Service work on the charging system

This section provides useful information and valuable tips surrounding all aspects of service work on the charging system, using a Volvo V60 equipped with Eco Start-Stop DRIVe as an example.

Automatic systems, such as the automatic start-stop system are installed to reduce the fuel consumption and emissions. There are different versions and requirements depending on the vehicle manufacturer. For this reason, highly efficient alternators with overrunning alternator pulley and starters with a reinforced structure for frequent starting processes are installed. The automatic start/stop function can only be guaranteed if all components of the start and charging system are operating correctly.

Important safety information
The following technical information and practical tips have been compiled by HELLA in order to provide professional support to vehicle workshops in their day-to-day work. The information provided on this website is intended for use by suitably qualified personnel only.


Charging system components: Overview

AGM battery with battery sensor

The abbreviation AGM stands for Absorbent Glass Mat (AGM) and means that the electrolyte in these batteries is bound in a glass fiber fleece. AGM batteries ensure higher start and supply reliability, are leak-proof and have been specially developed for vehicles with start-stop systems.


An intelligent battery sensor that has been additionally mounted on the battery's negative terminal monitors the battery condition. The measured values are transmitted to a higher-level control unit via the serial LIN communication interface.

Secondary battery

A second, so-called secondary battery has been additionally installed in this vehicle. Secondary batteries, also known as buffer or back-up batteries, support the vehicle electrical system when the engine has been switched off. The additional supply is managed by a battery control unit and it simultaneously prevents the batteries from discharging each other.

Alternator with overrunning alternator pulley

The alternator supplies power consumers with power while the engine is running and maintains the battery charge. The alternator output depends on the engine speed. The maximum alternator output is only generated above 2000 rpm. A charge regulator is installed in the alternator, which is also referred to as the alternator control unit. The charge controller has been connected to the engine control unit via a LIN interface. By mounting an overrunning alternator pulley only the driving force of one direction of rotation is transmitted to the alternator, thus reducing friction and wear.

Engine control unit

In this system, the charging system is intelligently controlled via the engine control unit (ECM). The central electronics module (CEM) sends a request about the desired charging voltage for the main battery to the engine control unit from where this request is forwarded to the alternator regulator. The charge signal lamp in the instrument cluster is controlled via the CAN network. At the same time, the engine control unit switches on the secondary battery via a relay for the charging process. The charging time of the secondary battery is calculated by the central electronics module and forwarded to the engine control unit.


The starter, also known as the starter motor, is required to start the internal combustion engine. Starters are electric motors that are briefly connected to the engine during the starting process via a ring gear that bring it to the desired starting speed. Depending on the design, the current consumption of the starter can be well over one hundred amperes. By reinforcing individual components, the starter has been designed for an increased number of start cycles over its entire service life

Service work on the charging system: Instructions

Before starting the alternator test, the batteries must be tested first. A simple check of the state of charge can be carried out with a commercially available voltmeter. The open-circuit voltage of the battery can be measured directly at the battery terminals by establishing the voltage. However, for this purpose, all electrical consumers in the vehicle must be switched off first. A good battery must show a voltage of 12.4 to 13.2 volts. 


If the measured value is below 12.4 volts, the battery must be charged and then checked again. In the event that the result after charging is still negative, check the battery with a battery tester and replace it if necessary. 


Before actual troubleshooting or diagnostics, carry out a brief visual inspection of the components and peripherals of the start/charging system in the engine compartment.


The following information and service work are shown on a Volvo V60 featuring Eco Start-Stop DRIVe, but can be applied to any other vehicle model with identical component systems.

Test with a battery tester

In order to better assess the overall condition of a battery, it is advisable to test the battery with a suitable battery tester, such as the BPC-TOOL. After a short time, a statement about the state of charge (SOC) and the state of health (SOH) of the battery becomes available on this tool. If both batteries are OK, as is the case in our example, you can start the alternator test.

Alternator test

First check the belt drive's and the electrical connections' condition and functions. 


Then check the power supply of the alternator. For this purpose, connect a suitable measuring device to alternator terminal B+ and the housing ground on the alternator. The measurement result must be equal to the battery voltage. The setpoint value in this case is the battery voltage with a maximum deviation of < 0.5 volts. If the voltage difference is greater, check the wiring for weak points, such as interruptions or contact resistance.


Checking the alternator charging voltage and charging current
Use a suitable measuring device and proceed as follows to measure the alternator output:

  • Connect the current clamp as closely as possible to alternator connection B+
  • Connect the red terminal (+) of the voltmeter to alternator terminal B+ and the black terminal (-) of the voltmeter to alternator ground
  • Start the engine and let it idle.
  • Then load the vehicle electrical system with approx. 40 amperes by switching on some electrical consumers. This can be achieved, for example, by switching on the low beam, interior blower, air-conditioning system and heated rear window
  • Increase the engine speed to approx. 2000-2500 rpm and read the measurement results


In this process, the control voltage must not drop below the setpoint value (13.7 to 15.0 volts) by more than 0.5 volts and the amperage must be higher than the total current consumption of all activated consumers.


Please observe the corresponding maintenance and repair instructions of the respective vehicle manufacturer for all tests on the starting and charging system!

Testing with the diagnostic unit/control unit diagnosis

In modern vehicles with battery management systems different functions can be opened to run tests in the vehicle, depending on the vehicle manufacturer and system. In our sample vehicle, any occurring faults in the charging system, engine control unit, and central central electronics module are recorded and stored in the fault memory. The alternator can be diagnosed either using the engine control unit as well as the central electronics module.


Many vehicle systems additionally provide digital measured values as parameters to enable fast diagnosis. Parameters indicate the current status or setpoint and actual values of the component. In this example, information about the IBS sensor and the battery's state of charge can be accessed.


The various diagnostic options have been illustrated using the mega macs 77 diagnostic unit as an example. The respective test depth and variety of functions can be set out differently depending on the vehicle manufacturer and is dependent on the relevant system configuration of the control unit.

Checking the charging system: Video

Service work on the charging system - Volvo V60

In this video we will show you the visual inspection before diagnostics, battery inspection, alternator inspection and how to perform an inspection using the mega macs 77 diagnostic unit.

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