Off Grid Solar

A handbook for Photovoltaics with Lead-Acid or Lithium-Ion batteries

Joseph P O'Connor

• Publisher: Old Sequoia Publishing
• Year: 2019
• ISBN:
  9781733454308
• Edition: 2
• Topic: Technology & Engineering
• Subtopic: Renewable Power Resources
• Index: Technology & Engineering

Off Grid Solar is a pocket guide and quick reference for anyone looking to build an electrical energy system with the free sunshine available to us all.

“The limitations to widespread solar are not centered on the cost, the efficiency, or the need for advancements in technology. The limitations are solely due to the lack of widespread knowledge on the subject.”

With that philosophy at heart, solar and battery expert Joseph P. O’Connor draws from his years of experience to craft a how-to guide needed by everyone around the world. Founder of OCON Energy Consulting, O’Connor has conducted and taught the installation/maintenance of Off Grid solar projects to people and communities as far away as the central African rainforest and right in his own backyard of Oakland, California. For anyone interested in harvesting their own solar energy completely detached from the grid, this is the book for you.

Thanks to straightforward explanations for readers of all levels, this book establishes a familiarity with the basic low cost components alongside the most innovative technologies. With this knowledge the reader can make better decisions based on their technology needs, rather than simply trusting the recommendations from others.

Need to keep costs down? How many solar panels are needed to refill the batteries? What is the best way to angle your solar panels? On projects both big and small, Joe’s got you covered.

Battery Selection

Now that you understand the load profile for your system and your local solar energy capabilities, you need to choose appropriate equipment for your off grid solar energy system. Before engineering a detailed system, there are a few things to do first. Start by selecting batteries, since they define how long you can provide power without sunshine available. After that, select the type of solar modules, charge controller, inverter, and balance of system components (i.e., all minor electrical and mechanical equipment, discussed later in this book).

Now is a good time for a refresher on voltage, current, and resistance. Voltage, the measure of electric potential, is measured in volts (V). Voltage measures the difference in electrical potential between two parts of a circuit and is commonly compared to pressure. For example, imagine two water buckets: one full of water, the other one empty. If a pipe connects the two near the bottom, the water will rush from the full bucket to the empty bucket, because of the water pressure. The same thing will happen if you connect a solar module and a battery. As long as the solar panel has a higher voltage, or “pressure,” it will push energy into the battery.

Current, also known as amperage, is the measure of electrical flow and is measured in amperes or amps (A). You can think of this as the number of electrons moving through a conductor in a given time period. One ampere is literally the measurement of 6 billion billion (6.2415 × 10¹⁸) electrons per second!

If a circuit has no voltage, then it has no current. Or in other words, if there is no difference in electric potential, then there will be no flow of electricity.

Electrical Resistance is a measure of how much a conductor opposes the passage of electrons. It represents the difficulty of electricity to flow and is measured in Ohms (Ω). Resistance is the ratio of voltage to current, so if you want low resistance then you want high voltage compared to current. See the Understanding Electricity chapter at the end of the book for a more detailed description of voltage, current, and resistance.

OK, now back to batteries, the energy containers of your off grid solar system. Lead-acid batteries are the most common type, and the majority of this chapter is about them. Lithium-ion batteries are popular in small products and electric vehicles (EVs) and are becoming more common in solar energy systems. When the battery system also includes the Battery Management System (BMS), safety equipment, and inverter, it is often referred to as an Energy Storage System (ESS) or Battery Energy Storage System (BESS).

Other Battery Types

There are many battery technologies in the world, each with their own unique benefits and limitations. However, I will only discuss the battery types that are relevant to an off grid solar energy system. Below is a list of less common battery chemistries. Only a few manufacturers support them, but you might find these battery types at a discount and want to use them regardless of other drawbacks.

Sodium-Ion batteries are made from nontoxic materials that are non-flammable. They tend to have a long cycle life and won’t be damaged when left in partial charge for long periods of time. They are heavy and large for the amount of energy that they provide, similar in size and weight to lead-acid batteries. Their biggest disadvantage is that they must be charged or discharged slowly due to their high internal resistance.

Nickel-Iron (NiFe) batteries are a very old technology invented by Swedish inventor Waldemar Jungner back in 1899 and commercialized by Thomas Edison in 1901. NiFe Batteries have a very long lifecycle, lasting up to 30 years, and are very tolerant to abuse from overcharging, over-discharging, and short circuiting. The trouble with NiFe batteries is that they are 3-4 times less efficient compared to other battery types. Also, many inverters and charge controls can’t handle the large voltage difference between the fully charged and discharged state.

Nickel-Cadmium (NiCd) batteries are rugged and have a relatively high cycle life but they are made with toxic cadmium, which can cause serious problems if not disposed of properly. Recently, NiCd batteries have begun to be replaced by Nickel-Metal-Hydride (NiMH) batteries, which are similar to NiCd but with slightly improved performance. NiCd and NiMH batteries are good for extremely low maintenance systems.

Zinc-Air batteries have similarities to fuel cells. When charging a zinc-air battery, the electricity converts the zinc oxide to zinc and oxygen. When zinc is separated away from oxygen there is potential energy available. During the discharging process, the battery combines the zinc and oxygen which generates an electric charge. Zinc batteries have been studied for a very long time, but few companies have found ways to commercialize the battery technology.

To learn more about batteries I recommend reading the content on www.BatteryUniversity.com

Lead-Acid Batteries

There are many trade-offs when selecting a battery and that makes every project different. Common, relatively inexpensive lead-acid batteries can be suitable for most systems. Lithium-ion batteries could be suitable for your system, but as a new technology they might not be as readily available. In the next few years, lithium-based batteries will become more cost effective and will likely replace the less efficient lead-acid technology.

If lead-acid batteries are maintained properly, they will function at 80-90% efficiency. It is important to store a full charge whenever possible, because this will extend battery life and maintain a higher efficiency. Lead-acid batteries can be damaged if overcharged or over-discharged. In this chapter, I will explain how to properly maintain batteries to increase their usable life.

Buy Deep-Cycle Batteries, Not Car Batteries

Batteries designed to start a car engine are NOT recommended for use with a solar PV system. Starter batteries are readily available around the world and relatively inexpensive because of the automobile industry, but they will stop functioning within 3 to 12 months if used with a solar energy system.

For solar charging applications, you want a Deep-Cycle battery, similar to those used for boats or electric vehicles—a typical car battery will not work. Sometimes called motive power or traction batteries, deep-cycle batteries are designed to be deeply discharged regularly using a large portion of their capacity. Deeply discharging a starter battery too frequently harms the thin plates, so deep-cycle batteries are constructed with thicker plates and different chemistry to handle deeper charging cycles.

Starter batteries used in automobiles deliver short, high-current bursts to start an engine and are meant to discharge only a small amount of their capacity. They are also called Starting, Lighting and Ignition (SLI) batteries. Starter batteries are designed to stay nearly 100% fully charged most of the time. The internal architecture of these types of batteries has a large number of thin plates for increased surface area, which provides a quick burst of current when needed. If batteries like these are used for a PV system, they will quickly fail because the internal architecture is not designed for the deep charge and discharge cycles that are common in an off grid PV system.

Lead-Acid Battery Types

Now that you know to look for a Deep-Cycle battery, which one should you choose? It depends on how you plan to maintain your batteries. If you are setting up a remote solar energy system without anyone available to provide maintenance, then you should consider Valve-Regulated Lead-Acid (VRLA) batteries. If you plan on having someone maintain the batteries, and they are able to maintain them about once a month, then you should consider Flooded Lead-Acid batteries, since they are about half the cost of VRLA batteries.

Lead-acid batteries have large capacities and are available in many places around the world. The flooded type is less common and requires more maintenance compared to other batteries; however, it also tends to provide the lowest cost per kWh.

Sealed or Valve-Regulated Lead-Acid (VRLA)

There are two main types of “sealed” batteries: Gelled and Absorbed Gas Mat (AGM). They technically are not sealed but are valve regulated to allow for gasses to escape. If you want a low maintenance system, then a sealed battery may be your best bet. Since VRLA will not spill like flooded batteries can, you can mount them in many positions.

*Remember that all lead-acid batteries require proper ventilation even if they are labeled as “sealed” batteries.

Gel Batteries do perform better than AGM Batteries in high temperatures, but they need to be recharged very slowly, which is not optimal for solar. AGM batteries are typica...