Back in the the 80's and 90's there was one battery tester on the market that most new vehicle manufacturers recommended.  The Vat-40 and there was a reason why the tester worked and provided consistent and reliably results.  Even today with the new capacitance battery tester we still find the most reliable results from the  Vat-40 battery tester


An overview of the battery test procedure.


Read the CCA rating of the battery

Load the battery for 10 seconds at half the CCA rating. to remove the surface charge.

Wait 1 minute then load the battery again at half the CCA rating for 15 seconds.  If the voltage at the end of the time is above 9.8 volts the battery is good.  If the voltage is between 8.5 and 9.8 volts then you proceed to the second part of the battery test.  

The three minute test

 Place a battery charger on the battery at a 40 amp charge rate.  If the battery voltage goes above 15.5 volts at any time over the course of three minutes then the battery is bad.  This part of the procedure test the battery's ability to take a charge and when failing usually indicates sulfated plates.

If the load voltage goes below 8.5 volts then the battery needs to be charged before it can be tested.

To test the Alternator.

Most alternators put out 90 -130 amps.  The output is lower on a hot alternator.   Before we test the alternator we need to find out what the manufacturers out spec is.  We need to put the amp probe around the alternator output cable and then raise the engine speed to 2k rpm, load the battery until the voltage drops to 12 volts and read the alternator output current.  The other problem and alternator could have is a shorted diode.  A shorted diode can cause a draw in the electrical system and casue the battery to go dead if the car sits for a day or two.


The last thing the Vat-40 is usually used to test is the starter and starting system.