What is a Deep Cycle Battery?
Deep Cycle Batteries are the key component in various types of renewable energy systems that require the storage of electricity. A battery is essentially a storage vessel for electricity. It is a critical component heavily relied upon by the system as a whole. A battery bank can provide a relatively constant source of power when the grid is down, or during periods when your photovoltaic system is not producing power. Although batteries are not one hundred percent efficient, they are predictable and stable enough for reliable long-term service.
Think of Your Batteries Like a Bucket of Energy
Batteries are basically the only method to store direct current (DC) power produced from sources like solar panels, wind generators, micro-hydro or generators. Think of your batteries like a bucket of energy, where the voltage is equal to pressure, and amperage equates to flow rate. Imagine that we are slowly pouring water into a bucket that has a small hole on the bottom. As we pour the water into the bucket, its slow leak will mean that you’ll probably use 12 gallons of water to fill a 10 gallon bucket by the time it is full. In the same way, it takes more energy to charge a battery than it will actually store. The size of your bucket is analogous to the amp hour capacity of the battery bank. Amp hour is the unit of measurement used to express the storage capacity of deep cycle batteries. The Amp hour rating, written as Ah, will tell you how much amperage is available when discharged evenly over a 20-hour period. Twenty hours has been the standard time length for rating batteries, although shorter or longer time variables may be used depending on the application.
Battery technology has not changed much in the last 100 years. The standard construction method involves flooding lead plates in sulfuric acid. The chemical reaction between the positively charged lead plate and the negatively charged acid allows the battery to store and “give” electricity. The thickness of the lead plate is closely related to the lifespan of the battery because of a factor called “Positive Grid Corrosion”. The positive lead plate gradually wears away over time. Thicker plates are used in deep cycle batteries. This usually translates to a longer battery life. Although plate thickness is not the only factor related to longer lifespan, it is the most critical variable.
Battery Bank with Offgrid System
Most of the loss incurred in charging and discharging batteries is due to internal resistance, which is eventually wasted as heat. Efficiency ratios are relatively high considering that most lead acid batteries are 85 to 95 percent efficient at storing the energy they receive. Deep cycle batteries used in renewable energy applications are designed to provide many years of reliable performance with proper care and maintenance. Proper maintenance and usage play a major role in battery lifespan. Toiling over your battery bank daily with complex gadgets and a gallon of distilled water, however, is not necessary. The most common causes of premature battery failure include loss of electrolyte due to heat or overcharging, undercharging, excessive vibration, freezing or extremely high temperatures, and using tap water among other factors.
There are three basic stages in charging a battery: Bulk, Absorption, and Float. These terms signify different voltage and current variables involved in each stage of charging.
Bulk Charge: In the first stage of the process, current is sent to the batteries at the maximum safe rate they will accept until voltage is brought up to nearly 80-90 percent full charge level. There are limits on the amount of current the battery and/or wiring can take.
Absorption Charge: In the second stage, voltage peaks and stabilizes and current begins to taper off as internal resistance rises. The charge controller puts out maximum voltage at this stage.
Float Charge: This can also be referred to as trickle charging or a maintenance charge. In this stage, voltage is reduced to lower levels in order to reduce gassing and prolong battery life. The main purpose of this stage is basically to maintain the battery’s charge in a controlled manner. In Pulse Width Modulation (PWM) the charger sends small, short charging cycles or “pulses” when it senses small drops in voltage.
Fortunately most of what goes into a battery is recyclable. Over ninety five percent of lead-acid cells used under the hood of vehicles will most likely end up being recycled into another battery. Europe takes the lead in efficiency by recycling nearly one hundred percent of their starter batteries.
Determining battery state of charge
There are a few ways to determine the state of charge on a battery, each with their own level of accuracy. As there is no direct method to measure a battery’s state of charge, there are numerous ways to go about it. One way to gauge a battery is by measuring its static voltage and comparing it to a standardized chart. This is the least accurate method, but it only involves an inexpensive digital meter. Another method of gauging the battery involves measuring the density or specific gravity of the sulfuric acid electrolyte. This is the most accurate test, yet it is only applicable to the flooded types. This method involves measuring the cell’s electrolyte density with a battery hydrometer. Electrolyte density is lower when the battery is discharged and higher as the cells are charged. The battery’s chemical reactions affect the density of the electrolyte at a constant rate that is predictable enough to get a good indication of the cell’s state of charge. Using an amp-hour meter one can also obtain an accurate indication of the battery’s state of charge. Amp-hour meters keep track of all power moving in and out of the battery by time, and the state of charge is determined by comparing flow rates.
Amp-Hour rating & Capacity
All deep cycle batteries are classified and rated in amp-hours. Amp-hours is the term used to describe a standardized rate of discharge measuring current relative to time. It is calculated by multiplying amps and hours. The generally accepted rating time period for most manufacturers is 20 hours. This means that the battery will provide the rated amperage for about 20 hours until it is down to 10.5 volts or completely dead. Some battery manufacturers will use 100 hours as the standard to make them look better, yet it can be useful in long-term backup calculations.
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Types of Batteries
There are three main types of batteries that are commonly used in renewable energy systems, each with their own advantages and disadvantages. Flooded or “wet” batteries are the most cost efficient and the most widely used batteries in photovoltaic applications. They require regular maintenance and need to be used in a vented location, and are extremely well suited for renewable energy applications. Sealed batteries come in two varieties, the gel cell and Absorbed Glass Mat (AGM) type. The gel cell uses a silica additive in its electrolyte solution that causes it to stiffen or gel, eliminating some of the issues with venting and spillage. The Absorbed Glass Mat construction method suspends the electrolyte in close proximity with the plate’s active material. These batteries are sealed, requiring virtually no maintenance. They are more suitable for remote applications where regular maintenance is difficult, or enclosed locations where venting is an issue.
• Flooded Lead Acid
Flooded Lead Acid batteries are the most commonly used batteries, and have the longest track record in solar electric systems. They usually have the longest life and the lowest cost per amp-hour of any of the other choices. The downside is that they do require regular maintenance in the form of watering, equalizing charges and keeping the terminals clean. These cells are often referred to as “wet” cells, and they come in two varieties: the serviceable, and the maintenance-free type (which means they are designed to die as soon as the warranty runs out). The serviceable wet cells come with removable caps, and are the smarter choice, as they allow you to check their status with a hydrometer.
• Gelled Electrolyte Sealed Lead Acid
Gel sealed batteries use silica to stiffen or “gel” the electrolyte solution, greatly reducing the gasses, and volatility of the cell. Since all matter expands and contracts with heat, batteries are not truly sealed, but are "valve regulated". This means that a tiny valve maintains slight positive pressure. AGM batteries are slowly phasing out gel technology, but there still are many applications for the gel cells. The recharge voltage for charging Gel cells are usually lower than the other styles of lead acid batteries, and should be charged at a slower rate. When they are charged too fast, gas pockets will form on the plates and force the gelled electrolyte away from the plate, decreasing the capacity until the gas finds its way to the top of the battery and recombines with the electrolyte.
• Sealed Absorbed Glass Mat (AGM)
Absorbed Glass Mat (AGM) is a class of valve-regulated lead acid battery (VLRA) in which the electrolyte is held in glass mats as opposed to freely flooding the plates. This is achieved by weaving very thin glass fibers into a mat to increase surface area enough to hold sufficient electrolyte for the lifetime of the cell. The advantages to using the AGM batteries are many, yet these batteries are typically twice the cost of their flooded-cell counterpart. On the plus side, these cells can hold roughly 1.5 times the amp hour capacity of a similar size flooded battery due to their higher power density. Another factor that improves their efficiency is the higher lead purity used in AGM cells. Because of their sandwich construction, each plate no longer has to support its own weight. Their low internal resistance allows them to be charged and discharged much faster than other types. AGM cells function well in colder temperatures and are highly resistant to vibration. There are many advantages to using the AGM cells over their flooded counterpart that are beyond the scope of this article.
Deep Cycle versus Shallow Cycle
Lead acid batteries are designed to absorb and give up electricity by using a reversible chemical reaction. In battery lingo, a cycle on a battery occurs when you discharge your battery and then charge it back to the same level. How deep a battery is discharged is referred to as Depth of Discharge (DOD).
Starting Lighting and ignition
Automotive starting, lighting, and ignition batteries (SLI) have a short or “shallow” depth of discharge, as they are designed to produce a high amount of current in a very short time. These batteries are not recommended for use in a photovoltaic system, as they would quickly be ruined by the deep cycles required for extended use.
Deep cycle batteries are designed with thicker lead plates, which have less overall surface area than their thinner SLI counterpart. Because of the reduced availability of surface area for chemical reactions, deep cycle batteries produce less current than an SLI type battery, yet they produce that current for longer periods of time. Deep cycle batteries can be discharged up to 80 percent DOD without damage depending on the model. In order to increase battery life, manufacturers recommend discharging deep-cycle batteries only down to 50 percent in order to increase battery life.
Maintenance & Monitoring
Proper maintenance and monitoring will greatly extend the life of your batteries. Flooded batteries need to be checked regularly to make sure electrolyte levels are full. The chemical reaction releases gases, as water molecules are split into hydrogen and oxygen. This, in turn, consumes water and creates the need to replace it regularly. Only distilled water should ever be used in batteries, and you should never add any kind of acid solution. The connections from battery to battery and to the charging and load circuits should always be kept clean and free of corrosion. Corrosion is created upon charging, when a slight acid mist forms as the electrolyte bubbles. Corrosion buildup will create a good deal of electrical resistance, eventually contributing to a shortened battery life and malfunctions. A good way to keep up on the terminals is to regularly clean them with a baking soda solution
Companies world-wide are quickly adjusting to the increased global market for solar systems by developing batteries that are better suited for photovoltaic systems. At some distant point in the future, it is likely that lead-acid batteries will become extinct, as newer technologies in lithium ion and Nickel metal hydride continue to evolve. Because lead-acid batteries are under the hood of virtually every car, advancements in lead-acid technology, however are still being made. New developments in lead-acid technology usually originate in the auto industry. Efficiency ratings are constantly going up, as new sensors and improved materials are helping batteries achieve longer lifespan.
The Trojan battery company, one of the world’s leading manufacturer of deep-cycle batteries, is deeply involved in the research and development of renewable energy (RE) applications for deep cycle batteries. Trojan recently set up a web page dedicated solely to the RE market, (http://www.trojanbatteryre.com/index.html), and keep expanding their AGM product line.