1992 Dodge Caravan Guage Reading

I don't know what it's called, but the guage that shows a battery, and "8" on lower side and "18" on upper side is reading always at one line below the "18".

Does this mean that the Voltage Regulator needs to be replaced, and how long can I drive around like this without serious trouble occuring?

Can I keep driving indefinately, and what could happen to the engine?

Money definately a problem, and what is the cheapest thing I can do to repair it?

Thank-you,

Ellie