How long to these pacemakers last? Have people had to get the plutonium "batteries" replaced?
* Pacemakers are hermetically sealed, usually laser-welded in a titanium case. Adding a replaceable battery with seals would complicate this arrangement.
* By the time the battery winds down, there may be a newer, better pacemaker on the market that fits the patient's needs.
* Since a battery replacement necessitates surgery, you might as well replace the whole unit and get all new parts rather than put an old one back in that may be reaching MTTF.
As for how long the cells last, modern Lithium Thionyl Chloride cells last 5-10 years depending on the pacemaker. They probably actually last longer, manufacturers are pretty conservative with lifetime estimates.
As for plutonium supplies like those in the article, they don't really die since they're thermoelectric. The amount of current you can draw will be proportional to the amount of heat generated by the isotope and the temperature gradient. Since the heat is related to the amount of isotope remaining, the power available will follow an exponential decay. The half-life of Pu-238 is about 88 years, so the battery will last a very long time. The exact lifetime depends on how much current the pacemaker takes to operate.
Most people had these devices replaced with more modern versions, but there are still a few people who have the old plutonium devices which were implanted decades ago.
Not so sure. I don't think the plutonium decay is affected by the current drawn via the thermocouple - it'll just keep putting out the same power (well, the same decaying power curve anyway) no matter how much current you try and draw. I suspect the thermocouple's voltage will just sag so the maximum drawn power will be the thermal output less the efficiency of the conversion (which, I guess, might be non-linear with current?).
But I'm pretty sure whether you draw zero Amps or short circuit the output, you'll still have precisely 50% of your plutonium left after 88 years, right?
True.
The power requirements can still vary between patients (due to varying quality of the electrical contacts, which tends to deteriorate over time), so there is an individual component, it's just not based on the power drawn from the battery.
What I meant (and should have expressed more clearly) is that the pacemaker will have some minimum current it needs to be able to draw from the TEG to operate, and after the plutonium has decayed to a certain point it won't be able to source enough current to operate, so the pacemaker will brown out.
> Easier to replace the whole pacemaker than the battery
unfortunately, this statement:
> As for how long the cells last, modern Lithium Thionyl Chloride cells last 5-10 years depending on the pacemaker. They probably actually last longer, manufacturers are pretty conservative with lifetime estimates.
only partially applies in this case. That pacemaker has a lot of work to do (stimulate every single heart beat), and depending on the quality of the electrodes on the heart, and the cables that lead to them, some needed to be replaced after two or three years. If one lasts 7 years, that is a very welcome respite, but quite the exception.
That said, pacemakers have come a long way. The first on that this person had implanted had a fixed beat. The current generation has sensors for oxygen saturation and movement, and be configured and maintained through a wireless interface, they log unusual events etc.
And with it, I'm likely healthier than without it & the underlying issue.
Samsung includes charging of medical device[1] in their wireless charging patent. Although the patent[2] seems to focus on the charging method/devices, rather than where they are located.
1. http://patentimages.storage.googleapis.com/US20140084858A1/U...
Wireless charging (for a pacemaker) has the issue that you've got to push the energy through quite a lot of flesh, with consequent efficiency and heat issues.
Wireless charging is absolutely an option for other devices implanted closer to the skin, though.
I highly recommend this podcast episode with one of the Voyager project managers: https://www.listentospacepod.com/episodes/2016/8/5/episode-5... (about 20 minutes long, iirc)
(In fact, I'd highly recommend the whole podcast, not just this episode, but this one actually talks about power consumption and battery life of Voyager).
Just like apple limits power draw of it's devices to avoid possible reboots when too much current is demanded from an old battery.
You have to remember that Voyager is way past its originally planned mission life span, and that the nuclear battery was more than fine for the planned life span.
Batteries tend to be heavy, not operate well at very low temperatures, and wear down quicker than the 40+ years since construction of the space craft.
From the perspective of the original mission design, even with incredible foresight, it seems to be not worth the trade-offs.
Last paragraph on page 2... quite a long damned time; by 35 years it was still going strong.