Testing Alternators and Car Batteries

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I have had numerous request about testing car alternators and batteries.
Here are a few Simple Tests to determine if these parts are OK,
and all that is required is a Digital Voltmeter and probably Two people.
One to watch the meter and one to start and stop the car.

USE CAUTION: To Prevent getting to near the Fan Blade or other moving parts,
or Accidently Shorting out the battery across a Ring on your Finger.


One Simple Battery Test:

1) Connect the voltmeter directly across the "Battery Terminals". (NOT to the Cable Clamps) 2) While watching the meter, Start the Car. 3) The Battery Voltage should Never fall below about 10 Volts. If it does: a) The battery is either "Low in Charge". b) Or The Battery is "Low in Water". (The water leven in the battery should be about 1/4 of an inch, Above the Plates.) "DO NOT" Fill it up FULL. c) Or The battery has a "High Internal Resistance". (Possibly Sulfided Plates and probably Time for New Battery?)
**NOW Repeat This Test BUT: 1) Connect the voltmeter directly across the "Battery Clamps on the Battery Cables". 2) This Reading should be Within 0.1 Volt Maximum, the Same as 3 Above. ***If NOT, Clean Battery Terminals and Clamps. 2a)If Necessary, Repeat Test to Varify it is now OK.
**NOW Repeat This Test Again, BUT: 1) Connect the Voltmeter Directly Across the Positive Terminal of the Starter and it Case Ground. 2) If the voltmeter reading is More than 1/2 Volts Lower than the Previous Test, ** You Probably have a Cable or Connectorp Problem! Check the Connections between the Cable and the Cable Connectors. ON BOTH ENDS.

One Very Simple Alternator Test:

1) Start Car and run at a normal idle. 2) Connect a Voltmeter Directly Across the battery Terminals. 3) Turn on Headlights to High Beam. 4) check the battery Voltage. *** The Battery voltage Should read between about 13.8 to 14.2 volts. (Possibly up to 14.5 Volts.) 5) Increase motor speed to about 1500 RPM. The Alternator should now produce a battery voltage between 14.2 and 14.6 Volts. *** If it is Lower than 14.2 volts, the Alternator probably has one or more Bad Diodes. 6) Now, Maintain the 1500 RPM and Turn off the Headlights. Run this test for One Full Minute. "The Battery Voltage should NEVER Exceed about 14.8 Volts". ***If it Does, the Alternators Voltage Regulator is probably Faulty, OVER CHARGING the Battery.

These Test are "Meant to be a Guideline". The Voltage Measurements above may not be accurate in all cases. For Example: "A Bad Starter Could Easily Pull the battery BELOW that 10 Volts". It is Impossible to take into account, All Possible Situations.

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