This may be in the 'you were afraid to ask' category. There are many devices which work on batteries or alternatively when plugged into the mains. You can recharge the batteries. Obviously when you are using batteries alone they have to have been charged. Now I am finding that while some devices (e.g. a radio) when attached to mains work equally well whether the battery is charged or not and think the charging goes on while you are using it, but others, particularly shavers and trimmers do not work, or hardly, on the mains until the battery is recharged which can take hours at least and can be inconvenient. I have just verified this experimentally. I do not remember that this ever used to happen. Is my impression that things have changed in this respect accurate, or is my memory at fault? And in any case why are things like this now? What is the arrangement and principles? And if it has changed, why? I would have thought it natural to put battery in parallel with mains supply (reduced and rectified in the device, the motor working with DC) and that is how it is done, but maybe I have some egregious misconceptions?