Charging voltage is probably just 13.6 and amps taper to a trickle after 20 minutes. Even worse in cold conditions. Control panel "full" was either just measuring the charging voltage or the surface charge right after. Battery could have still been below 50 percent.
Yes could be the CO detector just needs replaced but until you get a voltage reading we will not know. If more than five years old it may as well get replaced while you figure out your charging issues.
IIRC the low voltage alarm is close to 10 volts.
