Why do the voltage battery light keep coming on after a new battery has been installed?

Ah, you have made one of the most common and easiest mistakes to make in automotive maintenance. Because the 'battery voltage' idiot light came on, you assumed that the battery was bad. Did you have an automotive electrical specialist test the electrical system on your car? Did you have the battery tested at a battery shop? Did you know that there are 3 primary parts to an automotive electrical system, and anyone of them can be responsible for the system operating below required voltage levels? Most modern automobiles generate electricity with a device called an alternator, which is driven by a belt from the crankshaft. The alternator generates Direct Current, at voltages up to 20 volts, in some cases. To keep the alternator from frying the battery, a Voltage Regulator is used to control the output of the alternator. This insures that the battery is only charged with a current of 18 volts, or less. Most modern alterators have the voltage regulator inside, but on older cars, the voltage regulator is mounted on the fender or the firewall, somewhere in the engine compartment. So, the alternator generates the electricity used to charge the battery, but the voltage regulator controls the output of the alternator. If the alternator is going bad, it may not be generating enough voltage to charge the battery. The battery is usually a 12 volt battery, and must see more than 12 volts to charge. Or, the alternator output might be reduced to an unusable level by the voltage regulator. Before you replace the entire electrical system, take the car to an automotive electrical specialist for testing. They can determine exactly which component is not working, and repair it. (Note: If the voltage regulator is bad, this DOES NOT mean that the entire alternator must be replaced! In almost all cases, the voltage regulator can be replaced by disassembling the alternator. If the specialist insists that the alternator must be replaced, I would recommend getting a second opinion, even though testing is often not free. Voltage regulators are fairly cheap, and can be replaced by a competent technician without too much difficulty. Alternators are expensive.)