Depth of Discharge is how much energy is discharged from the battery before it is charged to 100% again. A common measurement is battery cycle life, at 50% D.O.D (Depth of Discharge).
A 50% DOD the battery has 50% of its energy capacity discharged, before it begins charging again. This is common in a solar situation where during the day the sun charges the battery, then during the rest of the day, the energy stored in the battery is used to power appliances. As the battery goes through more charge and discharge cycles it begins to degrade and the amount of energy the battery can hold decreases. So, while the battery is fully charged it doesn’t hold as much charge as when the battery was new.
Most batteries are rated by the amount of cycles it takes before the battery can only store 60% of the energy it could when it was new.
If we assume we are using the battery in a solar installation where the battery charges during the day and is discharged when the sun is down. We can estimate how long the battery will last for.
The graph compares how many charge cycles with a 50% DOD before the battery reaches 60% of its original capacity.
The amount of cycles the battery can handle also depends on how much you discharge it each cycle. If you only discharge it by 30%, using only 30% of the energy it can hold, the battery. As shown in the graph the Cycle life is much longer.
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