When the battery is running low, it gives an acoustic and vibration signal and the battery symbol lights up orange. Whenever this happens, I charge it. And if I'm away and that happens, I've been able to clean my teeth easily after the signal a few times. It's a great piece of equipment!

I don't even know if there is an acoustic signal. But you notice quite automatically when the motor is getting weaker. Then I put it on the charger.