Diagnosis
The battery isn't holding a charge. This is a common wear item, especially if it's been frequently drained to zero or left plugged in constantly for months.
How to Fix It
You'll need to replace the battery pack. It's a standard, user-replaceable part. 1. Turn the monitor off. 2. Slide the battery cover on the back downwards and remove it. 3. Unplug the battery's white connector inside. 4. Pull the battery out and order the exact replacement (search "Infant Optics DXR-8 battery"). Installation is the reverse.
If That Didn't Work
If a new battery also dies quickly, there might be a setting draining it. Turn off the monitor's screen by pressing and holding the power button until the screen blanks, using it just as an audio monitor. Also, lower screen brightness.
Heads Up
To prolong the new battery's life, try not to leave the monitor plugged in 24/7. Let it run on battery occasionally, and avoid draining it to 0%.