Diagnosis
The lithium-ion battery pack is degrading. This is normal after a couple of years of regular use, as they have a finite number of charge cycles.
How to Fix It
Your best bet is to replace the battery pack. 1) Order the specific replacement battery for the SpinWave 2039A. 2) Remove the old pack by pressing the release button and pulling it straight out of the compartment on the back. 3) Insert the new battery until it clicks. 4) Give it a full, uninterrupted charge (red light turns green) before first use.
If That Didn't Work
If a new battery behaves the same way, the problem could be in the mop's internal power management board, but that's far less common.
Heads Up
To maximize battery life, try not to store the mop with the battery completely dead or always plugged in. An occasional full discharge is okay, but constant deep discharges shorten its life.