Imagine performing diagnostics on a truck and not having to worry your laptop’s battery. No matter what, you know you will have enough power to work. That is the convenience and flexibility a DC to DC battery charger offers.
With this setup, no converter is needed which means diesel technicians are able to plug their computers directly into cigarette lighter ports. It saves the tech from having to carry a DC (truck voltage) to AC converter which then plugs into the standard laptop AC to DC charger.
This allows the technician to be able to maintain computer battery charge in case the computer battery is running low & still travel lighter, not having to carry both his normal AC charger but also a DC to AC converter. Saves space & minimizes the “cable clutter”. This way, a technician in the field never has to worry about the charge on his laptop computer.