Make sure that battery is fully charged with no “surface charge”.
12.4 Volts indicate that the battery at least 75% charged.
12.6V is a 100% charged battery.
Battery may have a “surface charge” which will make load testing a battery ineffective. Usually with open circuit battery its more than 12.6V.
To remove a “surface charge”, turn on your headlight for 1 minute.
Wait for 2 minutes allowing the battery to recover and cool down.
After that you can do a usual 15 seconds load test.
Start the car while monitoring battery voltage and cranking amps.
Most 4 and 6 cylinder engines will drow close to 500 amps when starter solenoid contacts close.
If at cranking, battery voltage stayed above 8.5V, batter passed the load test.
Minimum Battery Charge Rate Test
Connect the amp clamp at the negative cable of the battery. This will read only current that alternator suppliers for the battery, not for engine operation and accessories.
Make sure the battery is fully charged and accessories are off.
Start the vehicle and scope a minute of data about alternator output current and voltage.
Wait until alternator brings battery back into a full charge state (usually within a minute).
Watch the minimum charge rate. Most all new batteries will have a charge rate of 2 to 3 amps after the battery is full charged. At times, a sulfated (bad) battery can have a charge rate of 10 to 15 amps, or higher and can never  be satisfied. If this happens, this is a sure way to kill a good generator.
Components
Battery current sensor
Rough idle? Check load on a battery upstream of a battery current sensor, bypassing the sensor and forcing computer to use insufficient charging strategy. Subwoofer power cable?
Tip. If Alternator is not charging, it can be a failed alternator pulley (OAP or OAD)
Test. Rotate alternator pulley in normal operation direction. The pulley should lock up and turn alternator shaft
Rotate alternator pulley in opposite direction, it should spin smoothly without the alternator shaft turning
Rotate the pulley in the free spin direction, it should feel smooth not rough
Important! Donât test alternator by disconnecting a battery. It creates a large voltage spike called âload dumpâ that can destroy sensitive electronics.
The windings of an alternator have a large inductance.
When the vehicle battery is being charged, the alternator supplies it with a large current, the magnitude of which is controlled by the current in the field winding.
If the battery becomes disconnected while it is being charged the alternator load suddenly decreases.
However the alternator’s regulator cannot quickly cause the field current to decrease sufficiently, so the alternator continues to generate a large current. This large current causes the voltage on the vehicle bus to increase significantly — well above the normal and regulated level.
Rough idle? Check if there is a load on a battery upstream of a battery current sensor, bypassing the sensor and forcing computer to use insufficient charging strategy. Subwoofer power cable?
Inverted installation of the current sensor.
Battery Type
Conventional lead-acid battery is not suitable for Ford’s Smart Charging systems. Only silver-calcium battery is.
Intake/battery temperature sensor
Colder battery – higher charging output
Vehicle Specific Inputs:
RPM monitoring
No RPM signal during engine running – no charge (1993 Dodge Ram 5.9L DSL)
MISC
Electrical noise?
Wrong spark plugs
On 2005-2008 Corvette alternator may stop charging at 4000 rpms and above. This can be caused by racing spark plugs that induce high voltage potential to an adjacent alternator sense wire. GM doesn’t know if that situation had caused voltage regulator to be confused or it made it think that the system is overcharged causing it to logically lock-up. OEM spark plugs fixed the problem. Note: this system should not stop charging unless excitation is removed.
