Using the example above, let's assume that an after-market, 400 watt @ 69% efficiency high-power audio system, 20 amp electric winch, or 276 watts of lights is installed that adds an additional 20 amps of load each. With a total electrical load of 34 amps, at RPM below 2500, the battery will never be recharged with an 90 amp system. While the engine is running in this case, the battery must make up the deficit. The solution is to upgrade the charging system to 120% or more of the new worst-case load. In this example and based on stop-and-go driving habits, a high output charging system capable of 105 amps or more would be required to keep the battery fully charged. High alternator temperatures can further reduce the maximum output of a charging system, so cooling and sizing based on the continuous load matters. kills alternators, so Bosch, for example, has water cooled models available.
If the battery in your car is bad or near the end of its useful life, the alternator may not supply the required amount of alternating current (AC) to keep the vehicle operating properly. If any other components in the charging system are malfunctioning, the entire system will suffer from poor performance or system failure.
Batteries almost never fail at a good time. To prolong battery life, battery, terminals and cable ends should be kept clean and free from corrosion. The battery and starting/charging system should be periodically tested for proper performance. Many testers utilized today are able to predict when a battery is near the end of its useful life.
The vehicle's electrical load is normally satisfied first by the charging system and then any remaining power is used to recharge the battery. For example, if the total electrical load is 14 amps and the charging system is producing 35 amps at 2500 RPM, then up to 11 amps will be available for recharging the battery, which will take approximately six minutes. If the charging system is operating at say a maximum capacity of 90 amps at 5000 RPM, then the battery usually will be recharged within two minutes. Now let us assume that the engine is idling and the charging system is only capable of producing 10 amps. Four amps from the car battery are required to make up the difference to satisfy the 14 amp electrical load and the battery is being discharged further. This is why making short trips, driving in stop-and-go traffic, or during bad weather when there is a heavier electrical load, the starting battery may never get recharged and may even become "completely" discharged.