Alternator voltage and current – 12 V

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

How to perform the test

View connection guidance notes.

  1. Connect PicoScope Channel A to the vehicle battery terminals.
  2. Connect the high amp clamp to PicoScope Channel  B, switch on and zero the clamp.
  3. Connect the clamp onto the battery positive lead.
  4. Start the engine and run at idle.
  5. Switch on electrical auxiliaries (headlights, heaters, etc.).
  6. Minimize the help page. You will see that PicoScope has displayed an example waveform and is preset to capture your waveform.
  7. Start the scope to see live data.
  8. With your live waveforms on screen stop the scope.
  9. Turn off the engine.
  10. Use the Waveform Buffer, Zoom and Measurements tools to examine your waveform.​


The orientation of the clamp relative to the wire will determine whether it has a positive or negative output. If a live waveform does not appear on your screen, or appears to be inverted, try reversing the orientation of the clamp.

Example waveform

Waveform notes

These known good waveforms have the following characteristics:

  • A stable battery voltage (Channel A) around 14.5 V, with no spikes or significant ripples.
  • An AC waveform (Channel B) with a DC average around 48 A and even peaks and troughs.

Waveform Library

Go to the drop-down menu bar at the lower left corner of the Waveform Library window and select, Alternator voltage or Alternator current

Further guidance

The function of the alternator is to provide energy to power any electrical systems operating when the engine is running. Furthermore, the alternator must have sufficient output to replace charge within the battery after cranking and engine start.

Increases in alternator output have a number of adverse effects on vehicle operation:

  • The requirement for a stronger magnetic field places more resistive torque on the engine, a drain on engine output which leads to an increase in fuel consumption and an inevitable increase in emissions. This has led to the development of smart alternator systems (please see our guided test for smart alternators).
  • Heat becomes an issue for the alternator, with manufacturers using ducted air cooling or water-cooled alternators to regulate their temperature.

An alternator with a faulty control regulator can increase voltage proportional to RPM, causing damage to the battery and to connected electrical systems.

Symptoms / causes of a faulty alternator;


  • Noise and vibration from dry or damaged bearings due to dirt ingress or heat
  • Loss of output from loss of electrical contact between brushes and slip rings due to frictional wear.


  • Reduced or loss of output due to heat / vibration effects on windings and rectifier
  • Excessive output leading to component and battery damage due to faulty control unit, again caused by heat or vibration.


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


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