History of Atomic Batteries
Until recently we did not have the materials to build nano nuclear battery technology. There were many limitations in how much power we could extract from a nuclear battery in the old days. With today's new technologies we can now make extremely small nano nuclear batteries that produce power in multi watt level and produce little to no heat in day to day operation. The new Nuclear Technology makes them viable in all kinds of electronic devices.
They NAGs can be scaled to reach power levels even into the thousands of watts if required. There are some limitations as to size and power level also ther are some limits before the devices have to be air or water cooled.