Current and voltage at idle – 12 V

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

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. 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 the battery.

How to perform the test

Accessories

PicoScope settings

  • Channel A: ± 20 V DC
  • Channel B: ±100 A DC
  • Timebase: 200 ms/div
  • Sample rate: 1 ms
  1. Connect a BNC test lead to Channel A on your scope.
  2. Install the clips on your test leads and connect them to the battery terminals: black to the negative terminal, red to the positive terminal.
  3. Set the current clamp to the 200 A range.
  4. Connect your current clamp to Channel B on your scope and switch on the clamp.
  5. "Zero" the clamp.
  6. Connect the clamp to the alternator B+ lead.
  7. Start the engine and run at 1500 rpm.
  8. Switch on electrical auxiliaries (headlights, heaters, etc.). Some vehicles may require the engine to run for 60 seconds or more before charging commences.
  9. 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 the flow of the alternator current.

Example waveform

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.

Diagnosis

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

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:

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 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 style o 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.

Technical information

You can read more about alternators in our Online training section under Technical information – The alternator.

AT001-6

Disclaimer
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

    £259.00

  • PicoScope Battery Clip

    £2.75

  • Large Dolphin/Gator Clips

    £10.00

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

    £179.00

  • Premium Test Lead: BNC to 4 mm, 3 m

    £47.00

Help us improve our tests

We know that our PicoScope users are clever and creative and we’d love to receive your ideas for improvement on this test. Click the Add comment button to leave your feedback.

Add comment

Your email address will not be published. Required fields are marked *

Guided test: Current and voltage at idle – 12 V