assuming the alternators charging it at 14v, if you fully disconnect the battery after a long run (so begin with a fully charged battery and unplug it from any potential amp drain), leave it overnight, it should retain the same 12.4 - 12.6v for afew days.
if it goes below 12.4 or even drops out completely, the batterys dead & unable to hold charge. new battery.
if it holds charge fine but runs flat when connected to the car, there's a high amp load draining it.
check ur boot latch & light is proper shut off (easy to forget bout), all interior & exterior lights off.
