13.5 at idle is on the low side but still ok. An alternator in full
charging mode should be around 14.2 volts. They are typically set up to
hit that at about 1000 to 1200 rpm. Below that speed, the voltage will
drop with engine speed until it hits battery voltage of about 12.2
volts. Sounds like you may be just fine, have a bit too low of idle
speed, or even a blown diode or two. Try bumping the idle up a bit or
bring it in to any auto parts place and have them test it.

13.5  volts is good and it changes according to ambient temp.may go up a .5 or
go down a .25 of a volt
get a battery