Devices to be used as batteries are qualified through the following elements of appreciation:
As of 2007, compact designs (automotive) mostly use electrochemical batteries.
In the latest years, technological improvements gave new arguments in favor of this application of the flywheel concept.
While ball bearings or active magnetic bearings would cause a significant power leakage, a suspension with passive magnetic bearings (similar to Inductrack) is workable. By letting the flywheel spin in vacuum, energy loss only occurs by eddy currents and allows typical half-life of a few months when unused.
The passive loops can also be powered to spin up the rotor, and conversely used to draw electrical energy from the device.
Under these assumptions, you would spin the rotor at the highest possible speed before it breaks.
Two problems remain though: coping with gyroscopic effect and safety in case of rotor explosion. When the flywheel is stationary, and can be buried, it does not really matter.
In the hypothesis it would be used in a car, two contra-rotative vertical axis flywheels may cancel their effects when the car tilts back/forth or left/right, and just increases stress on the magnetic bearing.
The risk of rotor explosion at 60,000 to 200,000 RPM can be moderated by using a material that pulverises rather than creates shards at rupture. Counter-intuitively, carbon fiber is more suitable than dense metals such as lead for this purpose.