2.73 |
Calculate Charge Time: The V11 has a rated power capacity of 11Wh. 2.27W was being transferred from the panels to the V11, and assuming that 75% 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 a look inside the circuitry that is designed to protect the battery.
Solar Charger Tutorial – Part 1
How do Solar Chargers work?
A solar charger is a portable power system made up of a solar panel and an external battery pack. For Voltaic Systems, we pair one of our high performance, monocrystalline solar panels with a solar optimized lihtium ion battery pack. When paired together these systems use solar energy to charge your electronics anywhere you need power. So how do these systems work?
How do Solar Charger Work? The Short Answer…
When sunlight hits solar panels, the solar cells generate electricity. This electricity flows into a lithium ion battery pack with stores and regulates power to your devices when plugged in.
For a complete look, you can see our entire collection of solar chargershere.
How do Solar Chargers Work? The Long Answer…
We’ve created a four 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 Amazon and you can get a pretty good idea of how 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 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), 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:
- Multimeter
- Solar Panel – we use our 2 Watt 6 Volt solar panel that uses Monocrystalline cells, but you can use any panel you have lying around with any type of cells
- Sunlight – alternatively, you could use a couple high-powered incandescent bulbs but then you don’t get to spend the afternoon outside
1. 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.
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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.
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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.
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We measured got 0.33 Amps or 330 mAmps which is close to our specification of 333mA.
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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:
| Position | Voltage (V) | Current (A) |
| Directly Facing Sun | 6.82 |
0.33 |
| Angled 15 degrees | 6.81 | 0.32 |
| Angled 30 degrees | 6.78 | 0.32 |
| Angled 45 degrees | 6.73 | 0.29 |
| Angled 90 degrees | 6.07 | .06 |
| Angled 180 degrees | 5.89 | 0.03 |
| Finger on Corner (half of cell) | 6.79 | .2 |
| Fingers on Cell (full cell) | 6.74 | .04 |
| Faint Shadow | 6.78 | 0.25 |
| Close Shadows on Panel | 5.78 | .03 |
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Thumb covering half a cell
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Thumb covering whole cell
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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%, but a heavy shadow over all or part of the panel drop panel output by 90%.
Move on to Part 4 of our Tutorial – How do charge circuits protect batteries? In this tutorial we explain how built-in circuits protect both our batteries and your devices.
Review Part 3 of our Tutorial – How (and why) do I store power? In this tutorial we explain how to store solar energy in batteries for use when there is no sunlight available.
Review Part 2 of our Tutorial – How do I measure total output? Connect solar panels to loads and measure how much power is being generated.
Review Part 1 of our Tutorial – How do solar chargers work? Understanding the basics of generating solar electricity and how to measure Voltage and Current in different conditions.
This post was updated from its original post in September 2011.
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