I was inside my cluster last year and don't recall seeing a battery.
The cluster needs a +12v constant power to keep the clock/trip-o-meter. Pull the cable off the back of the cluster, check the voltage at pin #2 (red wire) You should have about 12v. Verify that you have a good ground as well, check the resistance of pin 13 (black) to the negative term on the battery. You should have less than an ohm or two.