Dead battery

Well tonight I was going out for a cruise around town and my car wouldn't start, Earlier today the dashlights would dim and the turn signals wouldn't work unless I reved the engine up. Got her jumped and the Voltage meter in the car was showing -20 until I reved up the engine to higher rpms just to get it to 0. A few days ago it was fine but now it's acting up. Gonna go down to the local parts store and see if they can test her. Any Ideas?