The vacuum will not start; E12 or twelve flashes appear; the unit was stored for a long time without use.

Safety first: Inspect the battery for swelling, cracks, or leakage. Do not attempt to charge a physically damaged battery.

Step-by-step checks:

• 1. Try a long initial charge: Connect the battery to the original Amazon Basics charger and leave it connected for at least 3–4 hours, even if LEDs do not light immediately. Some BMS circuits need time to recover from deep discharge.
• 2. Check charger LED behavior: If the charger LED never turns on or blinks, test the charger with another compatible battery if possible.
• 3. Clean terminals: Gently clean the battery and vacuum contacts with a dry cloth. Light oxidation can increase resistance and prevent proper charging.

When to stop: If E12 persists and the battery will not accept a charge, the pack is likely permanently damaged. Replace the battery pack; do not attempt to open or revive it with improvised methods.