Deconstructing hype on "nuclear battery"

I came across this article through my network, Nano-diamond self-charging batteries could disrupt energy as we know it, which included some eye-opening claims:

California company NDB says its nano-diamond batteries will absolutely upend the energy equation, acting like tiny nuclear generators. They will blow any energy density comparison out of the water, lasting anywhere from a decade to 28,000 years without ever needing a charge. They will offer higher power density than lithium-ion. They will be nigh-on indestructible and totally safe in an electric car crash. And in some applications, like electric cars, they stand to be considerably cheaper than current lithium-ion packs despite their huge advantages.

Needless to say, it immediately tripped off my bullshit detector. My prior with breakthrough new energy technologies is: “if it sounds too good to be true, it probably is”. Just for fun, I did some back-of-the-envelope calculations.

This source from Forbes says that one gram diamond (5 carat) of pure C-14 can generate 15 J of energy per day, so about 17 microwatt. This corresponds to an upper limit power density of ~ 0.17 W/kg. For reference, lithium ion battery has specific power of 250-340 W/kg (source: wikipedia), so such C14 “nuclear battery” is about 1/2000th of the specific power (measured by W/kg) as lithium battery.

Now let’s talk about cost… growing lab-made diamond is not cheap. I am far from an expert in the diamond growing business, but I’ve had experience in CVD machines, and that CVD process is most definitely not cheap. Some cursory search seem to indicate that the lab-based diamond can be as cheap as $800 / carat (with just plain vanilla C12-based methane precursor). To grow C14 diamond, you’ll need to get your own C14 enriched methane, which is more expensive than unenriched methane. Let’s be generous and say you can get your C14 core, C12 shell diamond for as cheap as $1000 / carat, that's still $5000 / gram for a nuclear battery that can only generate at best 17 microwatts, on par with the power consumption of a modern pacemaker.

Count me as skeptical that such battery can see its impact in climate change. That’s not to say that they can’t have very niche applications. Maybe a pacemaker for an ultra long-living, rare species?