After we caught sight of some very cool folks online making their very own homemade nuclear batteries, we knew we had to give it a go!
The device in question is actually known as a “Radioisotope Photovoltaic Generator” or “Photobetavoltaic Generator,” and it’s a pretty clever design: Glowing glass pills filled with Tritium gas and coated in a phosphorescent material are sandwiched between two photovoltaic cells. The beta radiation emitted by the Tritium is blocked by the glass walls of the pill, but they cause the phosphorescent coating to glow. This light has no trouble passing through the glass and lands on the photovoltaic cells where it produces a small amount of electricity! Commercial Tritium batteries — like those produced by City Labs — cut out the middleman by using betavoltaic (as opposed to photovoltaic) cells in direct contact with Tritium gas. Those commercial devices are more rugged and efficient, but come with a very large price tag (thousands of USD per unit). On the other hand, homemade devices constructed using the packing tape sandwich method are as cheap as it gets, but aren’t particularly rugged.
This kit provides a good compromise between price and quality of construction. We’ve designed carrier boards for both of the photovoltaic cells which we supply pre-soldered and fixed to the boards with chipbonder. We’ve also designed and cast custom silicone rubber grommets which cushion the Tritium vials between the two photovoltaic cells, preventing the cells from getting scratched or chipped and — most importantly — the vials from getting broken. The carrier boards also allow you to choose whether the photovoltaic cells are wired in series or parallel and provide you with breadboard compatible pin spacing for the battery terminals.
So how much power does it make? Not much. The best measurement we were able to make suggests that the maximum power point of this arrangement is 25nW, if you can build a circuit that loads it properly (between 20 and 30nA at about 0.6v). Further experiments show that the battery will — given enough time — charge a capacitor to a voltage of 1.5 volts. Using this battery to do any work will likely involve a clever energy harvesting circuit, but keep in mind that this tiny amount of power will continue to flow for about 20 years — the half life of Tritium being 12 years. It’s also worth noting that these measurements were taken using green colored tritium tubes from aliexpress, your mileage may vary with the color and quality of your Tritium glow tubes. Feel free to experiment!
Because the silicone gasket supplied with this kit is not perfectly opaque, you will need to take any measurements in a light-proof enclosure (we made a little box out of copper foil tape) otherwise you will be getting inflated readings from ambient light striking the photovoltaics. You may also fill in the gaps around your gasket using an opaque epoxy resin. In practice, though, it may be beneficial to grab whatever “bonus energy” you can and only rely on the tritium vials when there’s nothing else.