Explanation to Question 1:
Testing the charging voltage is the best way to verify the condition of the generator and the charging system.
It's important to differentiate between voltage and current tests when discussing generators and charging systems. Voltage follows current, not the other way around. To have generator current, you must have generator voltage. Without voltage there is no current. You can't get current from a dead (0.0V) battery because there is no voltage to push/pull the electrons. You can't get current from a dead (0.0V) generator.
Test the generator on the vehicle by watching the output voltage under all operating conditions of the vehicle's electrical system. If the vehicle's electrical loads are drawing generator current and the generator output voltage is staying high enough the generator is doing its job. The actual current the generator is generating is not the issue. If the electrical loads work and the charging voltage is good, the current has to be there. It's Ohm's Law and George knew what he was talking about.
Explanation to Question 2:
High ambient temperature lowers generator output voltage because the battery charges easily when warm. If the charging voltage did not decrease, the battery would be severely damaged from overcharging. The only reason the charging voltage changes with temperature is to meet the battery's charging needs, which changes with temperature changes.
Explanation to Question 3:
The opposite applies at low ambient temperature. Batteries do not charge very well when cold so the charging voltage rises in cold weather to ensure the battery charges properly.
Explanation to Question 4:
High electrical load lowers generator output voltage. It's just like placing a load on a battery and watching battery voltage decrease. If the electrical load is very heavy, charging voltage may be lower than normal and undercharge the battery.
Explanation to Question 5:
High electrical load increases generator output current and generator heat increases.
Explanation to Question 6:
Low electrical load allows generator output voltage to rise to a set point determined by the voltage regulator.
Explanation to Question 7:
Low electrical load on generator lowers output current and generator heat decreases.
Explanation to Question 8:
Generator/battery charging current should decrease as the battery charges. Internal battery resistance rises as the battery charges up until current holds fairly steady at only a few amps.
Explanation to Question 9:
The two factors that determine battery charging current are battery state of charge and internal battery resistance. Both increase as the battery charges, which lowers the charging current through the battery.
Explanation to Question 10:
If a vehicle has a history of generator failures, it may indicate the battery is at fault. Check battery charging current and note that there is a decrease in battery charging current until a stable reading is obtained that won't overload the generator when all accessories - such as headlights, AC blower, radio and windshield wipers - are turned ON. If battery charging current stays high after several minutes of engine run, the battery may be defective although it has no difficulty starting the engine. Compare the total generator output rating with total electrical system drain to determine if the generator is being overloaded by a battery asking for too much charging current when fully charged.
Actual number of amps a battery should pull when fully charged is hard to determine, but the number should be as low as possible. Check a few other vehicles with similar battery brands and group size to see what other batteries draw from the generator when charged. Then a normal battery draw can be established within 5 amps for the vehicle in question.
Almost all techs check a battery in discharge with a load test. Almost nobody checks a battery in the charging mode, which can put the generator at risk.
Now Let's Test the Charging System
Here's the general idea: Measure the charging voltage first, then load the system and see how low the voltage drops. If the generator is strong, it will maintain sufficient output voltage under load. Then take the load off the generator and let the output voltage find its normal high voltage to make sure the voltage regulator is doing its job. Now let's do it.
- Connect DMM to the battery terminals. Note battery OCV (open circuit voltage). It should be very close to 12.66 volts. It may be higher if the engine has been running in the past few hours because of surface charge on the battery.
- Start the engine and run a warm engine at 1500 rpm. Note the higher voltage at the battery terminals. This is the initial value of charging voltage. It should be in the range of 13.80 to 14.80 volts.
- Turn ON high-beam headlights, AC/heater high blower, windshield wipers and radio. As each load is turned ON, the charging voltage will dip as the generator delivers increasing current to the loads. The voltage should stay above 13.50 volts for a healthy charging system under load. Below 13.10 volts enters the area of a weak charging system. Maybe it's nothing more than a loose generator belt or a corroded connection. If you live in climates where rear window defrosters are often used, add the load of the defroster to see how low the charging voltage dips with the defroster ON. If the charging voltage dips below 13.10 volts, you know the battery isn't getting enough charging voltage to charge while the defroster is ON.
- Let the vehicle come to idle. On most vehicles this amount of electrical load on the generator will pull the charging system below 13.10 volts. If the charging system can hold above 13.10 volts, consider it a strong generator. Now apply the brakes. The charging voltage at this point is what will charge the battery under heavy electrical load while sitting at a stoplight. You need at least 13.10 volts. Lower means the battery is not charging enough under these conditions. If the voltage drops below 12.66 volts it means the battery is actually discharging during this time. If the driver goes through a lot of stop-and-go driving in this condition, a dead battery will eventually result. Don't be surprised to see a lot of vehicles drop below 13.10 volts at a stoplight with the brake lights on and heavy electrical load because the generator only puts out 50 to 60 amps on smaller cars. The charging voltage should rise as the rpms are increased.
- Turn OFF all electrical loads turned ON (high-beam headlights, AC/heater high blower, windshield wipers, radio and defroster).
- Run engine at 2,000 rpm and watch the charging voltage rise. It should rise no higher than 15.10 volts if the weather is very cold. If the weather is very hot the voltage may only rise to about 13.80 volts. How high the actual charging voltage gets at this point is determined by the engineers who designed the vehicle for a specific OEM battery. The voltage regulator, which is temperature sensitive, determines how high the charging voltage should be - higher in cold weather and lower in hot weather. If the charging voltage continues to rise past the numbers just given, the vehicle is overcharging. The voltage regulator may be defective if there is a bad connection between the generator and the battery. Check battery water and add distilled water if low. Remember to consider ambient temperature when evaluating the charging voltage for overcharge. In hot weather the charging voltage should be lower than in cold weather.
Practice this procedure on several vehicles and write down the numbers to learn how the charging system performs on different vehicles at different ambient temperatures. A little practice makes perfect at checking charging systems on the vehicle.
Answers:
- test generator voltage
- lowers voltage
- raises voltage
- lowers voltage
- increases current
- voltage rises
- lowers current
- decrease
- battery state of charge and internal resistance
- battery charging current
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