Current Usable Battery Capacity

The battery management system displays the current usable battery capacity expressed as a percentage of the nominal capacity as the value SOH (state of health). After commissioning, the Sunny Island adopts the set nominal capacity (Parameter Rated battery capacity) as the usable battery capacity and thus sets the state of health initially to 100%.

During operation, the Sunny Island learns to define the adopted state of health of 100% more and more precisely. This learning process only works in Sunny Island systems in which the battery is operated cyclically. Furthermore, there must also be regular idle phases during which the battery is not charged and is only operated under low electrical loads. A low electrical load is approximately 1.5% of the nominal capacity for a ten-hour electric discharge, e.g. 150 W for a 10 kW battery. It is therefore important to regularly check the state of health, especially in the first few months after the Sunny Island has been commissioned.

The Sunny Island is able to determine the state of health with an accuracy of ±15%. To determine a more accurate state of health, the battery capacity must be measured. For this, electrically qualified persons with special measuring equipment must be on site, e.g. members of the battery manufacturer service staff.

Effect of the battery temperature on the current usable battery capacity

The current usable battery capacity depends on the temperature of the battery. At temperatures of 20°C and below, the usable capacity of a battery drops significantly. The battery management system corrects the usable battery capacity state of health by -1% per °C, starting from 20°C.