How to Check the Charging System

Phil Krolick – Automotive

Back to Instructor Webpages

AU3.297 Home Page 

Test Battery - Starting - Charging as one System

Defects in the Charging system may cause the battery to go bad.

Defects in the battery may wear out the starter AND the generator.

Defects in the starting system may wear out the battery AND the generator.

Parasitic Draw may wear out the battery AND the starter AND the generator.

Defects in the Battery and Starting Circuit, or a Parasitic Draw will place added strain on the generator.  This will contribute to a failed charging system.  It is important to check ALL these systems any time you are working with the charging system. 

#1)  Test the Battery

How to Load Test a Battery  How to Use a Conductance Tester 

#2)  Test the Starting Circuit

How to Test the Starting System    Starter Testing PowerPoint 

#3)  Test for Parasitic Draw

Remember to check the fuse in your meter and save the memory.

#4)  Test the Charging System

When testing a charging system, look for:

Too Much Power


Overcharging systems will show over 14.8 Volts.

If the generator puts out too many amps the system voltage will begin to rise.

Symptoms of an Overcharging system are:

How do you check for too much Power?

With the engine running at a high idle, and less than 8 amps coming in to the battery, the charging voltage should be less than 14.8 volts

1)  Run the engine with all electric accessories Turned OFF

You are looking for a system that puts out to much power.  If you have all the accessories turned OFF it is easier to spot an over-charging system. 

2)  Monitor the Amps going to the Battery

Set up your inductive amp probe to accurately measure all amps entering the battery. 

When less than (about) 4 amps are entering the battery the system voltage should remain fairly stable as the battery is only accepting a small trickle charge.  

When over 8 amps are entering the battery, it is still actively charging.  As the battery gets closer to a full charge, either the system voltage will continue to rise, or the amps entering the battery will start to go lower. 

If over 8 amps are entering the battery during your test, you must allow the system to run long enough to see that the amperage is actively reducing (normal system) or see the voltage continue to rise (potential over-charge)

 3)  Monitor the Volts at the Battery

Voltage should be less than 14.8 Volts when the battery is accepting less than 8 amps.

REMEMBER that hot engines should charge at a lower voltage and cold engines charge at higher voltage.  IF the engine is COLD it may show slightly higher than 14.8 volts and have nothing wrong. 

These things can cause Too Much Power to come from the charging system:
     Defective Voltage Regulator
     Volt drop in the Voltage Sensing wire to the Voltage Regulator
     Volt drop in the Voltage Regulator Ground (if it has an external regulator)

Volt drop in the wire that the regulator uses to "see" the system voltage will fool a regulator into over-charging.  If the regulator is external to the alternator, having volt drop on the ground side of the voltage regulator will also cause over-charging. 


Not Enough Power


Undercharging systems will show under 13.8 Volts.

If the generator cannot put out enough amps, the system voltage will begin to decrease. 

Symptoms of an Undercharging system are:


How do you check for not enough power coming from the charging system?

1)  Run the engine at 2,000 - 2,500 RPM

The generator is not designed to put out maximum power at idle. 

2)  Turn on ALL electrical accessories. 

Be sure to place headlamps on bright (unless fog-lamps require lamps to be on dim).  Turn the heater blower motor on high speed. Turn the A/C on MAX (coldest setting).  Turn ON the rear window defogger.  Leave interior lights ON.

Be sure to look for additional, and add on, accessories and turn them on also.

3)  Monitor the Amps going to the Battery

If the battery is undercharged it may be accepting extra amps.  If the voltage is just a little low, but there are well over 8 amps entering the battery the voltage may slowly rise.  Watch it long enough to see if the amps go down or the volts going up.

4)   Monitor the Volts at the Battery 

Battery volts should be over 13.8 volts

REMEMBER that hot systems should charge at a lower voltage and cold should charge at higher voltage.  Testing with a high amp demand on the generator will cause it to really heat up.  IF the generator is HOT, the battery may show slightly lower than 13.8 volts and not be undercharging.

These things can cause Not Enough Power (or no power) to come from the charging system
     Defective Alternator
     Defective Voltage Regulator
     Volt Drop in the Wiring
     Loose (or Missing, or glazed and slipping) Drive Belt
     Extra accessories that require more power than the alternator can generate   

Many problems can cause undercharging.  Do not overlook a loose or glazed and slipping generator drive belt.  Be sure to volt drop the main (B+) generator charge wire and battery.  Also check volt drop from battery negative terminal and the case of the generator. Be sure to do these volt drop tests with all accessories turned ON!

If there is an external regulator see if it is providing a "full field".  "A" circuits should have 1 volt or less on the wire that is controlling the field (rotor).  "B" circuits should have close to battery volts on the wire that is controlling the field (rotor).

You can "full field" many alternators to see if they are capable of charging more.  I recommend you do this only under the supervision of an experienced technician until you get good.  Any time you "Full Field" an alternator make sure the battery voltage stays under 15 volts.  If it gets that high (with all loads ON) you can be sure the generator is not the cause of your undercharge.

Honda Electronic Load Detection (ELD)

 Many Honda systems use Electronic Load Detection or ELD.  On these systems the alternator may look like it is undercharging when it is working just fine!
Link to More details on Honda ELD Charging.


 Wrong Kind of Power

AC Voltage

Check this with an oscilloscope

If the generator puts out A/C voltage the vehicle control modules can make errors.

The Wrong Kind Of Power primarily comes from:
  defective diodes or defects in the Stator windings.

The oscilloscope pattern is often called a "Diode Ripple" .  Practice this with many different alternators to get good at finding and understanding this ripple pattern.

Remember to load the alternator with vehicle accessories.  Diodes may appear good until a heavy load is placed on them.  

Alternators that "Leak" A/C voltage can confuse on-board vehicle computers.  It is not uncommon for a "noisy" alternator to cause the engine to run funny. 


Trouble Free Power

A thorough inspection will help avoid future problems.

These things can be checked to insure Trouble Free Power
Alternator belt tension and condition
    Battery condition, especially corroded terminals, hold-downs, and cleanliness
    Wiring harness condition and routing

Loose belts will cause them to Glaze over and lose traction.
Over tightening belts will wear out bearings in water-pumps, power steering pumps, generators etc.
Belts that are glazed may slip even at the correct tension and do not always squeal

Replace glazed belts.      Replaced cracked belts

Modern belts can wear out without showing any signs of cracking, recommend a new belt if replacing the generator.  (be sure to check the belt tensioner!)

A good article on belts is at:

A good strong light is important when inspecting components to ensure they are not worn, corroded, or loose! 

After Repairs

Any time you make repairs to the charging system it is important to retest:
1) the battery.  2) the starting system  3) the charging system.  4)  Parasitic Draw 

A defect in any of these four areas will cause extra wear on battery, starting, and charging systems. 


Other pages in this section: 1_Welcome | aa-planning-ignore | Alternate Battery Homework | Alternate Start-Charge Homework | AU3.317 Electrical Systems & Engine Performance | Battery Lab 11 | Battery Safety | Begin your Electical & Engine Performance class | Captivate | Charging Systems Lab | CKP - CMP Sensor Lab | Classroom Presentations | Compression Testing | Current Ramp Injectors and Ignition | Diagnose with Diagram and Voltmeter | Final Exam Preparation | Grading | Homework | Homework 2 - Spark Plugs | Homework and Lab Opportunities | How to Disable an Engine | Ignition Lab 12 | Lab #1 Awareness for Lab Safety | Lab ideas | Lab Work Order Instructions | Learn More about Electrical Diagnostics | Learning Tools | Learning Units | Master list On-Line Learning Tools | NATEF Lab Assignments | old - Powerpoint Lessons | OLD - Reading Assignments | old electric lab | old_learning_Units | old_Unit_5 | Online Quiz | Practice with Wiring Diagram and Voltmeter | Professional Wiring Repair | Relay Diagnostics | Save Your Memory! | Scan Tool Intro | Solder | storage | Unit 1 | Unit 2 | Unit 3 | Unit 3 learning materials | Unit 4 | Unit 5 | Unit_6 | Why will this motor not operate? | Wiring Diagram Extra Credit

Other sections in this website: Syllabi | Applied Electrical | Mechanical Processes | HV3.297 Electrical & Electronic Systems | AU3.296 Steering Suspension & Brakes | Introduction to Engine Performance | Introduction to Braking Systems | AU3.316 Drive Train Service
Show all of Phil Krolick's web pages.

Instructors can Logon here