In contemporary off-grid solar power systems, deep cycle batteries are indispensable. Unlike conventional car starter batteries that provide short, intense bursts of power, deep cycle batteries are engineered for prolonged discharging, making them ideal for consistent energy supply in solar installations.
The Mechanics of a Deep Cycle Battery
A deep cycle battery encompasses three key components: the anode (+), the cathode (-), and the electrolyte. Inside, a chemical reaction leads to electron accumulation at the anode, while the cathode experiences a reduction of electrons. This imbalance prompts electrons to move towards the cathode, with the electrolyte acting as a barrier to prevent direct electron flow. This mechanism is central to the battery’s power generation.
Deep cycle batteries come in various forms, including flooded, gel, and AGM (Absorbed Glass Mat) types. Flooded batteries are akin to standard car batteries but are designed for different operational demands. Gel batteries contain a specialized gel electrolyte, whereas AGM batteries use an acid suspended within a glass mat separator.
Rating and Capacity
Deep cycle batteries are evaluated based on voltage and ampere-hours (Ah). The ampere-hour rating represents the energy storage capacity, indicating how much power the battery can hold and its discharge rate. Notably, the capacity diminishes if discharged rapidly. For instance, a battery discharged in one hour may see its capacity halved, impacting its lifecycle.
Batteries also have a 'C' rating, indicating the time over which they can deliver their rated capacity. A C20 battery, for example, discharges its capacity over 20 hours. In large off-grid systems, batteries often have a C100 rating, discharging over 100 hours and potentially lasting over 15 years with proper maintenance.
Battery Life and Cycle Rating
Selecting a deep cycle battery requires careful consideration of its cycle rating, which denotes the number of complete charge and discharge cycles it can undergo. The International Electrotechnical Commission's IEC 896-2 standard, which tests batteries based on a 100% discharge, provides a reliable basis for comparison, despite the general recommendation against fully discharging batteries.
Types of Deep Cycle Batteries
AGM Batteries
Absorbent glass mat (AGM) is a type of lead acid deep cycle battery where the electrolyte is absorbed into a mat made from fibreglass. The plates in an AGM battery can be flat or wound in a spiral formation. The resistance inside an AGM is low and they can handle higher temperatures and will discharge slower than most.
AGM batteries have a built-in valve that opens when the battery is recharged at high voltage, and this allows some of the battery material to escape, decreasing the overall battery capacity. There is a gas diffuser that allows the hydrogen to escape safely, they are also maintenance free and can be mounted in any position, unlike lead-acid batteries that need to be kept upright.
- AGM batteries are sealed and easy to move around safely.
- No maintenance is required such as fluid level monitoring.
- They can be mounted almost anywhere safely.
- The low internal resistance means they can be fully charged at a lower voltage and will take a more substantial charge current, taking about 3-4 hours to get a nearly full charge from an alternator.
- It is possible to get a much deeper discharge from AGM batteries with less risk of damage.
- AGM batteries will self-discharge at around 3% every month if not used, even after 12 months laying idle they can be put back into service without any damage or loss of working ability.
- AGM batteries are reliable and robust power units.
Sealed Lead Acid Batteries
Sealed lead acid batteries are also referred to as valve regulated lead acid (VRLA) batteries, recombinant batteries, and often referred to as maintenance-free lead-acid batteries. Types of VRLA batteries:
- Absorbed glass mat (AGM).
- Gel Cells.
The phrase "sealed lead acid battery" can be a bit misleading as a sealed battery would be dangerous due to pressure risks when the battery is overcharged. In these batteries, there is always some safety valve to prevent a hydrogen or gas pressure explosion.
Flooded Lead Acid Batteries
The oldest kind of rechargeable deep cycle battery in use today is the flooded lead-acid battery or wet cell battery. These types of batteries have a liquid inside and are not sealed, so they need to be stored and used in an upright position to prevent fluid from escaping. They also produce hydrogen gas, so the need for proper ventilation is essential for safety.
A flooded lead-acid battery will require regular maintenance checks specifically a fluid level check to make sure the lead is covered, and they can be topped up with distilled water to maintain the correct level.
Gel Batteries
Gel batteries or Gel Cells are valve regulated deep cycle lead acid batteries with a gel type substance acting as the electrolyte, unlike flooded lead-acid batteries they can be stored and used in any position. Gel batteries are maintenance-free and can take high temperatures and impacts without damage that would otherwise damage flooded lead-acid types.
Lithium-ion Batteries
A lithium-ion is a member of a family of rechargeable battery types in which lithium ions move from the negative electrode to the positive electrode during discharge and back when charging.
Li-ion batteries are the latest and greatest home energy storage devices there are at the moment, and when the price comes down will bring about the new revolution in home energy storage. There is a big buzz about this type of battery in the media with the big announcement from the Tesla company about their Power Wall home energy storage device.
Some of the advantages of Lithium-ion Batteries are:
- Lightweight
- High energy density
- Lower self-power discharge
- Lower battery maintenance
- No “power memory effect.”
- Increased charge cycle life
Battery Maintenance
Off-grid deep cycle batteries are designed to last a long time and have much thicker lead plates than average. It's not good practice to discharge a deep cycle battery below 20% of its full charge due to internal resistance creating heat when recharged. This is where a good quality charge controller with a low voltage cut off system can help prevent too much discharge resulting in damage and shorter battery life.
Battery Safety
Batteries can explode if a malfunction occurs or by misuse by humans such as trying to recharge a non-rechargeable battery or causing a short circuit. Explosions are likely to happen when a short circuit makes a large electrical current, the risk of an explosion is also due to the presence of hydrogen gas exploding after being ignited by a spark or other ignition source or merely a build-up of gas pressure due to a faulty or block release valve.
