Diagnosis
The rechargeable lithium-ion battery is degrading. This is normal wear and tear after a couple of years of daily use and charge cycles.
How to Fix It
Before declaring the battery dead, do a full recalibration charge. 1) Use it until it completely dies and won't turn on. 2) Place it on the charger and let it charge, uninterrupted, for a full 24 hours. 3) Use it normally until it dies again. 4) Charge it fully again. This process can sometimes help the battery management system recalibrate and report capacity more accurately.
If That Didn't Work
If the runtime is still very short (like dying after two 2-minute sessions), the battery's capacity is simply gone. The battery is sealed inside the waterproof handle, so user replacement isn't an option.
Heads Up
Heat is a battery killer. Don't leave the charging base plugged in and the handle on it 24/7 in a hot bathroom. Once it's fully charged (green light), take it off the base. Constant trickle charging in a warm environment accelerates battery decline.