Reading like a 5/6 year old battery, is it a "name" brand, or what they had when you needed a battery?
When you get engine started test alternator voltage at battery, should be 14.5-14.9v just after start up
Then after a few minutes of running it should read 13.5-13.8v
If it stays above 14volts then voltage regulator is stuck and is/has "cooked" that battery
Car Batteries are pretty finicky, they have many thin plates so they can release high amps quickly, starter motors need 60-80amps, and they can do it in COLD temps
But because of the thin plates they can't be drained very far before causing damage, or overcharged to long or it will cause damage
Deep cycle batteries have fewer and thicker plates, they can be drained down to almost 0 volts, but can't release high amps as fast, especially COLD, also they don't charge up real fast, they like slow re-charging, but also react poorly to overcharging