charging problem

[chaparral02]

what i dont get is my first measurements were ok, yeah??....13v.3 at idle. but
after a 40 minute ride why is it 12.3v at idle, then 10 mins later
12.7v?

Doesnt that say theres an intermittent charging problem? shouldnt
it at all times (at idle) be above those measurements?

Some Bikes alternators have a very low output when the bike is at idle rpm

Example
The alternator output at idle is maybe to  low to keep up with supply when  the rad fans and lights are on at the same time so it starts draining the power from the battery