The charging current is so low that leaving a toothbrush handle on constant charge will not be an issue regarding product longevity or safety. The induction charging current depends on the toothbrush model being charged, but in all cases, it is low (10s of milliamps). Therefore, the battery is never really regarded as full even when it reaches its maximum capacity, it is simultaneously losing charge, and at this stage, it is in “maintenance mode”. 

For the lower-end Vitality model, which has a NI-MH battery, the Oral B website recommends that “for optimal performance, you can keep your Vitality brush always on the connected charger when not in use”. This will also apply to other models with NI-MH batteries.

Higher-end models with li-ion batteries will usually have overcharging protections for additional safety.