Safety: Avoid repeated rapid restarts; this stresses the battery further.

Charge fully: Ensure the battery has been on charge for the full recommended time.
Test in low power mode: Run the vacuum in standard mode only. If it still stops quickly, the battery cannot sustain load.
Try another battery: If available, test with a different Hoover H-Free battery. If the vacuum works normally, your original battery is faulty.
Replace battery pack: Fit a new genuine Hoover H-Free battery pack.

LEDs on battery show full, but vacuum stops after a few seconds of use

Possible Causes