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/DDE). The software in the IBS controls the communication with the higher-level engine control unit (DME/DDE).

• 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/DDE after restarting the engine.

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

• 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/DDE 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.