Deep cycle batteries can be used to store excessive energy collected in days and provide power for nights or raining days. Note that normal car batteries are especially designed for high initial cranking amps (usually 200 to 400 amps for five to 15 seconds) to start a car and for shallow (10% or less) discharges. They are not designed for deep cycle discharges. So you need deep cycle (and marine) batteries, which are designed for prolonged discharges at lower current.
The most important consideration in buying a deep cycle battery is its capacity, which should meet or exceed your requirements. Battery capacity is usually rated in Ampere-Hour (AH). For example, a fully charged 110 AH battery can provide a 11 ampere current for approximately 10 hours. If your system needs to keep a 18 Watt PC running during 10-hour nights, you need a 12Volt battery of at least
18 Watt / 12 Volt * 10 Hour = 15 AH.
But considering that the battery charging/discharging efficiency is not 100%, and you need to save energy for raining days, you may want to use a battery with much higher capacity. In the system we built, we use a 98 AH battery to provide continuous power for a 18 Watt system even in 2~3 consecutive raining days.
Click here for more information on batteries.
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