Rep Power: 0

Once the car is waiting at a redlight i have my turbo timer on the volt settings and i see it drop from 13.5V to 12.1V

If i put the car in park, while still stopped the volts will jump up then once in drive it'll drop. I also notice lower levels while coasting, but once I accelerate a little bit itll jump back up.

couple things to note,
1. I need to clean the battery connection cables, little bit of battery corrosion on it from the old battery.
2. Cheaper-side aftermarket alternator, destroyed my OEM alt with a sound system (no longer connected)
3. the ground and power cables are looking a bit old, going from the alternator. maybe have to replace soon or in future
4. Battery is new when i got the alternator.

not sure what is drawing so much power when it's stopped or while its coasting, I would maybe say the aftermarket injectors or something but those aren't even going when stopped.

 
Rep Power: 126

That alternator isn’t doing you any favours. See if you can grab a junkyard one if you don’t want to pay OEM prices.

 
Rep Power: 222

This, could also be the computer stopping the charge while at a stop for fuel efficiency reasons

 
Rep Power: 367

hondas do adjust charging depending on electrical load. If not much in use, it drops the load.

 
Rep Power: 0

Could going to 11.9v-12.2v be unhealthy for the car at all?

 
Rep Power: 126

No. That’s a normal level of fluctuation.

 
Rep Power: 367

lots of people that installs oil pressure gauges freaks out when they see the low pressures at idle (which is absolutely normal)