Why isn't my battery charging?
A 12V battery needs a higher voltage to charge. You're using a 12V HDD power supply with outputs a constant voltage to charge a battery that already presents 12V across its terminals. No current will flow from the power supply to the battery. If it was discharged even more however, you will be able to charge it up to 12V (Whatever the equivalent State of Charge corresponding to that voltage is).

Try using  a 14V supply, and preferably with a current limitation (@3Amp MAX) feature so that you won't damage it.