According to page CHG-27 of the 2009 FSM:
Quote:
Regulated output voltage [V] 14.1 - 14.7*
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and
Quote:
*: Adjustment range of power generation voltage variable control is 11.4 - 15.6 V.
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The 13.5V you are reading is probably the fully-charged battery voltage (~13.2V) plus a few tenths due to meter accuracy.