if I had to guess.......I would say the battery is not doing this.the extended crank yes maybe.12.3 volts Is engine off no load voltage I imagine......what is your charging voltage?after car starts it runs off your alternator.I have seen crank sensors and ignition modules do this.trouble codes are just a start if there is any.looking at scan data and understanding what you are looking at is more important than just codes.lots of times especially on older vehicles codes are not there.