Unfortunately it not that simple.... you maybe measuring the charger voltage.
Not sure about your slow charger, but a modern 'smart' charger will first charge at 14+V for a few hours (or longer), until it sees that the battery is charged. Then it reduces the voltage to a floating charge of ~13v.
If the battery is good or bad and charged with the charger floating at ~13V, as soon as you disconnect the charger and measure the voltage, you'll be measuring the floating voltage not the 'resting' voltage of the battery.
Previously if you were measuring 14.5V, you're measuring the battery voltage soon after the charger was still charging the battery, not the resting voltage.
Best to disconnect the charger and let the battery rest, for a few hours, before taking it's voltage. You can notice the voltage dropping, even on a good battery.
I remember my dad had a rhyme for a battery's charge. "12.7 to heaven, 12.6 bit sick, 12.5 barely alive"
The fact it needed a charge at all, would say to me, you'll be replacing it soon.