Battery state of charge

The APM with the intelligent battery sensor determines the state of charge during driving and when the vehicle is at a standstill on the basis of measured data:

• Driving:
  • Balancing the charge and discharge current of the battery.
  • Calculation of the current characteristics on engine start to ascertain the battery condition.

  While the vehicle is being driven, the IBS transfers the data across the bit-serial data interface (BSD) to the engine control unit (DME). The software in the IBS controls the communication with the higher-level engine control unit (DME).

• Vehicle standstill

  When the vehicle is at a standstill, the measured values (open-circuit voltage measurement) are queried in cycles to detect energy losses. The measured values are entered in the IBS in the memory and transferred to the DME after restarting the engine.

For the history of the state of charge, the following values are stored in the DME:

• Battery state of charge of the last 5 days.
• Charging status histogram showing periods in the ranges 0 - 20 %, 20 - 40 %, 40 - 60 %, 60 - 80 % and 80 - 100 %. The charging status histogram is reset in the following cases: programming the DME or registering a battery replacement.

The battery voltage measured by the IBS approaches the actual open-circuit voltage slowly after the vehicle has "gone asleep". The accuracy of the measured value increases with the duration of the rest phase: The measured state of charge is reliable after a rest phase of at least 3 hours. However, an insufficient rest phase or standby current violation may result in the state of charge not being properly determined: the state of charge is implausible.