What is a deep cycle battery?
In the context of renewable energy, when batteries are referred to it usually means deep cycle batteries.
Deep cycle batteries are an energy storage units in which a chemical reaction occurs that develops voltage and results in electricity. These batteries are designed to be cycled (discharged and recharged) many times.
While a car battery is designed to deliver a burst of energy for a short time, a deep cycle battery provides power at a steady rate over a long period.
Among conventional deep cycle batteries, the flooded battery is the most common, which is similar to the standard lead acid battery in your car. The gel batteries, as the name suggests, have a gel-like substance in them and the AGM batteries consist of acid suspended in a glass mat separator.
While Flooded, AGM and gel batteries are used most frequently in off-grid scenarios; next generation lithium-ion battery systems will experience significant uptake among grid-connected households.
Deep cycle battery ratings
Up until recently there were two ways batteries were generally rated: volts and amps.
Amp hours (Ah) are the rated capacity available in chemical energy inside a battery that is converted into electrical energy. It also refers to the amount of energy that the battery can store, or conversely, it can be seen as the discharge rate, which measures the time it takes to discharge a battery before it needs recharging.
The capacity of the battery is reduced if the battery is discharged at a shorter period, for instance over 1 hour. The amp hour capacity will be reduced by about 50% and so will the amount of cycles.
Where the battery is discharged at a constant rate of current over a number of hours, this is referred to as the “C” rating. For example, many small batteries are rated at the C20 rate, this means that they will deliver their amp hour capacity if discharged over 20 hours. The types of batteries in large stand alone power systems are rated at the C100 rate which means that they are designed to be discharged over 100 hours or 4 days. This will give you a life span typically of about 15 years.
You can learn about battery voltages and state of charge here.
More recently, with the advent of the home energy storage revolution; batteries for residential applications are often referred to by their kilowatt-hour (kWh) capacity.
Selecting a battery can be confusing. While all will claim to be particularly well suited to a energy storage purpose, all deep cycle batteries are not created equal, even within their own type such as AGM, Gel or Sealed Lead Acid.
As a deep cycle battery can be quite an investment, you’ll want one that will last the distance.
One of the ways to determine this among conventional types is the cycle rating; that is how many times it can be discharged and recharged. One of the best benchmarks for this is the IEC 896-2, based on a 100% discharge.
While discharging a battery 100% is not recommended as it will significantly decrease the life of any deep cycle battery, the IEC 896-2 provides a good baseline for drawing comparisons between brand X and Y or even different battery lines from the same manufacturer.
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