Twin-battery power supply

In order to ensure that the car is capable of starting, two batteries are fitted in the vehicle. The power supply is divided into two circuits: the vehicle electrical system and power supply for the starting process.

Brief description of components

Power module

The power module monitors the vehicle electrical system and the battery charge level of the vehicle battery. The power module controls the activation and deactivation of consuming devices according to specified priorities. While the vehicle is being driven, no consumer shutdown takes place, as the engine control module always outputs a positive charge balance. When the vehicle is stationary, however, the priority deactivation is effective.

Auxiliary module 1

Auxiliary module 1 activates the vehicle battery depending on the charge status of the starter battery for charge by the alternator(s). Auxiliary module 1 is wakened across a wake-up line by the power module. The power supply of auxiliary module 1 comes from the starter battery.

Auxiliary module 1 contains functions for:

• Optimum charging of the starter battery
  
• Increase in the starting capability of the vehicles, also with a defect in the vehicle electrical system
  
• Diagnosis
  

Isolating relay

The isolating relay separates the starter battery from the vehicle electrical system. The separation protects the starter battery against discharge by the vehicle battery. The isolating relay is activated by auxiliary module 1 depending on the values of the current and voltage measurements and the terminal statuses.

Clip-on ammeter

The clip-on ammeter used to measure the current flowing between the starter battery and the vehicle battery.

Main functions

Closing the isolating relay

The isolating relay is closed

• at a starter battery voltage ≥ 13.5 V with terminal R and after 5 minutes (charge via external-start support point),
  
• when the engine is running (but not during the starting process)
  
• after opening to charge the vehicle battery if the voltage of the vehicle battery is 200 mV greater than that of the starter battery for 2 minutes
  
• in the event of a failure of the alternator(s)
  

Opening the isolating relay

The isolating relay is opened

• when the engine is running and a current flow >0.5 A is present for longer than 5 seconds towards the vehicle battery
  
• during the starting process
  
• in stationary operation (engine is off).
  

Emergency operation

A starting process is also possible when the vehicle battery is flat is or missing. The state of flat or missing vehicle battery is detected when:

1. the engine is not running and the voltage of the vehicle battery U_BB < (11 ± 0.2) V.
   
2. the engine is running and
   - safety battery terminal (SBK) vehicle battery has triggered or
   - fuse of vehicle battery cable is defective.
   

In these cases, the electronics unit of the Car Access System (CAS) is supplied with voltage by the starter battery. Bus communication with auxiliary module 1 is possible. The wake-up signal from the power module can be transferred to auxiliary module 1.

After receipt of the wake-up signal, auxiliary module 1 evaluates the voltage of the vehicle battery. With U_BB < (11 ±0.2) V, the isolating relay is closed. The starting process is carried out. If 'engine running' is not detected within 5 minutes, the isolating relay opens.

As of 'engine running', the isolating relay is activated as follows:

1. If charging current is measured at the starter battery (I_sz ≤ 0.5 A), normal driving operation is set.
   
2. If no charging current (I_BB = 0 A) is measured at the vehicle battery, the isolating relay remains closed. Deactivation does not take place in the case of discharge current of < 0.5 A.
   

Check Control messages

Auxiliary module 1 can issue the following Check Control messages for display on the Control Display:

• Disconnect charge cable
  
• Starter line fault
  

Diagnosis

All inputs and outputs that are part of the auxiliary module 1 functions can have their status checked by diagnosis. The outputs can be activated individually.

In the event of a fault, an appropriate entry is made in the fault memory and, if appropriate, a Check Control message is issued.

Notes for Service department

Charging the Battery

A a battery charger can be connected in the engine compartment to the battery grounding point or directly to the battery at the back. The power module detects an external battery charger after 1 hour and a battery voltage > 13.2 V without engine operation. Following detection of external battery charging, a battery charge level of 80% is reported to the diagnosis function even if the charge level is higher or lower than that figure.

Battery charging function, charging socket

A trickle charger can also be connected to the charging socket.