Diagnosis
Severe battery degradation. Lithium-ion batteries in the handle have a finite lifespan, typically 2-3 years with daily use and charging.
How to Fix It
1. Perform a full recalibration. Drain the battery completely until the brush will not turn on. 2. Place it on the charger for a full, uninterrupted 24-hour cycle. 3. Use it until it dies again. Repeat this full drain/full charge cycle twice. This can sometimes recalibrate the battery meter.
If That Didn't Work
The battery cells are beyond calibration. The only solution is battery replacement, which is not user-serviceable. Contact Colgate Hum support for warranty or out-of-warranty replacement options.
Heads Up
For longest battery life, avoid leaving the brush on the charger continuously for days. The optimal range is between 20% and 80% charge.