My 99 Expedition (5.4L) was not cranking and with the help of the older posts on this forum (Thanks a lot Forum Members....You Rock!!!), I was able to diagnose the problem (bad starter relay). After replacing it, the vehicle started right up, but the battery light stayed on. The battery terminals were pretty corroded (with all of that acid coming out of the battery) earlier but I had cleaned them now. Have already tried resetting by disconnecting/reconnecting the battery...and it didn't resolve the issue.

Could it be the battery that is not fully charged? A bad battery? or an alternator?

Your help is really appreciated!!