A battery light could come from a few problems. The first thing to check is your voltage. If you have 13.5-14.7 volts with the engine on, and no accessories on, then you have a good regulator. If you turn on your headlights, and the voltage goes below 13 volts, then you could have either a bad regulator, or a bad diode. A battery light is usually an indicator of the voltage going either too high, too low, or a shorted diode. A blown diode will decrease your available output voltage, and you will have a light on at low RPMs, but will usually turn off at high RPMs. This is the opposite of what you are experiencing. A shorted diode will usually decrease your output at any RPM range, and cause a battery light, since it is puttting a load on your alternator's output on one of the stator's windings. Do you have an internal, or external regulator? If you have an external one, try to swap the regulator with a new one. If you do a voltage test, and it appears OK at idle, and with a load, try to increase the RPMs to the point when you will get that light to come on. I'm sure that you will see some sort of voltage drop if there is an actual problem with either the regulator, or diodes. If you have no output at all, and your voltage is below 13 volts, then your brushes are bad.
