2003 Dummy Light versus Gauge

Recently my dummy battery light has come on. I checked the voltage at the battery with the truck not running and it was 12.4 volts. I started my truck and again checked the voltage at the battery and it was 11.2 volts. This indicates to me that the alternator is not charging the battery and thus the alternator needs replacing. The funny thing is that the gauge in the dashboard for the alternator/battery indicates all is well. I have a new alternator on order, but I am still confused as to why the light is on, but the gauge indicates that the alternator is charging properly. Any thoughts?

Thanks.

MT

IIRC, the light measures the voltage at the battery. So if the battery is not dead or dying, then the light won't come on. You can go a couple days with a bad alternator but it would eventually kill the battery.