I just ordered a Noco Genius 10...Do I need to be concerned about overcharging my AGM battery?

 

No, I think all the Noco Genius models will go into a maintain mode once the battery is fully charged. My Genius 2 does

 

Nope. They are "smart". I have three (a 2 amp for my motorbike, 5 amp for my small tractor, 10 amp for my truck) and they all work as advertised. I used to only use Deltran (Battery Tender Jr, etc) but I use these now.

 

I have Noco 10 as well, with Noco quick connector installed... I use it only occasionally, and so far no issues.
My only complain is Noco 10 is limited to 10A, while if you working on the truck and trying to prevent battery from going low, you you won't be able to charge it - as truck engine off power consumption can easily go >8A

One thing is not clear to me though... when the truck is OFF, and you are charging the main 12V battery, does it charge the small aux battery at the same time?

 

One thing is not clear to me though... when the truck is OFF, and you are charging the main 12V battery, does it charge the small aux battery at the same time?[/QUOTE]

In a word - yes.
The system is set up to isolate the secondary 12v from load so it retains settings and powers systems temporarily while the main 12v is under heavy load but it receives charge otherwise when charging of the main 12v is occurring