Diagnosis
The lithium-ion battery inside the parent unit has degraded from repeated charge cycles. This is normal wear and tear after a year or two of daily use.
How to Fix It
Before replacing anything, do a full recalibration. Let the parent unit run until it completely powers off on its own from battery. Then, charge it uninterrupted for a full 8 hours while it's turned off. This can sometimes help the battery meter recalibrate.
If That Didn't Work
The battery itself needs to be replaced. The HB6550 uses a common 3.7V rechargeable battery pack that's user-replaceable. You'll need to open the battery compartment on the back (usually a small Phillips screw) and swap in a new one. Search for "HelloBaby HB6550 replacement battery."
Heads Up
When you open the compartment, note the connector type on the old battery (usually a small white plug). Make sure the new battery has the same connector. Don't attempt to use a different voltage battery.