The batteries come into play when you need to store the energy produced by your solar panels. But don't hurry to shell out your money for expensive batteries because your energy grid may act as a lossless battery if you have a grid-tied solar power system. If your local grid supports net metering you can offset your energy meter by the amount of energy your panels produce. Depending on your grid policies you can be credited with or paid for the energy you have produced. Your grid, in this case, is a free battery bank. In the off-grid scenario, you don't have this advantage.
If you have made a decision to go with a battery system you'll need to consider several things. Firstly, how much energy in Wh (Watt Hours) your battery is going to store? This depends on your energy usage patterns. To understand the yearly average of your energy consumption you can use an energy monitor or request utility bill report for the timespan of interest. Before investing in a huge battery bank system you may find it more cost effective to reconsider your energy spending habits by examining energy usage data to discover where most of the energy is spent. What's not less important is to make your house energy-efficient. Secondly, you'll need to know how many days on average you will rely on battery supply exclusively. It may happen due to grid outage or several days of rainy/cloudy weather when panels generate very low or no energy.
A single battery may be sufficient for several hours of autonomous supply. For several days of autonomous supply, you will need several big and expensive batteries. When choosing batteries, make sure the battery system's voltage is aligned with panels' voltage. It's recommended for the battery voltage to be 2-3% less than panels' output voltage to account for voltage drop or power fluctuations over time. Another important factor to look at is the battery's life expectancy. Life expectancy of a solar battery is rated by the number of cycles it can perform before it is disposed of. 6000 cycles battery means it can go from fully charged to fully
Lead Acid Battery
This is the oldest type of rechargeable batteries, low-cost, largely used in storage and backup systems like stand-alone power systems. Absorbed glass mat (AGM) and gel-cells models are common for this purpose and are collectively referred to as Valve-regulated lead-acid (VRLA) batteries. Gel-cell models have lower freezing and higher boiling temperatures than the liquid electrolytes used in more common wet cells and AGM batteries. Gel-cell batteries are commonly used with PV modules because they have reduced surge current. Both AGM and gel batteries are sealed, do not need watering, can be positioned in any direction and have valve-controlled gas blow off so they are essentially maintenance-free. A subcategory of VRLA batteries are the deep-cycle batteries which are designed to go through a deep discharge cycle using most of its capacity. Deep-cycle batteries are used for supplying energy to grid-tied and off-grid solar systems. They are designed to withstand many
Flooded Lead-Acid Batteries
Flooded batteries are called so because the plates are completely submerged in electrolyte fluid. Electrolyte level must be above the tops of the plate to avoid damaging the batteries. The water decomposes from the electrolyte during the charging process so the regular maintenance of flooded batteries requires inspection of electrolyte level and the addition of water.
Lithium-ion (Li-ion) batteries are the most commonly used storage technology because of the battery's higher specific energy density and higher voltage output per cell compared to other batteries. This battery consists of an electrolyte and 2 electrodes. Ionic movement causes the battery to charge when ions move from the positive electrode to the negative one and discharge when ions move in the opposite direction. Li-ion battery's lifespan is strongly related to the temperatures both ambient and resulting from high charge levels. The optimal temperature for these batteries is 25 ºC (77 ºF) with the temperatures higher and lower than that causing capacity loss.
The prismatic type of Li-ion batteries is commonly used for mobile devices. But we are more interested in their usage in solar. Packed in welded aluminum housings, the cells can deliver 20 - 30 Ah capacities and have applications in solar energy storage and electrical vehicles (EV). Prismatic cells provide better space utilization at the expense of higher manufacturing cost, lower energy density and higher inclination to swelling but these are minor disadvantages compared to the gains. Of all the types of Lithium batteries, lithium iron phosphate (LFP) batteries are particularity suited for backup power storage because they offer a longer cycle life than other lithium approaches. They have a constant discharge voltage of 3.2V which eliminates the need for voltage regulators. This means the battery can deliver virtually full power until it's discharged. Because of the nominal 3.2V output, 4 LFP batteries can be strung together to get a 12.8V battery.
A flow battery or redox flow battery uses 2 different electrolytes in 2 tanks separated by a membrane. The electric current occurs through the membrane while the 2 liquids continue to circulate in their respective tanks. Some of the advantages of flow batteries are practically unlimited cycles thanks to the membrane not being susceptible to degradation, little maintenance, electrolyte liquids can be reused, the storage capacity can be increased by adding more electrolytes, less reactive and easy to dispose of. The disadvantage is that the current implementations are less powerful and require more advanced circuitry.
Nickel Cadmium (Ni-Cd) Battery
Nickel Cadmium batteries have been a storage option since the early 20th century. They don't have as much energy density as other battery types but they provide reliable power and don't have complex maintenance requirements. Their main advantage is the property to maintain a practically constant voltage of 1.2V at high discharge rates. Large ventilated wet cell NiCd batteries are used in emergency lighting, standby power, solar energy storage systems and so on. Vented cell NiCd batteries are used when high discharge rates and large capacities are required. Unlike typical sealed NiCd batteries, vented cells have vents or a low-pressure release valve that releases the gases generated during overcharge or rapid discharge. The advantages of NiCd batteries include low maintenance, tolerance against big fluctuations of temperature, high cycle life. The disadvantage is that cadmium is a toxic substance and cannot be disposed of though it can be recycled.