I have used a toothbrush charger, but the coupling I got between them did not charge the battery very fast (left it on the charger 2 days). The coupling efficiency depends on the proper alignment of the electric/magnet field lines so 3D alignment and spacing are critical for efficiency. This lead me to duplicating the arrangement that is used in the car. In other words use the actual antenna in exactly the same physical layout. Being an engineer, it is easy for me to design and 3D print a nice setup to hold the key and antenna. It is also easy to gin up a circuit to supply the correct signal. My spare key will then reside fulltime in a optimum charger and I won't tie up my toothbrush base. Plus it gives me an interesting project to do.
AceEngineer
