why wont car start?gauges bounce.lights flicker.acts like dead battery as well

What is your question?

i drove my car to a few places with no problems. stopped at a store and shut it off. went in to grab a coffee and went back out to leave. car was off for about 15 minutes. went to start it but when i turned the key my gauges started bouncing around and lights would flicker. car will not start. checked my battery and it tested fine also checked wires and fuses. any idea what could be wrong?