Tritium-powered betavoltaic batteries are off-the-shelf products, and have existed for decades. You can buy them today. But they can only supply a current on th

Tritium-powered betavoltaic batteries are off-the-shelf products, and have existed for decades. You can buy them today. But they can only supply a current on the microamp scale, only useful for some niche sensor applications... On the upside, these batteries are fairly safe.