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.

How to perform the test

  1. Connect PicoScope Channel A and Channel B.
  2. Start the engine and run at idle.
  3. Switch on electrical auxiliaries (headlights, heaters, etc.).
  4. Minimise the guided test screen, select GO or press the space bar and start capturing in PicoScope.

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.

Example waveform

Waveform Notes

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

The library has more than 50 examples of alternator current and voltage testing, illustrating both good and bad waveforms across a wide range of vehicles.

Go to the Channel details dropdown bar at the bottom left of the library screen and select "alternator".

Technical information

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: Current and voltage at idle - 12 V