Testing 24V alternator current and voltage at idle

You will require a PicoScope to perform this test. A list of suitable accessories can be found at the bottom of this page.

For Smart Alternators test refer to the pre-set 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 placed upon the battery. Any charge taken from the battery must be replaced by the alternator. The output of the alternator is regulated and must neither under- or over-charge.

How to perform the test

PicoScope settings

  • Channel A: ± 50 V DC
  • Channel B: -100 A +200 A DC
  • Timebase: 20 ms/div
  • Set current clamp to 200 A range
  1. Connect a BNC test cable to Channel A of your scope.
  2. Install clips to your test leads and connect across the battery terminals: BLACK to negative, RED to positive terminal.
  3. Connect your current clamp to Channel B of 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 and heaters etc). Some vehicles may require the engine to run for 60 seconds or more before charging commences.
  8. Minimise this help file to reveal an "example waveform". From here you can run your scope by pressing either the space bar on your keyboard or the "start" button in PicoScope to start looking at live readings.

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

Example waveforms

In the waveforms 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 on the 24 V waveform can be attributed to induced spikes from the injection circuit and are of no concern.


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

Example: In the typical example to the right there should be a minimum of 20 amps output from the alternator when the engine is running and the headlamps and heated rear screen are switched on:

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.

The recommended regulated voltage varies slightly between motor manufacturers but typically for 12 volt vehicles is between 13.6 and 15.0 volts, and for 24 volt vehicles 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 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 Automotive tab on your PicoScope screen and select 'Charging and Starting > Charging Circuits > Alternator AC Ripple/Diode test' for this additional test procedure.

Technical information

The alternator

The alternator, as the name implies, produces an alternating current (AC) output, which is rectified to direct current (DC) to provide the correct type of voltage to replenish the battery, keeping it at full charge.

The field current, approximately six to eight amps, energizes the rotor which then induces electric current in the stator as it rotates. The rating of the alternator tends to be vehicle-specific, as a base model has less electrical demand than a vehicle with typical top-of-the-range accessories such as electric front and rear heated screens, heated mirrors, additional lighting, heated and electrical adjusted seats etc.

The alternator stator has three internal windings wound 120 degrees between phases and requires nine diodes in 'bridge' configuration to rectify the output. The voltage is controlled by a solid-state regulator that maintains the output in the range stated in the notes above. The output current is determined by the requirement at the time. For example, a battery that has just been subject to prolonged cranking draws a higher output from the alternator than when the battery is fully charged.

A good charging system has the following characteristics:
A fall in battery voltage should be accompanied by an increase in charging current and vice versa.

The regulated voltage can be measured on a multimeter, but this reading can appear correct even if the alternator has a diode fault that reduces the output by 33%. The only true way to monitor the alternator output is to observe the output voltage and current waveforms using an oscilloscope.


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 / Volts idling (24 V)