samlexsolar SCC-30AB, Owner'S Manual, Page: 20 / 48

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GENERAL INFORMATION: BATTERIES
Figure 3-5: Cycle life as a function of deep of discharge
Battery capacity
Previously, the storage capacity of a battery is expressed in Ah (Ampere-hours)
showing how many hours a certain current can be taken from the charged battery
until the battery is discharged, i.e. until the battery voltage drops to the discharge
voltage threshold.
Nowadays, it might be more favorable to express a battery capacity in dischargeable
energy, namely Wh (Watt-hours) or kWh (kilowatt-hours). However, these two ways of
expressing the battery capacity are equivalent because they are related via the battery
voltage, i.e. Ah × V = Wh.
Unfortunately, the capacity of a battery is not a constant quantity, but depends
on the amount of discharge current. The manufacturers, therefore, give the rated
capacity of their batteries always with regard to a certain discharge current.
The so-called Rated Battery Capacity refers to the capacity of the battery under given
standard conditions: it is common practical to define the rated capacity at 20 °C by
discharging the battery with a
Rated Battery Current (I
20
), which refers usually to a
constant current, with which the battery will be completely discharged in 20 hours.
Some battery manufacturers indicate the 100-hour discharge capacity for batteries
intended for PV applications. When comparing such capacity, it should be remembered
that, for a given battery, the 100-hour capacity is always at least 15 % higher than the
20-hour capacity .