2001 Pontiac Bonneville battery won't stay charged

Welcome to the forum, possibilities, faulty battery--faulty alternator-- faulty wiring circuits. Have the charging sysyem checked. Use a digitalvoltmeter take a reading across the battery posts, everything turned off. Take a second reading across the posts, engine running, the second reading should be higher than the first, if the alternator and circuits are good. 12.6 volts is considered a full charged battery, engine off, running, should be about a volt higher.