DIAGNOSIS: The rechargeable lithium-ion battery has reached the end of its lifespan, typically after 2-3 years of daily use. It can no longer hold sufficient capacity.
FIX: There is no user-replaceable battery. Your options are: 1) Use it plugged into the charger (impractical), or 2) Replace the handle. For a product this age, replacement is the most efficient path.
IF THAT DIDN'T WORK: Before concluding the battery is dead, perform a full reset. Leave it on the charger for 24 consecutive hours without using it. If runtime doesn't improve, the diagnosis is confirmed.
HEADS UP: Constantly running the battery down to zero before charging accelerates this degradation. Charge it when the green light starts flashing, not when it dies completely.