As vehicle electric systems become more and more complex, simple alternator regulation is usually no longer sufficient. For this reason, some vehicle manufacturers (e.g. Ford, BMW, Mazda, Peugeot) have integrated so-called intelligent alternator control units into their charging systems.
While in a conventional alternator, the built-in control unit determines the alternator rated voltage, this task is taken over by the engine control unit with these new systems. The function is explained below using a system built into a Ford vehicle as an example.
Ford “Smart Charge” alternator control
The alternator built into this system does not look significantly different from conventional alternators. In this case, there is also a voltage controller on the rear of the alternator. This controller is connected to the engine control unit (PCM) by means of two signal cables. Pulse-width modulated signals are transmitted between the alternator and PCM via these cables for communication purposes. On the basis of this information, the PCM monitors and controls the charge voltage. The function of the charge control lamp in the instrument cluster is controlled by the PCM. For the function test, the control lamp is switched on after the ignition has been switched on, and switched off again after the engine has been started provided the system function is fault-free.
1. Charge voltage regulation and calculation of battery temperature
Since warm batteries are more efficiently charged with low voltage and cold batteries with higher voltage, the charge voltage is adapted by the PCM depending on the battery temperature. Reference parameters for calculation of the battery temperature are the intake air and the coolant temperature.
The battery charging current is optimised through permanent calculation of the battery temperature and adjustment of the alternator output voltage.
2. Alternator switch-off when engine starts
When the engine is started, the alternator is deactivated by the PCM to reduce the engine drag torque. The alternator is only switched to the required value electronically by the PCM after the engine has been started up.
3. Increasing idling speed at low voltage and high electrical load
When the battery charge is very low or the electrical load in idling is very high, the PCM can gradually increase the speed up to 150 rpm in order to increase alternator performance.
4. Advance notification function for alternator load
The PCM receives information about imminent electrical load from the alternator via the signal cable, and can thus compensate the expected alternator torque by increased idling speed.
The PCM can guarantee greater idling stability on the basis of this information. By monitoring the vehicle electric system voltage, the PCM can change the charge current by changing the pulse-width modulated signal to the alternator.
5. Activating or deactivating electrical consumers
By linking the PCM with the central electronics module, the following consumers are activated or deactivated in the event of over-voltage or under-voltage depending on battery voltage:
- Heated windscreen
- Heated rear window
- Air-conditioning system (if appropriate)
- Auxiliary heater (if appropriate)
The low-voltage limit is around 10.3 V and the over-voltage limit is around 16.0 V.
Damage caused by excessive vehicle battery charge is reduced by activating individual consumers, and at the same time the charge voltage is kept within the specification. Switching the consumers on increases the engine load and thus serves to support the control unit at the same time in the warm-up phase. If the battery voltage falls below the limit value, the consumers are deactivated again to prevent excessive discharging of the battery.
6. Diagnosis and limp-home function
The diagnosis possibility of the Smart Charge system is implemented through the engine control unit (PCM).
System faults are stored in the engine control unit and can be read out using a diagnostic unit. After the ignition has been switched on, the system carries out a self-test. If a fault is detected in the “Smart Charge” system during the self-test, the charge control lamp is not switched off. The alternator is operated at a fixed charge voltage of 13.5 V if voltage regulation is not possible due to the fault. This enables the alternator to generate enough current to supply the vehicle systems.
During driving, the charge control lamp is only switched on if the PCM detects the following faults:
- Impermissible voltage
- Internal alternator fault
- Communication fault PCM/alternator
PCM: Engine control unit
GEM: Control unit for the central electronics
a: Communication cable for consumer control (CAN)
b: Communication cable for charge control lamp (CAN)
c: Monitoring signal alternator function
d: Control signal alternator cable