Holy cow, you are driving a car with fault lights actively on, and they are telling you that the electrical system is on the fritz, yet you drive anyhow?! That's the definition of insane. You should commit crimes, and point to this as a defense for your actions.

The BATTERY LIGHT is not a battery light at all, it is a CHARGING SYSTEM light. It _always- means that the alternator is not producing sufficient electricity to maintain vehicle operations, and that the vehicle is operating off of battery power. It will operate fine, until the battery is dead. You know it is dead when non-essential systems, such as the radio and the AC systems shut off as the car tries to keep enough power to keep the engine running.

I was driving in my F150 several years ago and the "battery light" came on. I thought I had about 30-ish minutes of operation before the truck shut down, and it was about 5 minutes to my destination, and 10 to 15 minutes home. I guessed wrong. Within a minute or two, the gauges on the dashboard stopped working and the radio switched itself off. I was able to make it to the right lane so I could park at the curb, but once the charging system failed, it was not long before the battery was toast. I assume a fresh battery might last longer than an old one. When I took the alternator out, one of the two brushes inside was worn down to a nub, and it was not capable of collecting electrical current from the stator.

An alternator makes AC current. There is a voltage regulator -- bridge rectifier circuit -- that converts the AC to DC so that the vehicle systems can use it. Some cars used to have generators that made DC current, but the industry has gone with alternators instead of generators. I do not know the reason why. Perhaps an alternator is cheaper to build, even with the cost of the voltage regulator added on.

The charging system light can mean that the alternator itself has failed, or that the voltage regulator has failed. Some cars allow the VR to be replaced separately from the entire alternator -- BMW is among them. You can buy the VR for less than half the cost of the entire alternator. An alternator is a far better buy as a new part as opposed to a rebuilt one.