Even a novice automotive technician should know that the primary purpose of the alternator is to maintain battery charge and keep the electrical system working properly. What you may not realize is the significance of the load placed on the engine by an alternator that is charging at full capacity.
The alternator, which is driven by the engine crankshaft (via serpentine belt), can diminish engine performance and fuel efficiency noticeably if allowed to operate at full capacity when maximum voltage/amperage is not required. In fact, if the engine were allowed to maintain idle RPM as the alternator increased its level of voltage output, engine vibration, a rough idle, and possibly even engine stall would occur.
Without getting into the complexities of the automotive alternator and how it functions, let’s address how alternator voltage output and engine adaptation strategy are calculated using the powertrain control module (PCM), body control module (BCM), and/or some type of dedicated charging system module.
Smartphones, smart cars, and smart TVs are relatively new to us but Chrysler began using the engine control module (ECM) to regulate alternator voltage/amperage during the 1980s. So, smart alternators have been around for a while.
Smart charging systems which are controlled using the PCM are often said to have a smart alternator. While many late model alternators are more complex than those used on older vehicles, they are not constructed with an integrated computer. However, some are equipped with electronic clutches (similar to an air conditioning compressor) which are controlled by the PCM. When battery voltage (and ground) are applied to a magnetic coil, it draws the clutch down on the alternator until centrifugal force forces it to engage the alternator pulley. When the voltage is interrupted, the clutch disengages from the alternator pulley and the engine is no longer burdened by the load. In most cases, the PCM or BCM monitor sensor signals which deliver engine speed (RPM), engine load percentage, alternator charging rate (at the alternator and at the battery), and battery temperature. This data is calculated by the PCM in order to determine how much voltage/amperage is needed to keep the alternator operating at peak efficiency – but not above it. The PCM then provides the alternator with the appropriate degree of voltage on the field current terminal. This strategy effectively regulates the output voltage and relieves engine load (caused by the alternator) during times when less voltage is required.
Among the more sophisticated forms of smart charging systems is the Regulated Voltage Control (RVC) system. This system is based upon the theory that the battery can function on an eighty-percent state of charge rate and that charging the battery any more than this is placing an excessive load on the engine. In this type of system, the PCM is programmed to monitor multiple factors before initiating a particular charging rate strategy. These strategies may include:
- Dead or Defective Battery Mode: Initiated when the battery shows signs of being defective or unable to hold a charge
- Start-Up Mode: Implemented when the battery needs more voltage/amperage at start-up
- Fuel Conservation Mode: Used when voltage/amperage requirements are minimized to conserve fuel
- Full Charge Mode: This charging system strategy is initiated when maximum voltage is required; for example when the air conditioner is on or the headlamps are on
Additionally, some vehicle smart charging systems add a deceleration mode that converts excessive voltage/amperage to braking power while allowing the alternator to maximize output without adversely affecting fuel efficiency.
In summary, the smart charging system uses onboard control modules to regulate alternator output so that engine load may be minimized and engine horsepower and fuel efficiency can be maximized.