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We know that you can start a fire with a battery. the better prepared you are the easier it is going to be.

I am not prepared and all I have in my pockets is my car keys, which includes a keyless entry fob that has a '3v Lithium Coin Cell Battery' is it possible to start a fire with this battery? All I have is the metal key ring, finger nail clippers, and a couple of other keys on the ring.

I don't have a tiny screwdriver to get the battery out, so I am going to have to use a rock to break the key fob. I suspect that banging two rocks together would be a more effective fire starter, but I might be wrong.

Can I start a fire with my key fob battery?

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With just the right equipment in a laboratory setting, you can use the energy in a key fob battery to cause a spark. You can then use that to start a fire under the right conditions.

However, that's not going to happen in any realistic back country conditions. The voltage and current capability of a key fob battery are just too low.

The reason you can sometimes use a car battery for starting a fire is because a car battery can deliver a massive amount of current. When shorting the battery with a small wire, the contact point gets so hot that some of the metal is vaporized, causing a spark. Sometimes small amounts of molten metal are also shed off. A key fob battery just doesn't have the oomph to do that.

In addition, it's not easy to start a fire with just a electric spark unless you have ignitable vapor. A few drops of stove fuel on tinder, then letting that vaporize for a few seconds can work, but trying to light solid tinder directly is very difficult. The reason this is sometimes successful with a car battery is due to the molten metal resulting from the high current, not the spark itself. With a car battery, you can even get small wire to glow, and then melt. A key fob battery isn't capable of that.

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    Steel wool is the easiest way to use a battery to start a fire. Even a PP3 (with a fairly high internal resistance) will get very dry tinder (or liquid fuel) going using steel wool. Now I want to experiment!
    – Chris H
    Commented Sep 15, 2018 at 14:15
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    @Chris: Yes, what you really want is very thin wire. Steel wool is a handy way to get such wire. Steel also has the advantage of oxidizing when heated enough. In this case, steel wool isn't available. Although I haven't tried it, I expect a key fob battery to be too week to get steel wool hot enough to start a fire. You need something with decent current capability, like a flashlight battery.
    – Olin Lathrop
    Commented Sep 15, 2018 at 14:20
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    Steel's high resistivity compared to electrical wire is also a help. Brass (from the keys) is also a poor conductor but I don't think there's any way to make a filament out of it with the constraints in the question
    – Chris H
    Commented Sep 15, 2018 at 16:09
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    electronics.stackexchange.com/questions/234901/… More keyfob battery notes. New battery momentary 0.1amp peak @ 3V so 0.3W and that’s best-case. So you’d better have made some good tinder (with your Survival Nailclipper)
    – mmcc
    Commented Sep 16, 2018 at 4:53
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    @mmcc: There is a very big difference in short circuit current and internal resistance between a AAA battery and a coin cell, even a large one like a CR2032. You're just not going to get "steel wool" hot enough to start a fire with just a small coin cell. I'm a electrical engineer, and I could design a circuit that would make a spark powered from only a coin cell. However, the required parts aren't available in the wilderness. Making a spark and heating wire to burning temperature are also two different things.
    – Olin Lathrop
    Commented Sep 16, 2018 at 13:54
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A bit more than a comment on Olin's answer:

I've done an experiment, and no, you can't.

I took:

  • a brand new CR2025
  • some steel wool to make it easier; this is a nice firelighting filament with bigger batteries
  • a tissue

I shorted the cell using the steel wool, held in a pad of tissue to provide tinder and keep the heat in. It got fairly warm for a couple of minutes, even through the tissue, but when it cooled and I opened the tissue it wasn't even blackened. If I couldn't get close with the contents of a house and garage at my disposal, you're not going to be able to do it in the field.

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  • Agreed. I tried too (CR2016 and a single strand of fine steel wool) and got nothing.
    – mmcc
    Commented Sep 17, 2018 at 23:47

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