I sure do load test new batteries, as well as check them with
hydrometer.
The reason is I have bought new batteries with either a dry cell or a
shorted one.
Nothing is perfect.
Even a battery.
So the like new alternator is putting out 14.3 volts with all the
accessories on?
You have something drawing off that battery.
If the light fixture had a dead short that would do it.
But anything going to ground would drain it right away.