Alternator - current and voltage at idle - 12 V

The purpose of this test is to assess the charging rate of the alternator in relation to the electrical load on the battery.

For testing of Smart Alternators, refer to the pre-sets for Smart Charging and the Smart Charging Help Topic.

Connection guidance

Connection for diagnostic work will vary dependent on application.

Technicians should whenever possible gain access to the test circuit without damage to seals and insulation. If this is not possible then make sure appropriate repairs are completed.

General connection advice

PicoScope offers a range of options within the test kits.

Dependent on difficulty of access, choose from:

  1. Breakout leads.
  2. Back-pinning probes.

Testing sensors and actuators (to include relevant circuit/connectors):

  • When testing a sensor, it is desirable to gain access at the control unit.
  • When testing an actuator, it is desirable to gain access at the actuator.

How to perform the test

  1. Connect PicoScope Channel A.
  2. Connect and zero the high amp current clamp to Channel B before connecting to the vehicle.
  3. Start the engine and run at idle.
  4. Switch on electrical auxiliaries (headlights, heaters, etc.).
  5. Minimise the help page and with the example waveform on your screen PicoScope has already selected suitable scales for you to capture a waveform.
  6. Select GO or press the space bar to see live data.

Note: If the current Channel B is reading a negative value, check that the arrow located on the inside of the jaw on the current clamp is facing the direction of the flow of the alternator current.

Waveform notes

Channel A (Average 14.45 V) indicates a stable battery voltage with no spikes or significant ripples.

Channel B (Average 48.63 A) indicates the current flowing into the system with even peaks.

Refer to vehicle technical data for specific test conditions and results.

Waveform Library

Further guidance

Typical values:

2 x headlamps at 60 watts 120 watts
Heated rear screen 120 watts
Total 240 watts
240 watts divided by 12 volts = 20 amps.

In a typical example, there should be a minimum of 20 amps output from the alternator when the engine is running, with the headlamps and the heated rear screen switched on.

The recommended regulated voltage varies slightly between motor manufacturers but 12-volt vehicles are typically between 13.6 and 15.0 volts. It is equally important that the system is neither under- or over-charging.

The current available from the alternator may vary depending on the type of alternator installed, engine speed, battery load, state of charge and drive pulley arrangement.

PicoScope can reveal some alternator faults, such as a faulty diode, that cannot be detected by simply checking the output current or the regulated voltage. Please refer to our Guided Test Alternator AC ripple/diode test for this additional test procedure.

You can read more about alternators in our Online training section under Technical information – The alternator.


This help topic is subject to changes without notification. The information within is carefully checked and considered to be correct. This information is an example of our investigations and findings and is not a definitive procedure. Pico Technology accepts no responsibility for inaccuracies. Each vehicle may be different and require unique test settings.

Suitable accessories

  • 200 A / 2000 A (high amps) DC current clamp


  • PicoScope Battery Clip


  • Large Dolphin/Gator Clips


  • Premium Test Leads: Set of four leads 3 m (TA125 - TA128)


  • Premium Test Lead: BNC to 4 mm, 3 m


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Guided test: Alternator - current and voltage at idle - 12 V