1997 Chevy Blazer short? Battery won't hold over nite cgar

Hello and merry christmas and thanks for your help.I hope. Any way, mechanic put meter on battery and alternator, says battery is good and alternator is charging at 12 1/2 volts. Battery connections are clean and tight, 'check engine light'. Not on, no interior lights are on, ran truck for an hour to charge battery, started up 4 hours later but 'not' like usual and could tell the battery was drawing down when I started it up and did not hold the charge over nite, tried to start it this morning. Click click click click. Hope I have given you enough information, thanks and have a happy holiday