Hmmm...
Carbon-14 has a specific activity of 62.5mCi per mmole (2.31GBq per mmole)
The half-life of Carbon-14 is 5715 y
Hmmm...
https://www.snopes.com/fact-check/radioactive-diamond-batteries-real-thing/
...The actual amount of carbon-14 in each battery has yet to be decided but one battery, containing 1g of carbon-14, would deliver 15 Joules per day. This is less than an AA battery. Standard alkaline AA batteries are designed for short timeframe discharge: one battery weighing about 20g has an energy storage rating of 700J/g. If operated continuously, this would run out in 24 hours.
...Using carbon-14 the battery would take 5,730 years to reach 50 per cent power, which is about as long as human civilization has existed. However, it is unlikely that the diamond battery will provide direct power to an attached device. More likely is that it will be associated with a capacitor that will be ‘trickle charged’ by the battery and then discharge at set intervals, to power devices at set intervals or to continually power low draw devices.