[shabble]

For a short while they were used in implantable pacemakers as 'lifetime' power sources[1]

The main issue is that they're really rather inefficient. The efficiency of the thermocouple at converting thermal->electrical is only about 5-10%, and combined with the cost of shielding, expense of the radioisotope to begin with, and security/safety considerations, they're really only suitable for niche aerospace/defense applications.

Edit: I was curious if anyone had considered a stirling or other heat-engine driven by decay heat, and found https://en.wikipedia.org/wiki/Stirling_Radioisotope_Generato... which looks like it can hit 20+% efficiencies. The downside is that unlike thermo-electric/Seebeck effect converters, they have moving parts that could be a threat to reliability, which is the major issue when you're a planet away from the nearest repair tech.

[1] https://www.orau.org/ptp/collection/Miscellaneous/pacemaker....

[ceejayoz]

The bigger issue using it in pacemakers is it's actually beneficial to have a battery that needs replacing every 10 years or so - pacemaker tech gets better so fast that a 30 year old device would be crap compared to the current generation.

[banana_bread]

How much better really, if the current one wasn't working well you would find out real quick.

[ceejayoz]

More sophisticated programs (fewer false positives and false negatives), smaller devices, batteries that last longer between charges, etc.