Diagnosis
This is a known behavior, not necessarily a faulty battery. The camera is designed to run primarily on wall power and use the battery only as a brief backup. To preserve the battery's long-term health, the software often keeps it at a partial charge, like 80%, when constantly plugged in.
How to Fix It
First, confirm you're using the official power adapter and cable that came in the box. Off-brand chargers often don't provide enough consistent power for both operation and charging. If you have the right gear, try a full power cycle: 1) Unplug the camera. 2) Let it run on battery until it powers off (or until it gets below 10%). 3) Plug it back in and leave it alone for a full 24 hours. This can recalibrate the battery's internal meter.
If That Didn't Work
There might be a setting causing excessive activity. Go to the camera's settings in the app, then "Power Manager." Make sure "Surveillance Mode" is set to "Optimal Surveillance" and not "Customized," which might have overly aggressive recording schedules that keep the camera awake and draining power faster than the charger can replenish it.
Heads Up
Constantly keeping a lithium-ion battery at 100% charge while also being warm from operation can shorten its overall lifespan. Seeing it hover between 80-90% when always plugged in is normal and is actually a protective feature.