DIAGNOSIS A failing lithium-ion battery that can't hold a charge, or a background function like Bluetooth is draining it constantly.
FIX Turn off Bluetooth via the Oral-B app to see if battery life improves. Go to Settings > Brush Settings and disable Bluetooth connectivity. Use it as a regular brush for a week.
IF THAT DIDN'T WORK Perform a full battery recalibration. Drain the brush until it completely dies and won't turn on. Then charge it uninterrupted for a full 24 hours on the original magnetic charger.
HEADS UP Using non-genuine brush heads or excessive pressure can cause the motor to work harder, draining the battery much faster. Always use genuine iO heads.