Solar Energy Systems - Solar Battery Bank
Batteries store direct current electrical energy for later use. This energy storage comes at a cost, however, since batteries reduce the efficiency and output of the PV system, typically by about 10 percent for lead-acid batteries. Batteries also increase the complexity and cost of the system.
Types of batteries commonly used in PV systems are:
• Lead-acid batteries
- Flooded (a.k.a. Liquid vented)
- Sealed (a.k.a. Valve-Regulated Lead Acid)
o Absorbent glass mat
o Gel cell
• Alkaline batteries
- Nickel-cadmium
- Nickel-iron
Lead-Acid Batteries – Lead-acid batteries are most common in PV systems in general and sealed lead acid batteries are most commonly used in grid-connected systems. Sealed batteries are spill-proof and do not require periodic maintenance. Flooded leadacid batteries are usually the least expensive but require adding distilled water at least monthly to replenish water lost during the normal charging process.
There are two types of sealed lead acid batteries: sealed absorbent glass mat (AGM) and gel cell. AGM lead-acid batteries have become the industry standard, as they are maintenance free and particularly suited for grid-tied systems where batteries are typically kept at a full state of charge. Gel-cell batteries, designed for freeze-resistance, are generally a poor choice because any overcharging will permanently damage the battery.
Alkaline Batteries – Because of their relatively high cost, alkaline batteries are only recommended where extremely cold temperatures (-50oF or less) are anticipated or for certain commercial or industrial applications requiring their advantages over lead-acid batteries. These advantages include tolerance of freezing or high temperatures, low maintenance requirements, and the ability to be fully discharged or over-charged without harm.
Sizing Battery Banks – For grid-connected systems, batteries are usually sized for relatively short time periods with 8 hours being typical. Size may vary, however, depending on the particular needs of a facility and the length of power outages expected. For comparison, battery banks for off-grid systems are usually sized for one to three cloudy days.
Interaction with Solar Modules – The solar array must have a higher voltage than the battery bank in order to fully charge the batteries. For systems with battery back-up, pay particular attention to the rated voltage of the module, also called the maximum power point (Vmpp), in the electrical specifications. It is important that the voltage is high enough relative to the voltage of a fully charged battery. For example, rated voltages between 16.5V and 17.5V are typical for a 12V system using liquid lead-acid batteries. Higher voltages may be required for long wiring distances between the modules and the charge controller and battery bank.
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