A battery serves as the ENERGY STORAGE or ENERGY BUFFER in off-grid and hybrid solar power systems.
Also refered to as Battery Bank, typically multiple batteries are required in a solar power system. Battery bank stores the energy produced by solar panels during daytime and provides this stored energy during night time to the appliances, if required.
Solar batteries must be rechargeable so as to be able to store and provide electric charge many times during opeation. In a typical solar power application, a battery is regularly charged to 100% capacity and then discharged up to 80% of its capacity and then recharged to full capacity again. This process is refered to as Deep-Cycle charging-discharging and hence the name Deep-Cycle Batteries.
Deep-cycle batteries come in many different types, sizes and qualities. Some of the most commonly available batteries are:
• Lead-Acid Batteries – by far the most commonly used type and also the least expensive. These are also the most robust type. There are two main variants available:
◦ Dry – also called maintenance-free. Electrolyte is sealed in the battery housing therefore no need for refilling nor any danger of spillage. Common types are AGM and Gel.
• Lithium – high efficiency, high energy density and long-life. Available as LiFePO4 (lithium iron phosphate) type for renewable enery applications. These are gradually becomming popular in solar power applications, but high price and high sensitiviy to working conditions as well as danger of expolsion mean limited application scenarios. Nevertheless, a good companion for off-grid and hybrid solar power systems.
• Nickel – one of the oldest types of rechargeable batteries. Available as nickel cadmium and nicke metal hydride types. These have higher efficiency and higher energy density then lead-acid batteries, but are more expensive and more sensitive to operating conditions. These are now gradually phasing out.