DC-DC CHARGER RUNAWAY DISCHARGE WARNING.

THE EXPLANATION IS HERE:
This is not a problem I’m trying to solve. And neither is this a mystery as to why it happened. My diagnosis has been verified by professional auto electrical and DC charger manufacturers. It's not my opinion. It's my EXPERIENCE. And this is a warning to those who may be interested. NOTHING MORE!
If you charge the starter battery with AN EXTERNAL charger, the DC-DC will. WILL. OKAY. IT WILL . . . activate itself and begin charging the aux battery. THIS IS HOW DC-DC CHARGERS WORK!
If the external charger's capacity is less than your DC charger's it will drain the start battery in the process of charging the aux battery. IT WILL! This is also logical.
If you use a trigger cable from the ignition, and you leave the ignition on with the engine not turning, the DC charger will stay on until a pre-determined low voltage is reached, and switches off. That voltage may be too low to start the engine. I'm not guessing.
Now, you will only know if the full starter battery discharge has happened IF AND ONLY IF the charge is interrupted BEFORE THE CHARGE OF ALL BATTERIES IS COMPLETE. If you do not interrupt the charge, you won't know and all will seem fine. But the start battery has undergone a full discharge and recharge.
So. To avoid a runaway discharge, just charge the aux battery first. It's that simple! Or you may be caught with a flat start battery.
Now if you disagree, get this. There is nothing to disagree with! I am recounting an experience and presenting a warning. Use it OR don't use it. That's for you to decide.