Current and voltage at idle – 24 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 current clamp to PicoScope Channel B, switch on and zero the clamp.
  3. Connect the clamp onto the battery positive lead, observe the current directional arrow, if incorrect the waveform will be inverted.
  4. Start the engine and run at idle.
  5. Switch on electrical auxiliaries (headlights, heaters, etc.).
  6. Minimise the help page and with the example waveform on your screen PicoScope has already selected suitable scales for you to capture a waveform.
  7. Start the scope to see live data.
  8. With your live waveform on screen stop the scope.
  9. Turn off the engine.
  10. Use the Waveform Buffer, Zoom and Measuring tools to examine your waveform.

Example waveforms

In the waveform below, Channel A indicates a stable battery voltage with no spikes or ripples. Channel B indicates the current flowing into the system with even peaks.

Note: The periodic spikes evident can be attributed to induced spikes from the injection circuit and are of no concern.

Waveform Library

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

Further guidance

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

In a typical example, there should be a minimum of 10 amps output from the alternator when the engine is running and the headlamps and heater blower are switched on max:

Typical values:

2 x headlamps at 60 watts 120 watts
Heater blower max speed 120 watts
Total 240 watts
240 watts divided by 24 volts = 10 amps.

The recommended regulated voltage varies slightly between motor manufacturers but 24-volt vehicles are typically between 26 and 28 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 style of the charging system, 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 Lead: BNC to 4 mm, 3 m


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


  • Premium Test Lead: BNC to 4 mm, 5 m


  • Premium Test Leads: Set of four color leads 5 m (TA199 - TA202)


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Guided test: Current and voltage at idle - 24 V