high voltage stock 6.1 alternator

did a search, found a bunch. but all about frying Regulators or voltage up and downs. I have constant 15Volts. Maybe my Batterie is too small with 55AH which is also mounted in the trunk and grounded to the Chassis only? So light, fuel pump running drops Batterie voltage and the Regulator powers against that to charge the Batterie?
also read about a HP Regulator http://store.alternatorparts.com/partnoc8313.aspx

will try a bigger ( 72AH ) battery today and route a ground cable to the engine. will see what happens from there