DIAGNOSIS Rapid runtime depletion is typically a failing battery cell. The V15 uses a Nickel-Cobalt-Aluminum battery pack. Individual cell failure reduces total capacity dramatically.
FIX First, confirm the behavior. Fully charge until the LED turns solid blue. Run on Eco mode with a non-motorized tool. Time it. If runtime is under 15 minutes, the battery is degraded.
IF THAT DIDN'T WORK Rule out parasitic drain. Remove the battery from the vacuum after every use. Storing it on the dock with a minor fault can cause continuous low-power drain, deepening discharge cycles.
HEADS UP Batteries are consumable. Expect diminished capacity after 2-3 years of regular use. Using Max mode frequently accelerates degradation due to high heat generation. Replacement batteries must be genuine Dyson for correct communication with the BMS.