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I'm working on a wearable project that is really space constrained. Originally, I had planned to use a Li-Ion battery, but I opted out of it because the person wearing it won't be able to take the wearable off in case there is a catastrophic battery failure.

I would use a triple-A battery, but those are too large.

What battery can I find that is smaller than a triple-A battery, but still fits the following constraints:

  • Safter than Li-Ion
  • Smaller and lighter than triple-A
  • Nominal voltage of 3.0V (I would be willing to use 2 1.5V cells if they are small enough)
  • Charge capacity >= ~50mAh
  • Capable of delivering up to 300mA in very short bursts for rf transmission (about 30 milliseconds). Otherwise the "resting" current consumption of the system will be on the order of 10μA.

I've found plenty of small batteries that satisfy one or 2 of these requirements above, but I can't seem to find one that fits all 5 except the 110mAh li-ion that I linked above.

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    \$\begingroup\$ There are many coin cells out there. But if you are asking specifics the Q would be off-topic (shopping). Without any details about what the power requirements are for example, or if there are any specific safety concerns, its quite broad. Worth pointing out most that I've come across are rated for a most 2mA continuous draw (maybe 10mA peak). \$\endgroup\$
    – Tom Carpenter
    Jan 20, 2016 at 2:16
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    \$\begingroup\$ Ordinary CR2032 button (okay, 'coin') Lithium cells are not considered safe enough to be shipped on passenger aircraft- never heard of any problems, but I've not researched it in depth. I guess you could use alkaline button cells like two 1.55V 357 button cells, but they don't hold very much energy. If your users are going to have to buy batteries you might want to check out what exactly is for sale locally. \$\endgroup\$
    – Spehro Pefhany
    Jan 20, 2016 at 4:45
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    \$\begingroup\$ Presumably you can use a decoupling capacitor to supply the short bursts of high current? And the predominant safety issue with button/coin cells seems to be related to their ingestion. They are known for particularly bad outcomes (along with magnets) when swallowed, which is due to the fact that they corrode very readily. I would guess that this device isn't intended to be worn internally, however. \$\endgroup\$
    – Oleksandr R.
    Jan 20, 2016 at 23:16
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    \$\begingroup\$ You can check out silver oxide cells, which claim to be suitable for low continuous drain with pulse on demand. You may have to dig for data or do some testing. They're more expensive than similar alkaline cells. \$\endgroup\$
    – Spehro Pefhany
    Jan 20, 2016 at 23:22
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    \$\begingroup\$ I think 10mF is enough, not 100mF. And a supercapacitor of this capacitance is very unlikely to explode or set on fire. Seiko makes some products that could be of interest to you. \$\endgroup\$
    – Oleksandr R.
    Jan 20, 2016 at 23:28

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I've never heard of a "safe" battery for sale.

  1. Lead acid - Gee, Lead and Sulfuric acid... totally not safe.
  2. Alkaline - not too dangerous to you (read: Less dangerous but still caustic), but when they leak, your electronics are gone.
  3. Nickel Cadmium - Cadmium, that's poisonous if it breaks.
  4. Nickel Metal Hydride - Hydrides are extremely reactive.
  5. Lithium anything - Lithium will literally burst into flames on uncontrolled contact with oxygen. So don't puncture one.

There are a few prototypes for safe batteries going on. Al-ion, nickel film protected Li-ions, etc. But, those aren't available.

Instead, You should make sure that the Li chemistry you're using isn't Li-FePO3 (this one is the most dangerous of the Li-ions), and protect the battery from over-heating by creating a circuit that cuts off current from the battery if the battery becomes too hot. Heat is the most dangerous thing that Li-ions go through, since they are generally very well sealed against outside oxygen getting to the lithium. Also using a standardized connector so that the battery can be replaced after a few years will help give the project some longevity and keep it safer. As batteries age they become more prone to failure, catastrophic or otherwise.

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  • \$\begingroup\$ I think the constituents of alkaline batteries are not so much toxic as the potassium hydroxide electrolyte is corrosive. I still wouldn't advocate eating such a battery, but I think one can get away without any serious injury even if it fails in ordinary use. Perhaps not if the potassium hydroxide goes in your eye... but we are getting toward increasingly unlikely scenarios. \$\endgroup\$
    – Oleksandr R.
    Jan 21, 2016 at 13:42
  • \$\begingroup\$ Potassium Hydroxide is high reactive and corrosive to your skin as well every other cell in your body. That's not exactly toxic, more caustic... I'll change that. \$\endgroup\$
    – Dave
    Jan 21, 2016 at 13:44
  • \$\begingroup\$ Yes, I know, but it is a question of amount, and the fact it is dispersed in a manganese dioxide (zinc oxide when discharged) paste. KOH is a minor constituent of an already tiny battery. This limits the possible contact with one's skin and other cells in the body. As I said, I wouldn't advocate eating these batteries, or implanting them inside the body. But they are used in hearing aids and watches, so mere proximity to the skin and the faint possibility of leakage (they are sealed) do not seem to be serious risk factors in practice. \$\endgroup\$
    – Oleksandr R.
    Jan 21, 2016 at 13:53
  • \$\begingroup\$ @OleksandrR., I'm not talking about the average case. Lithium batteries aren't exactly renowned for bursting into flames for no reason (excepting LiFePO3). NiMH aren't exactly known for killing people either. I'm just pointing out that there isn't a "safe" option. There is no battery that has no draw backs (yet, looking forward to Al-ions). The point is to work with what you've got, and pick the best option, and make it as safe as possible. To be honest, Alkaline won't even fit the project. I just named it to point out that there isn't a completely safe battery. \$\endgroup\$
    – Dave
    Jan 21, 2016 at 14:06
  • \$\begingroup\$ I would appreciate your source of information on Li-FePO4 batteries. I had read that, unlike Li-ion, they would NOT catch fire, NOT explode, and so on if punctured, heated, etc. See batteryuniversity.com/learn/article/… which indicates "Its strength lays in long life and superior safety, but it has a lower capacity than cobalt-based Li-ion systems. A further tradeoff is the lower nominal voltage of 3.3V/cell rather than the customary 3.6V/cell of other Li-ion systems." What leads you to believe that it is the "most dangerous of the Li-ions"? \$\endgroup\$
    – Technophile
    Oct 16, 2018 at 18:46

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