Batteries enable using solar electricity even when the system isn’t producing it. Solar applications typically use deep-cycle batteries because they can endure repeated and deep discharges, typical in off-grid renewable energy systems. Even if the system is connected to the grid, power will not be available without batteries during an outage.
Again, solar batteries must provide long periods of deep discharge during evening and night, and fully recharge in sunlight during the day. Unlike car batteries, deep cycles can be gradually discharged by as much as half of their capacity and then gradually recharge. There are two types of deep-cycle batteries: flooded and sealed. Flooded lead-acid batteries (FLA), also called wet cells, are commonly used in renewable-energy applications. They are the least expensive deepcycle battery, though they last the longest and come in a variety of sizes.
FLA batteries use a sulphuric-acid solution that reacts with the lead plates in the cells to produce electricity. However, hydrogen and oxygen are released when FLAs are recharged, which can expose the “flooded” plates and lead to explosive gas. Therefore, these batteries require regular monitoring and maintenance, such as adding distilled water and properly ventilating gases to the outside air.
Sealed batteries (absorbed glass mat and gell cell) use non-fluid electrolyte contained in inaccessible cells. One advantage is they can be oriented any way: upright, sideways, and sometimes even upside-down. In a sealed battery, the electrolyte is suspended in either an absorbed glass mat (AGM) or in a gel.
Because these batteries are sealed, escaped moisture can’t be replaced, so they don’t last as long as FLAs. They also cost more. However, sealed deepcycle batteries are great for applications that require frequent battery handling, or in unattended photovoltaic systems in remote locations. Gel-cell batteries are typically even more expensive than AGMs, but demonstrate better performance in cold or very deep-discharge applications and also don’t require much maintenance. However, they are sensitive to overcharging.
Another important consideration when choosing a battery is size and weight constraint. Batteries are sized by their nominal voltage and (Ah) of storage. These can range from 20 to 2,000 Ah, the latter of which can weigh several hundred pounds. With proper maintenance and monitoring a deepcycle battery can last four to ten years.
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