Reducing 16.5VDC battery output to the range of 13.8VDC +- 15% without introducing noise for 15 Amp load
Two silicon diodes in series will knock 1.2V off the supply, getting you 15.3V 

perhaps use a 15A rectifier bridge connected in reverse, add a heat-sink.







simulate this circuit &ndash; Schematic created using CircuitLab