1979 D100 318 won't start under its own power

i have a 79 d100 that won't start under its own power i hook up a power jumper or jump it with another battery it fires up, but won't start using its own power the battery reads 12 to 13 volts on the meter . i replaced the voltage reg.

How old is the battery? Sounds to me like it might have a dead cell in it or something and needs replaced since it starts up when you jump it.