How do external alternator control units work?

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.

Functional scope

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

Schematic diagram

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

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5 Comments

Filed under Electronics, External Alternator Control Unit

5 responses to “How do external alternator control units work?

  1. lee

    What voltages are measured at “c & d” with respect to the alternator charging and not charging? Thank you

    • Hi lee,

      The only value that can be read out is the PMW signal.

      However, there are no actual values or debit values available, as the car manufacturer did not describe it.
      Mostly the garage or workshop does not need this information, as the diagnostic possibility is implemented in the ECU and can be read out using a tester.

      * In some cases only can be read with a Ford tester (depending of the ECU software)

      Pete

  2. Volmar

    Very interesting description of the Smart Charge System. I currently have the Ford Galaxy 2.0 Ecoboost with this kind of charging system. I have a 12V digit voltmeter to see how the voltage area is driving. For certain time I had a voltage of >13V but sometimes around 12V (and very short time while driving I also had just 11.8V). Then it becomes cold weather I had voltage range between 14 and 15.xV.
    What I am worrying is that I had voltage value of 5°C). No alert in my cockpit where showed (no charging error symbol). My question is, is my voltage range in an acceptable range or there might be a charging error (although no error messages in my Convers+)?
    Regards
    Alex

  3. Volmar

    ** Sorry some typo correction **
    Very interesting description of the Smart Charge System. I currently have the Ford Galaxy 2.0 Ecoboost with this kind of charging system. I have a 12V digit voltmeter to see how the voltage area is driving. For certain time I had a voltage of >13V but sometimes around 12V (and very short time while driving I also had just 11.8V). Then it becomes cold weather I had voltage range between 14 and 15.xV.
    What I am worrying is that I had voltage value of <12V. No alert in my cockpit where showed (no charging error symbol). My question is, is my voltage range in an acceptable range or there might be a charging error (although no error messages in my Convers+)?
    Regards
    Alex

    • Hi Alex,

      The battery charging voltage must be higher in cold weather and lower in warm weather in order to accommodate the chemical processes which take place inside the battery.

      Normally, the charging voltage will be up to 14.2 V, especially when the engine is running without much load. With some load on the engine, the voltage may drop a bit.

      If the voltage shows below 12 V constantly while the engine is running (without much load), I would advise that you have the charging system checked. There can be a few causes for voltage dropping below 12 V while the engine is running. (e.g, wear & tear on the alternator components, poor connection, etc).

      Hope this helps.

      Pete

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