Power management

The power management is on the one hand part of the electrical energy management system. Power management is software stored in the DME or DDE and in the intelligent battery sensor that is used for controlling the vehicle's energy requirements.

Power management comprises the functions controlled by the software in the DME / DDE and in the IBS:

• Variable charge voltage for the battery by adapting the charge voltage from the alternator to that required by the battery
• Increased idling speed to boost the alternator's output
• Reduction of load peaks through power reduction when the vehicle's electrical system is unable to provide the energy needed (vehicle electrical system deficiency)
• Auxiliary consumers switched off via CAN messages when engine has reached its limit of starting capability
• Off-load current monitoring

Power management links the input signals with the characteristic curves stored in an EPROM (Erasable Programmable Read-Only Memory) and generates the output signals to control energy requirements.

• Power management components:
  • DME or DDE
  • EPROM
  • Microcontroller
• Power management input signals:
  • Battery voltage (U)
  • Current (I ±)
  • Temperature (T)
• Characteristic curves
  • Battery voltage (U)
  • Current (I ±)
  • Temperature (T)
• Output signals
  • Idle-speed control
  • Nominal value for charging voltage
  • Auxiliary consumer shutdown
  • Load peak reduction

Power management registers the battery charge state and the battery condition.

• Battery charge balance 

  The charge balance of the battery is determined by the charge quantity flowing into and out of the battery. Two counters are provided in the power management to give a running balance of the battery's charge state. One of the counters counts the charge quantity taken up by the battery. Another counter counts the charge quantity discharged from the battery. At the factory, the counters are calibrated for the battery fitted. The IBS transmits the data to power management in the DME / DDE. Data is transmitted via the bitserial data interface (BSD). The difference between the two charge levels is the battery charge state (SoC: "State of Charge"). Following an engine shutdown, the power management computes the current battery charge state for the next engine start.

• Battery condition 

  The battery condition (SoH: "State of Health") is derived from the drop in battery voltage and the current drawn during engine start. These data are measured by the IBS during the starting procedure. The average value of the starting current in the start phase and the value of the voltage dip are transmitted to the DME / DDE via the bit-serial data interface (BSD). The starting procedure is indicated to the IBS by currents greater than 200 ampres (A). The "engine running" signal is output by the DME / DDE as soon as the engine starts. The power management system calculates the battery's internal resistance from the average value of the starting current and the value of the voltage dip. Conclusions about the battery's condition can be drawn from its internal resistance.