Current and voltage at idle – 24 V

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

The purpose of this test is to assess the charging rate of the alternator in relation to the electrical load on the battery for a 24V system.

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 module.
  • When testing an actuator, it is desirable to gain access at the actuator.

How to perform the test


PicoScope settings

  • Channel A: ± 50 V DC
  • Channel B: -100 A +200 A DC
  • Timebase: 2 s/div


  1. Connect a BNC test cable to Channel A on your scope.
  2. Install clips to your test leads and connect them to the battery terminals: black to the negative terminal, red to the positive terminal.
  3. Connect your current clamp to Channel B on your scope and switch on the clamp.
  4. "Zero" the clamp.
  5. Connect the clamp to the alternator B+ lead as in Figure 1.
  6. Start the engine and run at 1500 rpm.
  7. Switch on electrical auxiliaries (headlights, heaters, etc.). Some vehicles may require the engine to run for 60 seconds or more before charging commences.
  8. Start capturing in PicoScope Automotive by clicking the Go button or pressing the space bar or your keyboard.

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 alternator current flow.

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