The Amp Hour rating on wet and AGM batteries is the number of amps that can be drawn over a 20 hour time period before the voltage drops to 10.5V. To get the actual rate that can be constantly drawn divide by 20 so your 14 amp hour battery can supply .7 amps. The total available amps drops a lot if you increase the rate of draw. That 14 AH battery would last less than 30 minutes on a 15 amp load. The voltage would start out above 13V and drop. It would be within 10% of 12 V (13.2V to 10.8V) for maybe 20 minutes.
Also to get the most use out of the battery it should not be discharged more than 50-60%. AGMs can be drawn a little lower so the actuall usable amp hours is about 7.