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

Sunlight is inconsistent and often nonexistent (think nightime) 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, measure power flow into 2 different types of batteries and how to estimate how long it will take to charge a battery.
As a simple example we made a simple circuit that powers a tone generator. When you remove power by shading the panel, the sound quickly dies out. If you add a capacitor (an component that stores electric charge) to the circuit, removing power doesn’t stop the sound. Larger capacitors with more reserve store even more power and keep the tone more or less constant even when power is removed.
Watch the short video:

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.
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:
a. measure the amount of power flowing into out batteries

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.

Measure the voltage at the panel (not connected).
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, 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’s 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 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 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 at 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 at 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:
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.

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 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.

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 2.73W, 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.

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.

Part 2 – Power Generation

Part 4 – Protecting the Battery

About The Author

22 Responses

  1. CK

    Hi! Thanks for giving me a lot of clue on how solar panel works. And I have a question here. You are connecting the battery in parallel to the solar panel right? What if I connect it in series?

    • admin

      The panel and the battery are connected in parallel, but since it is only two entities – a power source (the panel) and a load (the battery), there is really only one way to wire them together. If you start to add multiple solar panels, then you can connect the panels together in either parallel or series.

      • Karolina

        I bought 6 6 Multi-Cryatalline solar cells that are sppoused to be 4 watts, .5 volts and 8 amps. But when I measure the amps using multimeter the highest I ever get for amps is 3.5, even when sun is highest in the sky. Is this normal to get only half amps when measuring solar cells with meter or are these no good and I should return them? Anyone know? P.S. these are not broken or chipped.

  2. anjan

    Can I connect the solar panel directly to a voltmeter to measure the voltage without any batteries in between. Also what is a breadboard and do you sell one

  3. Justin

    Hi, This tutorial is great but I had a really hard time understanding where to put the Multimeter probes to get the different voltage readings and current readings. Could you give much higher resolution images so I can clearly see the connections and placement of the multimeter probes. Or what would be really great is a step but step video.

    Thank you for creating this.

  4. Yvonne chuah

    Hello,May I know how big the solar panel is?Thanks for kindly reply.

  5. mike abernathy

    thank you very much for these truly outstanding tutorials!

    I am working on a solar recharger for a glider and this really helps.

  6. Christos

    I was searching two days over the internet for the info you have at part one (about the shadow’s and the rest for current output)
    Your tutorial is great and you are amazing for sharing the info!!!!

    Thank you

  7. Raj

    Thanks for the amazing diy solar charger.
    I was wondering why li-ion batteries should not be used in this simple charger.
    Is it that nimh batteries are more safe than the li-ion chemistry ?

    • Voltaic Systems

      NiMh are a bit safer, but we were trying to show a variety of charging scenarios through the tutorials. Most of our products use LiIon / Lipos. One big downside of NiMH is that they charge less efficiently than Lipos and take more energy to store each Watt hour of power.

  8. Andrea

    Hi Philippines,
    Do you think a 5v panel will be enought th charge the nimh pack? Thanks in advance

  9. Andrea

    Hi Philip,
    Do you think a 5v panel will be enought th charge the nimh pack? Thanks in advance


Leave a Reply

Your email address will not be published.