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Solar Panels & Batteries
This third blog post is about Solar Electric Components. We are going look at some of the main components in a solar electric system: the solar panels and the batteries. Technology is moving forward at a very fast rate. Most of the generalities are true today, but there are likely to be exceptions.
If you missed the earlier blog posts, I recommend that you read them to get a good foundation:
Review of Series and Parallel Circuits
Wiring panels in series: Wiring the negative of one wire to the positive of the other, results in the current staying the same. The voltage increases.
Wiring panels in parallel: Wiring the two positives together and the two negatives together, results in the voltage staying the same. The current increases.
It s important to note that regardless of the way it is wired, the power, or watts, remains the same. Since Watts equals Volts x Amps, it doesn’t matter if it is series or parallel.
Wiring batteries in series, their voltage increases. Wiring batteries in parallel, the amp hours increases. You’ll often see rows of panels or batteries wired in series, each row is a string. You can then wire strings in parallel. This allows you to get both higher voltage (with series strings) and higher amps or amp hours (with parallel).
Here is an example using 12V 80AH batteries. You call 2 parallel rows of 4 batteries wired in series 2 strings of 4. Wiring four (4) 12V batteries in series equals 48V with the AH remaining at 80AH. Wiring 2 strings in parallel doubles the amp hours, resulting to a battery bank of 48V, 160AH. Multiplying V x AH, the battery bank can produce 7680WH (48V x 160AH) of electricity.
Solar Panels (PV Modules)
Solar panels (or PV modules) generate DC electricity when exposed to sunlight via the Photovoltaic Effect. This was first observed by a French physicist Edmond Becquerel in 1839. A simple explanation is that the photons from sunlight are absorbed by a semiconductor material, generally silicon. The negatively charged electrons are knocked loose from their atoms, and flow from the negative side to the positive side to recombine with available holes there. This creates a direct current flow. This flow of electrons can then be used to:
- directly power a DC device, like a pump or a fan
- can be used to charge a battery bank
- can be inverted to AC power to use in your home
Each solar cell generates about 0.5V. That’s not much for practical use. So multiple cells are wired together in series to create a higher voltage, creating a solar module, commonly referred to as a solar panel. A typical 12V solar panel has 36 cells in series. The larger a solar cell is, the higher the current. So the cells of a 200W panel are generally bigger than a 100W panel. Multiple solar modules wired together then creates a solar array.
You can see the difference in the look of a 12V module compared to a 24V module.
Each module has a label on the back, stating their specs. Here s an example of a Kyocera 140W 12V module.
It lists the rated outputs for the panel, as well as any certification it has. The ratings are actual outputs under standard test conditions, so the numbers you measure in the real world may be slightly different. Let s go over each of the specs for a solar module:
- Open Circuit Voltage (Voc) is the voltage you will measure when nothing but a voltmeter is connected to the solar panel. This is the highest voltage the module will output at 77 degrees Fahrenheit with the sunlight intensity at 1000 watts per square meter, which are just a few of the details of standard test conditions, or STC. The voltage will be higher when it is colder out, and lower when it is hotter.
- Short Circuit Current (Isc) is the amps output with no load on the panel. It is the highest current possible at STC. There are times when the output could be higher, for instance when the sun is coming out from behind a cloud, you can see the silver lining , where the edge of the cloud is magnifying the sunlight, causing the intensity to be brighter than STC.
This brings us to the two specs which are when the module is connected to a load, so more real world conditions. But still at the temperature and brightness listed under STC.
- Maximum Power Voltage (Vmp) is the actual voltage the module will output when connected.
- Maximum Power Current (Imp) is the amps output while under load. PV modules were originally designed to charge battery systems, so it is typical to see panels listed for what voltage battery bank it is able to charge.
Nominal voltage is a shorthand grouping term, originally based on battery voltages (for example, 12V, 24V, 48V). To charge a 48V battery bank, you simply wire four 12V modules or 2 24V modules in series to add up to 48V. In general, you can determine what nominal voltage the module is by the number of cells on the panel. A 12V nominal panel usually has 36 cells, and its Open Circuit voltage is about 22 volts, and its Maximum Power voltage is around 17 volts. However, as grid-tied solar systems that don’t use batteries have become more popular, you start to see different size nominal panels that don t logically line up with battery bank sizes. The most popular size modules used in grid-tied systems today are 60 cell, 20V modules. The wattage of available panels has been increasing, and many manufactures are achieving that by increasing the number of cells, increasing the voltage of the panels. As you recall, watts equals volts times amps, so increasing the volts while maintaining the same amps increases the watts. As such, there are 80 cells and higher available these days.
Solar panels have no way to store power, you use it or lose it. Batteries allow you to store power to use later, by using the power generated by the module or other power source to charge the batteries. Batteries used in a solar system MUST be deep cycle batteries. These are made very differently from car batteries. A car battery has the internal plates designed to send a short, high current blast to start the engine. It then gets recharged quickly by the alternator to send another short blast hours later. A deep cycle battery is designed differently. It is designed to be gradually charged and discharged over a course of hours. If you try to use a car battery for an off-grid system, it will work for a short amount of time, but you will very quickly kill the batteries. PV systems can have many batteries. There are losses inherent in batteries as they convert electrical energy to chemical and back to electrical: 5-15% of energy is lost to storage and extraction. You should never use more than 50% of the rated capacity, or you will quickly reduce the battery’s ability to hold a charge, and have to replace the bank. These losses need to be taken into account when calculating the size of your battery bank.
When selecting a battery for your system, there are 2 primary types of batteries available, flooded or sealed.
A flooded lead acid battery has removable vents that you must remove to check the specific gravity of the acid and add water on a regular schedule, usually once a month. Because it is not sealed, it is designed to output the hydrogen gas that is created during its charging process. Therefore the battery bank MUST be properly vented to the outside. If you are looking to have flooded batteries shipped, be aware that they are considered hazardous material, so additional precautions and expenses may be required. The advantage of these flooded batteries is that they are less expensive than a typical sealed battery, and a well maintained flooded battery will generally last longer than a typical sealed battery. But if you neglect the battery and do not handle the maintenance, you will quickly have a dead battery bank on you hands.
Sealed Lead Acid batteries are most commonly available as either AGM or Gel. This refers to the form of the electrolyte. An AGM battery has the electrolyte in a spongey mat, and the gel batteries have a thicker gel that keeps itself distributed within the battery. There are pros and cons to each of these designs that we won t get into, but in general , both types of sealed batteries are very similar. The biggest pro of sealed batteries is that since they are sealed, they won’t spill or outgas. This makes it a safer option than flooded batteries. They also don t require the monthly maintenance, just occasionally inspect them to see that they look to be in good shape. Sealed batteries are an excellent choice for battery backup systems that aren’t charged and discharged every day, but require a long standby period. They also do better in extreme cold. The downside to the sealed batteries is that they tend to be more expensive than a flooded battery, and have a shorter life than a well maintained flooded battery. But it you are not able or willing to maintain a flooded battery, sealed is definitely the way to go.
Just as Watts = Volts x Amps, Watt hours = Volts x Amp hours. This will come in handy when we size our battery bank. Many people ask how long they can run things from a battery. This depends on how deeply you discharge the battery, known as depth of discharge, and how quickly you are drawing current out of the battery. If you are drawing 2 amps out of a battery for 4 hours, this is using 8 amp-hours. You don’t want to use more than half of the stored power in a battery, so the most power you want to take out of a 92 amp hour battery is 46ah. So if you are running that same 2 amps for 23 hours, you would have drained the 92 ah battery to 50% depth of discharge. The amount of power that a battery can store varies based on a number of variables, including how fast you charge and discharge the battery. In this example, you see that if you charge and/or discharge the battery over 5 hours and a rate of 49 amps, it can store half the power than if you did the same over the course of 100 hours at 4.6A.
Most batteries are rated at 20 hour charge rate, basically how much power you can use during a day. If you are using power faster or slower than 20 hours, you must adjust the sizing accordingly. The amount of power that a battery can store varies based on a number of variables, including how fast you charge and discharge the battery. In this example, you see that if you charge and/or discharge the battery over 5 hours and a rate of 49 amps, it can store half the power than if you did the same over the course of 100 hours at 4.6A. Most batteries are rated at 20 hours, basically how much power you can use during a day. If you are using power faster or slower than 20 hours, you must adjust the sizing accordingly.
When selecting a battery, you must decide between flooded or sealed, what voltage battery, and how many amp hours. You need to keep in mind the size and weight of the batteries. Will they fit in your available space? Also note what terminals they have to connect to the battery cables.