Dim warning lights can mean a bad diode in the alternator. This causes the output to drop below battery voltage for short periods of time, too short for a meter to register. A scope on the alternator output will show either negative going spikes (short) or flat spots (open) in the output waveform. On average, the bulbs are receiving a small amount of current from the battery. Some auto electrical service manuals show pictures of these waveforms. The alternator will be able to maintain the battery under light load conditions but will not be able to deliver its' maximum rated output. The battery light will glow brighter right after starting or with heavy electrical load.