1991 ford ranger battery light

Another way you can test the to see if it is the alternator or battery is to take a multimeter and touch it to the battery terminals. With the truck off the meter should read at least 12.4 volts. With the truck on it should read between 13 and 14 volts. If when the truck is off it is lower than 12 volts its the battery. If it reads less than 13 volts when the truck is on, then it is the alternator.

Good luck!

Bad battery module for the light or a dieing cell in battery

batterys can be tested and wont show a dead cell at all but you will be told that the battery good
most batterys are refurbished batterys sometimes getting the luck of a good battery is 50/50 chance

heres a way to test the battery and alternator at the same time
start vehicle
disconnect battery positive post
if vehicle dies the alternator is at fault if vehicle doesnt die its the batterys life dead cell