Most of the PS are made by subcontractors like this one (Kuantech) and the schematics are very seldom distributed through OEM's.

I convinced that there are adjustable pots that you could trim in order to get the 13,8V output.(as in many desktop PS)
The best thing to do is to examine the IC datasheet and find where is the Vref and the resistors bridge / pot used to ajust the output voltage.
Sometimes you can also find IC's application notes with detailed schematics.