i agree with the others, 15+ for any period of time is too high, and indicates an issue with either your battery or charging system. ( i am a small engine mechanic by trade, but car charging systems are not that terribly different than lawn mower/compact utility tractor charging systems, all are 12v based, and anytime anything over 14.6-14/8 occurs, it may indicate an issue somewhere in the charging system ) probably gonna want to sort that out before installing the new radio.