time for a new battery

i have never noticed this before on any of my vehicles but yesterday at a stop light i had my blinker going and my battery guage would bounce every time my blinker would go. under speed it was fine but sitting at idle it bounced right along. does this mean it's time for a new battery or a new alternator? i have a 2002 z-71 with the 5.3. thanks, Derek

stop at autozone or advance auto or o'reillys and they test both for free