This is not a battery per se, it's really a generator. Over 20 years this thing is worth something like 20 watt-hours, but is far smaller than the equivalent primary cell (because it doesn't store energy, at least not chemically) and doesn't have issues with current leakage. It's meant for powering super long-lived low-power devices, one example that comes to mind that could benefit would be the container monitors used by the IAEA for monitoring nuclear material.