Solar Charging Batteries

Energy Farmer
Ever wanted to be a farmer of energy? Imagine if you could grow your own energy? Or be like one of those free-energy dreamers that believe we can harness energy from spinning magnets.

Keep in mind that the Law of Conservation of Energy says that energy can be neither created nor destroyed. What we can do is harness the energy and store it in a battery or whatever then we can do stuff with this renewable energy source. OooOooOoo. However, different applications require different types of materials, different designs, and different products which have their own requirements as well. So we'll have to focus on one main issue and go from there.

Solar Charging Battery
Let's keep it simple. We want to collect energy from the sun, store it in a battery, and use our cell phones to charge off from the battery. Easy enough. Three things that are essential: solar panels, battery charger, and battery. I already did some research and found a solar charging battery from Sparkfun! The SY-007 gets 0.6Watts optimal with full direct sunlight and has a 3500mAh battery. We can definitely get 1.5-2 full phone charges off of these babies. But wait... 0.6Watts? 3500mAh? What does this mean?!

Unit of power and battery capacity unit of measurements. Just think of running a load at 3.5Amps for 1 hour or 1Amp for 3.5hrs. Although the battery circuit was made to max discharge at 1Amp. We can spend a half a day talking about batteries if you want. It's fun. Maybe we could talk about the solar panels first, since that is where the party starts.

Solar Power
There's a lot of physics that go into collecting energy from the Sun. The solar panel material is a form of silicon. Some commercially available types are monocrystalline, polycrystalline, and amorphous. The most cost-efficient for us would be the monocrystalline.

To explain how it works: photons from Sun hits solar panels, causes some movements among the electrons, and they slowly flow in one direction through a wire (usually RED) going to a terminal of a device, for us it is the positive terminal of the battery. There is another wire which is usually BLACK that connects to ground or the negative of a battery. There are possibly diodes on the solar panel to prevent reverse current and on the battery to prevent leakage on that side. This is the basic idea behind charging a battery from solar panels.

But we can't just pick any ole solar panel or battery and assume that it'll work. It's more than that. The solar panels outputs a certain voltage and current depending on the amount of sunlight. Ideally we want the solar panel output voltage to be 1-2Volts greater than the battery. The more current the panels can output, the faster the batteries will charge up. 10milliamps current wont do us much good relative to the capacity of our battery, but 100mAs would be more reasonable. Think of it like 1 man pushing a 200 pound wall versus 10 men pushing the same wall. So the requirement for picking out solar panels is the voltage we need for our application and how much output current we want.

Battery Business
There is a variety of battery chemistries including lithium ions, lithium polymer, nickel cadmium, nickel metal hydride, those alkaline batteries, lithium iron phosphates, lead acid batteries (in cars), etc. Again, types of batteries are used specifically for different applications. We don't use alkaline batteries for our cellphones because they aren't rechargeable...And we don't use lithium polymers for our cars because it might go BOOM! But LiPo (lithium polymers) are perfect for our little solar charging battery application.

Let's talk chargers. Connecting a solar panel directly to a battery will probably charge it, yes. BUT charging the battery beyond its' limits will make it go BOOM! or render it broken, never to be useful ever again. That is why need need battery chargers. They can: 1. Regulate the voltage and current going into the batteries, 2. Shut down the charging process when the battery is at full capacity, and 3. Recharge the battery when it falls below certain voltage levels. A LiPo specific charger can do all that so you don't have to worry about overcharging.

Battery capacity will be a unit in Amp-hours. The higher the number (in Ah or mAh for milliAmp-hours) the more capacity it has, pretty simple. 1Ah is the same as 1000mAh. So say we have our 3.7V 3500mAh battery, and a device that runs 3.3V @100mA of current. We can definitely run our little 3.3V device for 35hours no problem. And then we want to switch out the device for one that's running at 1A of current; it will run for a good 3.5hours. Just keep in mind that the capacity tends to decrease with every full charge cycle (fully discharge and recharge) as well as a shelf current leakage, meaning if left alone, there will be a small current leak; which is why they say to charge your batteries before its first usage; otherwise you probably are draining the batteries to the point of no return. Yup, don't drain LiPo to the max for the reason that once it reaches a low enough voltage, it just doesn't come back to its nominal voltage. In that case, I would safely dispose of it.

Calculating Time to fully charge 3500mAh with 0.6W optimal from the solar panels
Power is measured in Watts or Joules per second (J/s).
In electrical terms Power P = V*I (voltage times current). Power also equals R*I^2 or V^2/R. Whatever you want... it's all the same. In our case me thinks P=V*I will be the simplest.

Let's say our charger will supply 4.5V to our battery. P=0.6W; V=4.5V
Current would be "I = P/V = 0.6W/4.5V =0.133A or 133mA.

Time = capactiy (3500mAh) divided by current (133mA)
t = 3500mAh/133mA = 26.3 hours

So it will take approximately 26.3hours of full sunlight to charge the 3500mAh battery from empty with the supplied solar panel in the SY-007. On a good day, you would get 4-6hours of Sun time. So let's say we get 4.5hours of Sun a day... 26.3hrs/4.5hrs = 5-6 days. This being calculated without taking into consideration the efficiency of the charger converting the solar panel voltages down to the charging voltage for the battery, (usually 75% efficiency is expected). So really, 8 days of optimal sunlight to fully charge this 3500mAh battery. WOW.

Conclusion
Although it is nice to think you're getting free energy from the Sun to power devices or storing energy in batteries, the effectiveness can be hidden. I certainly would like to recharge my batteries as fast as possible within the limits. That is where we bring in the option to pick and choose our materials to cater towards our application. We can definitely opt for using other solar panels (even configure multiple solar panels to increase the voltage or increase the current throughput), changing the charger to charge at a faster rate with greater efficiency, and changing the capacity of the battery (but I like high capacity batteries so 3500mAh is fine).