Using Miata BAttery on my sr20 s13/ Starting/charing problems!

[apexi240sx]

ever since i started using the Mx-5 Miata battery, i ve had a some problems, now its getting really serious, the battery is not holding the charge that
SR20DET charging system is putting out... And the battery Is NEW, a duralast from autozone, this the 2nd time i had to replace it, seems like when i leave the car parked for over 3 days with the battery connected, it seems like it drains it, dead, when i jump start it the car will start, then i go drive it, then i turn it off and it will not turn on again! i tested the charging system with an ohm meter and the alternator is charging fine 14.7 v but the battery is not holding the charge... is the miata battery suitable for the 240sx? (right ohms, cold cranking amps) im forced to run this battery cause of the FMIC...... has anyone had these probs with this battery?? or should i get a westco or optima Yellow to rid this prob