Diagnosis
The rechargeable battery inside the parent unit is worn out or wasn't calibrated, or a setting is killing it. These lithium-ion batteries degrade over a couple of years.
How to Fix It
First, try a full battery recalibration. Drain the parent unit until it powers itself off completely. Then, plug it into its charging dock and let it charge uninterrupted for a full 8 hours (overnight). Don't use it while charging this time. This can reset the battery meter. Also, lower the screen brightness (Menu > Settings > Brightness) and turn off any unnecessary sounds or alerts.
If That Didn't Work
The battery itself is likely failing. It's a common point of failure. You can contact Babysense for a replacement battery kit; the V43 parent unit battery is user-replaceable with a little screwdriver. They sell official ones.
Heads Up
Using a third-party USB charger or a fast-charger can sometimes damage the battery management circuit, leading to rapid drain. Always use the included white power adapter and charging dock if possible.