Comparison to electric batteries
Flywheels are not as adversely affected by temperature changes, can operate at a much wider temperature range, and are not subject to many of the common failures of chemical rechargeable batteries. They are also less potentially damaging to the environment, being largely made of inert or benign materials. Another advantage of flywheels is that by a simple measurement of the rotation speed it is possible to know the exact amount of energy stored.
Unlike most batteries which only operate for a finite period (for example roughly 36 months in the case of lithium ion polymer batteries), a flywheel potentially has an indefinite working lifespan. Flywheels built as part of James Watt steam engines have been continuously working for more than two hundred years. Working examples of ancient flywheels used mainly in milling and pottery can be found in many locations in Africa, Asia, and Europe.
Most modern flywheels are typically a sealed device that needs minimal maintenance throughout its service life. Magnetic bearing flywheels in a vacuum enclosure, such as the NASA model depicted above, do not need any bearing maintenance and are therefore superior to batteries both in terms of total lifetime and energy storage capacity. Flywheel systems with mechanical bearings will have a limited lifespan due to wear.
The arrangement of batteries can be designed to a wide variety of configurations, whereas a flywheel at a minimum must occupy a square surface area. Where space is a constraint for the application of energy storage (e.g. under trains in tunnels) the flywheel may not be a valid application.