Radioisotope Thermoelectric Generator

Is that a nuclear reactor on your pocket, or are you just happy to see me?

The fourth generation Radioisotope Thermoelectric Generators utilizing Promethium-147 as their heat source provide the best compromise between power density and duration.
Others using polonium-210 can generate 70 times the power output as promethium-147 and are safer to use, but have to be refueled after only 6 months.

Utility

Producing an incredible amount of electric power in a small package, fourth generation thermal generators are the first choice for power armors and robot vehicles.
Note:  Since Promethium-147 is a beta particle emitter, no special shielding is needed beyond the metal housing of the apparatus.  Polonium-210 is only an alpha particle emitter, and so it is relatively safe to handle with gloves.  Just don't eat it.

Manufacturing

Fourth generation radioisotope thermoelectric generators were invented before the great cataclysm, and were installed in Glitter Boy armor and other capital products like ships and aircraft.
The technology to develop them is lost; however, enough have survived that they can be replicated using advanced tools. The Coalition states, Free Quebec, Northern Gun and Bandito Arms all have access to the tooling necessary to reproduce them.
Access to the source material, however, is another issue entirely. Polonium-210 is a natural element found in the decay of Uranium-238. And while there are few natural sources of Uranium-238 in North America, there is still plenty of used material around to scavenge as a source of Polonium-210.
Promethium-147, on the other hand, is an artificial isotope created by the decay of Neodymium-147, which only has a half-life of 11 days. Neodymium is a relatively common metal, but an operating nuclear reactor with Neodymium is needed to create the Neodymium-147, and then the promethium-147 must be extracted as it is created before the neodymium-147 expires. Needless to say, only Northern Gun and The Coalition States have the infrastructure necessary to accomplish such a task.
Less powerful radioisotope thermoelectric generators can be made from scavenged nuclear material left over from defunct nuclear reactors, and the various ground zero sites across the continent from the detonation of nuclear warheads.