Does anyone know the symptoms of a catalytic converter going bad?

Mine seems to be running pretty good, but my milage has dropped 10 to 15 mpg. This is disturbing. I just had the 90,000 mile service done and the technician said that it's not throwing out any trouble codes. The IMA also seems to be discharging and charging quite often. When the battery is fully charged, it will drop down to half charged just going up an onramp on the freeway.

This is frustrating me.

Howard