What you seeing is the charger limiting charging current to stay under its 50watt power limit. It won't make any diff running it with DC unless the chargers output power increases using DC. The chargers I'm familiar with generally have a little more output power running as a DC charger, but its slight like 50watts on AC and 63watts on DC. This is what the Triton 2 specs show for example.

That would only improve charge rate marginally, 63w/16.8v = 3.8a. You will not be able to do a complete 1C charge rate on a 4S lipo with this charger.