yeah, ur stock electrical system is not made to handl 18v. First, ur alternator charges @ 14.4v, second the stock wiring and all is made for 12-14v, third all stock electronics are made for 12-14v. 18v should be charged by auxillary 18v alternator, and should only be used to power things that can handle it aka...modded amps, and **** like that. U sure ur not talking 16v...b/c most modded amps still will only take like 17v. But yeah, not a good idea. peace
NG