Mercury 150hp not charging when trolling

[mathews_hunter]

Had the shift linkage come lose on my kicker after it was serviced by a certified Mercury marina the other day. So I used my main 150 four stroke to troll yesterday.   After about 3 hours of trolling a System error for low voltage came across the screen.   My Battery was drained to 10.9v.  Pulled all the gear in put in neutral and brought rpms up to 3000 and my voltage picked up to 13volts.  I don’t understand why the alternator would not put out proper charging voltage when using the troll control feature on my Mercury gauge.  Has anyone ever had this issue?  Was running a Helix 9, VHF, and the fm radio.  The same loads I have when I troll all day long with my kicker.  The kicker is a 2016 9.9 Pro Kicker 4stroke and my main is a 2016 150 4stoke.