{"id":47979705,"title":"Show HN: Building a smartphone powered by nuclear decay: the power engineering","points":2,"user":"joshuashelvin","time":1777666593,"time_ago":"14 hours ago","type":"link","content":"

Every smartphone built in the last twenty years assumes the same thing: the battery will be charged once a day. Before I get to the math, sit with this for a moment: what would actually change if a phone never needed to be plugged in?

That is what I am building. The power source is betavoltaic — electricity generated continuously from the decay of a radioactive isotope, with no moving parts, no combustion, no charging infrastructure.

The device I am describing is not a current smartphone with a different power source. It is a device designed from the ground up around a power budget that current betavoltaic technology can sustain. Day One capability is meaningfully more limited than a 2026 flagship phone. That gap closes as the power architecture advances.

The source physics:

Tritium (H-3) decays by beta emission to helium-3.

The emitted electron carries a maximum energy of 18.6 keV, with a mean energy of approximately 5.7 keV. The neutrino carries the rest and is unrecoverable. The beta spectrum is continuous — a statistical distribution of electron energies up to the endpoint — which is why mean energy, not maximum, is the relevant figure for power calculations.

Tritium activity for power calculation: 1 Curie of tritium is 3.7 × 10¹⁰ disintegrations per second. At 5.7 keV mean energy per disintegration, 1 Ci of tritium releases approximately 1.6 × 10⁻¹⁹ J/eV × 5,700 eV × 3.7 × 10¹⁰ dis/s = approximately 33.8 mW of total beta energy. This is the upper bound on recoverable power — conversion efficiency determines how much becomes electricity.

Tritium's specific activity is 9,650 Ci/g. One gram of tritium therefore releases approximately 33.8 mW/Ci × 9,650 Ci/g = 326 W/g of total beta energy. Commercial betavoltaic cells at 1–4% conversion efficiency yield approximately 3.3–13 W/g electrical output — but tritium gas is diffuse and the actual cell includes a substrate, so power density per unit volume of assembled cell is far lower than per gram of tritium. Practical assembled cell power densities in current commercial products run 1–10 mW/cm².

City Labs in Miami produces tritium-on-silicon betavoltaic cells commercially under NRC license. Their current cells produce in the range of 50–300 μW/cm² of active junction area under typical loading. A mid-range figure of 150 μW/cm² is defensible for planning purposes with current commercial cells.

A multi-layer stacked cell module with 35 cm² of active junction area per layer and four layers — a total stack volume roughly comparable to a conventional battery pack — yields approximately 35 cm² × 4 layers × 150 μW/cm² = 21 mW total output. Accounting for interconnect losses and cell-to-cell variation, net output at the module terminals: approximately 15–18 mW continuous.

The conservative number — fewer layers, lower-yield cells — lands around 5–8 mW. I am designing the Day One architecture around 5 mW continuous as the floor, with 15 mW as the target as cell technology matures.

The decay curve is governed by the half-life\nP(t) = P₀ × (1/2)^(t/12.32)

At t = 5 years: P = P₀ × 0.755. At t = 8 years: P = P₀ × 0.629. At t = 12 years: P = P₀ × 0.499. The device at year 12 produces half its launch output. That output is predictable to high precision years in advance. No battery ages on a curve this clean.

The decay curve and upgrade cycle:

A device launched at 5 mW reaches the Day One capability floor at year 12. No consumer electronics device in history has been designed around a decade of predictable, transparent degradation rather than a two-year replacement cycle driven by software obsolescence. This one is.

What I'm looking for:

I'm the founder. I'm raising a pre-seed round to sign two co-founders: one with betavoltaic or nuclear battery hardware experience who can lead the prototype program. If you're that person, or you know them, I'd like to talk.

For the full essay and technical whitepaper, or to contact me directly, visit shelvin.com.","url":"item?id=47979705","comments":[],"comments_count":0}