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The most important part of the system is the battery/battery bank, which stores the energy generated from your system. There are many different types of batteries on the market for various applications. Solar systems work with deep cycle batteries and not common cold cranking amp (CCA) automotive batteries. Deep cycle batteries can be charged and discharged at a slow rate and are ideal for solar setups. This section will talk about the purpose of each, along with how you would go about sizing them.
The main purpose of the battery is to store energy produced by the panel. Without this component, an off-grid system will be incomplete. Batteries are generally rated by a voltage level, mostly 6V or 12V, and an Amp-Hour rating. This Amp-Hours rating is important to know the capacity of the battery. Most small systems are 12V. RV and Boats generally are 12V systems as well. A set of batteries connected together is called a battery bank.
To understand battery sizing, we must understand the capacity of the battery. Batteries being measured in Amp-Hours and Volts, need to be calculated into Watt-Hours to tell you the energy. It is important to have both information.
Imagine you have 2 x 100AH batteries, one being 6V and the other being 12V. The 6V battery will be measured at 6V x 100 AH = 600 Watt Hours. The 12V battery will be measured at 12V x 100 AH = 1200 Watt Hours. As you can see even though they have the same Amp-Hours, their energy is different.
To size a battery, we need to first understand our consumption. We can calculate consumption as Wattage of Appliance x Hours of Run Time. Once we get this Watt-Hour value, we can divide it by 12V to get the battery in terms of Amp Hours. We also prefer to double this value, as we recommend a depth-of-discharge of only 50% to help preserve the life. This means we don’t recommend draining the battery under 50%.
Let’s take an example of a 35 Watt fan running for 6 hours. We have 35 Watts x 6 Hours = 210 Watt Hours. We then do 210 Watt-Hours/12V = 17.5 Amp-Hours. We then want to double the value for DOD of 50%, so we have 17.5 Amp-Hours x 2 = 35 Amp Hours. Our battery size would be 35AH at 12V. Keep in mind this is in 12V. To see this same battery in 24V, you would do 210 Watt-Hours/24V = 8.75 AH. You can then double this to 17.5 Amp Hours. This batter would be 17.5 AH at 24V. You can utilize multiple batteries in series, parallel, or series-parallel as mentioned in Section 2.4 to get a desired battery bank size.
The most common battery type we recommend are Lead Acid Batteries. You will generally see a Sealed or Flooded Lead Acid battery. It is important to make sure that your controller is compatible with your battery type. We offer three types of compatible deep cycle batteries: AGM, Gel, and Lithium-Iron Phosphate. The AGM and GEL batteries have very similar characteristics whereas the Lithium-Iron battery is vastly different. We recommend researching the types to see which type is best for your application as they range in size, weight, and cost.
Once you choose a battery type and size, you can then start putting your system together. The battery is where all the energy is stored in the system and following all the safety precautions and guides is necessary. Please email or call our Renogy team here for any support.