low battery power and rev counter fault?

I have a kawasaki gpz500s 2000 model and have suffered starting problems i think due to a low battery. After bump starting and running the bike the rev counter needle bounces around the dial then hits the red line when off the throttle even though the engine is not revving that high. Also when left idling the engine seems to misfire and run very rough. The battery is no longer holding a charge.