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Solar Battery Backup: Solar Batteries and Solar Battery Bank in Solar Power Systems
This video will show: basics of batteries and their application in solar electric systems, the most important battery features, what kinds of batteries are used in solar electric systems, how to connect batteries by revealing tips and what to avoid.
Batteries are devices capable to produce and store DC electricity. A battery cell is a container, usually filled with diluted acid used as electrolyte, with two plates of positive and negative polarity, immersed into the electrolyte. Such a battery cell is called a ‘wet cell’. There are also ‘dry cells’, which do not contain liquid electrolyte. Battery cells connected together form a battery. Batteries connected together form a battery bank. Battery banks are widely used in solar electric systems, especially in off-grid ones, to store the solar generated electricity.
In photovoltaic systems batteries are used to replace the photovoltaic generator:
at night, during cloudy weather, or, when the solar array is disconnected for repair and maintenance works.
Which solar systems use a battery bank? A grid-tied solar system does not use a battery bank unless it is provided with power backup. Battery banks are typical for stand-alone solar electric systems. There are however certain exceptions.
The ability of a battery to store DC electricity is called ‘capacity’. Capacity is measured in amperes-hours or Amps-hours. After DC electricity is put for storage in a battery, the later can render it as DC voltage. The higher the capacity, the longer the battery is able to deliver the rated voltage. After some time however, the battery is no longer able to deliver the stated voltage. Such a battery is said to be in a state of ‘discharge’. To deliver the stated, or the rated, voltage, the battery needs to be ‘recharged’. The shorter the discharge or charge period, the shorter the battery life. In a solar system the battery bank is recharged by the solar array. Batteries used in solar systems are rechargeable. Every battery has its life duration which means that it cannot be discharged and recharged for limitless number of times. Another important battery parameter is ‘Depth of Discharge’. It is the grade down to which the capacity of a battery can be reduced. The lower the depth of discharge, the longer the battery life. Vehicle batteries are not designed for frequent and deep discharge, as is the case with photovoltaics. For this reason vehicle batteries are not recommended for photovoltaic systems.
In battery-based photovoltaic systems the following types of batteries are used: Lead-acid batteries, which can be flooded or sealed, and Alkaline batteries, which are always sealed. Alkaline batteries can be nickel-cadmium, used for small portable appliances or daily loads, and nickel-iron.
We come to the most popular battery type: flooded lead-acid batteries.
Flooded lead-acid batteries are the best choice for residential photovoltaic systems. Lead-acid batteries comprise multiple individual cells with a nominal voltage of two volts each. Lead-acid batteries contain so-called ‘wet cells’ whose electrolyte is in a fluid state.
Valve-regulated lead-acid batteries are also known as sealed lead-acid batteries. They have the following benefits: do not require maintenance by the user, produce less hydrogen gas during the recharge process, have more vertical construction rather than horizontal and thus are suitable for locating in limited room space. Valve-regulated lead-acid batteries however are not widely used due to the following reasons: their cost is greater, they have a shorter life span, and¬ required charging voltage should not be exceeded since the process might result in generating too much hydrogen gas which cannot be fully released.
Sealed or gel batteries are suitable for mobile applications of small, low-current stand-alone photovoltaic systems. They are: Easy for handling and transportation, safety, as there is no acid and no gassing during charging, and lack of maintenance, that is, no need to add distilled water.
Batteries are used connected in a certain way. The question is why.
We connect batteries: to obtain higher output voltage, to obtain higher capacity, or to obtain bother higher output voltage and higher capacity.