98 Plymouth Voyager Battery Light Coming ON

First of all, It sounds to me you have a problem with current drain some where, I would inspect the alternators wiring, Also battery terminals wiring for corrioded breaks into the wires, If their is any kind of breaks would cause problems, Get some heat wrap tubing to cover the cracked wires, If wires are corioded, I would cut them and replace them with correct gauge of wiring.

Good Luck.