DIAGNOSIS The battery is either failing to hold a proper charge or the vacuum is being used in a high-power mode that consumes the battery much faster than you expect.
FIX First, rule out mode confusion and reset the battery.
- Press the power button once for Eco mode (longest runtime), twice for Medium (Auto), three times for Boost (max suction, ~5-8 min runtime). You might be accidentally in Boost.
- For a reset: drain the battery completely until the vacuum stops. Then charge it uninterrupted for at least 4 hours, or until the charger light turns off.
- Always use the official Dyson charger and wall outlet.
IF THAT DIDN'T WORK The battery itself is likely degraded. Lithium-ion batteries have a finite lifespan. If it's under warranty, contact Dyson. If not, you'll need to purchase an official replacement battery—third-party ones often don't communicate correctly with the V15's smart system.
HEADS UP Never store the vacuum for long periods (like a month or more) with a fully depleted or a 100% charged battery. Store it with about half a charge for battery health.