Flywheel "batteries" have high power density but low energy density. Said another way... they can deliver large amounts of power for very short periods but are lousy for delivering small/medium amounts of power for a long time. It's better to think of them like capacitors than batteries. They cover or absorb spikes but do not last long.
Only very specialized flywheels have anything approaching battery performance over long time periods. They are made out of carbon fiber and spun in hard vacuum over magnetic bearings. It isn't something a DIY'r could rig up in his basement.
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