Battery sensor
The battery's SOC (State Of Charge) is calculated from the battery voltage after the battery has been at rest for several hours. Once the car is put in use again, the SOC calculated from the rest potential is used, this value is then corrected with the current (charging/discharging) through the battery.
The battery sensor (343) measures the current magnitude and its direction to determine by how much current the battery is charging or discharging. This value forms the basis for calculating the battery's degree of charge, SOC, during operations.
The battery's degree of charge is defined as the battery's present capacity divided by its capacity as set out in the specification. The capacity is measured in ampere-hours, Ah. During the manufacture of the car the battery capacity (Ah) is programmed into the battery sensor. When the battery is replaced the capacity of the new battery needs to be programmed into the new one, if its capacity is different from the factory value.
The current is calculated by measuring the voltage drop over a current shunt built into the battery sensor. To be able to calculate the SOC correctly, the battery voltage and battery temperature must be known. The battery sensor measures the voltage and temperature. To achieve a correct battery temperature, the external temperature is used with a correction to the temperature measured by the battery sensor, this information comes from BCM via the I-bus.
The optimal charge voltage for any given situation is calculated based on the battery voltage, battery current, temperature and SOC. The battery must be charged optimally, neither too much nor too little, in order to reduce fuel consumption, high degree of charge and long battery life.
Should a situation arise where the alternator's charging current is insufficient to maintain the energy balance in the system (decreasing system voltage) then the BCM will request increased idling to increase the alternator's charging capacity. On the other hand, the alternator may, under certain circumstances when the battery SOC is high and the car's own consumption is low, turn off the alternator. This gives a fuel saving.