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The average 98.6 degree Fahrenheit body heat of a human being when transmitted through a cloth pocket to a cell phone inside is enough to speed up chemical processes inside the phone's battery. As a result, it runs down at a faster rate.
20 Facts You Didn't Know 5 Minutes Ago, YouTube, 7+ million views. The claim appears in the section starting circa 12m45s.

I'm skeptical.

Question: Can human body heat cause a mobile phone's battery to "run down at a faster rate"?

I didn't immediately find the answer using this Google search, although it seems clear that batteries can be damaged by heat.

Rebecca J. Stones
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    If anything, it should cause it to run down slower: it's well known that a warm battery can provide more charge than a cold one. – Mark Nov 01 '23 at 21:23
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    @Mark do you have a source for that? I have only ever heard of evidence to the contrary, and I think that would go against conventional expectations. – PC Luddite Nov 02 '23 at 02:40
  • @PCLuddite I think Mark is mistakenly overgeneralizing. Alkaline batteries and lead acid ones perform considerably worse in sub-freezing temperatures. Lithium ones appear to be minimally impacted at those temperatures. https://www.alexhibbert.com/blogx/2017/2/20/lithium-and-alkaline-battery-cold-testing https://volts.ca/blogs/educational/batteries-that-work-best-in-cold-weather – Dan Is Fiddling By Firelight Nov 02 '23 at 06:12
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    @Mark's point is valid - see the citation in my answer. Which effects dominate will depend heavily on the usage – Chris H Nov 02 '23 at 15:59
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    Not even that really? Everybody in here seems to be forgetting that a powered on phone isn't just some random brick, even when idle. It still consumes energy, and depending on the internal components geometry the cpu can sink more or less of its heat into the battery (AFAICT that can easily amount to some ~15°C and ~10°C differential respectively at normal temperature and pressure). Unless you are putting the device inside your knickers while outside air is at most "chill", I really cannot see how the human body could significantly affect its internal hotness. – mirh Nov 03 '23 at 22:31

3 Answers3

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This diagram from this research paper shows that the self discharge rate of lithium ion batteries (such as used in phones) depends on the storage temperature:

enter image description here

20°C is about room temperature (just under 70° F.) A fully charged battery stored at that temperature for 12 weeks (three months) will still have over 90% percent of the stored energy still available.

The same battery stored at 40°C (104°F) will have less than 80% of its stored energy still available after the same time. That's a little over normal body temperature, so you'd have maybe 80% or a bit more if stored at body temperature.

Yes, body temperature does cause your battery to self discharge a little faster.

It is not, however, enough to be noticeable in normal use. It would take several days (more likely a couple of weeks) of disuse and storage at body temperature for you to be able to tell the difference.

Technically, your body heat causes the battery to discharge faster. In reality it is not enough to notice.

Other things than the battery self discharge contribute to rapid loss of available capacity.

For example, if you carry your phone in your pocket, then the touch screen will be constantly triggered by contact with your body. That will cause the phone processor to wake up more often to check for touch commands, possibly activating the screen backlight at the same time. The backlight also consumes current and contributes to the battery discharge.

Even more likely than triggering the touch screen in your pocket is that putting the phone (specifically, its antennas) next to your body in your pocket will partially block the cellphone network and WiFi reception, causing your phone to use higher transmit power to maintain contact with the phone and WiFi networks.

JRE
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    "In reality it is not enough to notice." And it's unlikely that a pocket will ever reach body temperature, so the effect is even less. – Ray Butterworth Nov 01 '23 at 13:17
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    For something like a mobile phone, the dominant source of charge loss is usage, not self-discharge. In that situation, the increased total capacity of a warm battery may overwhelm the increased self-discharge rate. – Mark Nov 01 '23 at 21:26
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    "the touch screen will be constantly triggered by contact with your body" The screen won't respond to fabric touching it, which is why you can't wear regular gloves and operate a modern phone. However, some phones (iPhones at least) will turn their screen on if you shake them a certain way, even without touching the screen (or back sensor). – Laurel Nov 01 '23 at 23:36
  • It's also worth noting that the battery will output more energy when it's hotter - my son's science experiment showed that alkaline AA's at 70C ran a USB fan over 2x longer than room temp, and the same has been shown for rechargeables online (there is a cost of degradation though). Pity processors use more power at heat as well :( – Stephen Nov 02 '23 at 03:51
  • "if you carry your phone in your pocket, then the touch screen will be constantly triggered by contact with your body" I don't find this to be the case; pockets keep things from touching your body. I don't see many glowing pockets. – Schwern Nov 02 '23 at 03:55
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    @Laurel But then butt dialing shouldn't be happening if that's the case, yet it happens. – Nelson Nov 02 '23 at 06:29
  • @Stephen Do processors use more power when hot?? Correlation is not causation -- they are hot because of the power they consume. In machines with fans high-spinning fans will consume additional power when the processor is hot, but I don't think the processor or anything else in fanless machines does. – Peter - Reinstate Monica Nov 02 '23 at 08:45
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    @Laurel There is a situation where the phone reacts to a fabric. Sweaty fabric, to be precise. The last time i "butt dialed someone", I had the phone in my gym shorts and was climbing a rock wall in the northern italy. The screen kept getting pressed even though it was facing away from my body because of all the sweat permeating my shorts. – mishan Nov 02 '23 at 08:52
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    "Touch screen wake up" can and, in my opinion, should be disabled. either way, like others said, fabric won't trigger it, so a misfire should not be a regular occurrence. – osiris Nov 02 '23 at 09:08
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    Normally, the touch screen is turned off when the proximity sensor tells the phone that there’s a massive object close to it, like in the pocket. It’s the same mechanism that turns it off when you press the phone to your ear. – Holger Nov 02 '23 at 09:21
  • @Nelson &others - having the name I have, I used to be the recipient of many in-pocket calls, back in the days when phones had buttons. I've hardly received any in over a decade now – Aaron F Nov 02 '23 at 09:23
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    Most phones will have the touchscreen off when the screen is off, and will only respond to buttons or stronger taps and/or movements. – jcaron Nov 02 '23 at 12:45
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    To add: more processing-intensive applications like games can and will make your phone and its battery warmer than body heat. Fast charging will too, although given that it's charging up you wouldn't be able to notice the loss. – cthulhu Nov 02 '23 at 12:46
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    @Peter-ReinstateMonica As I understand it, lower temperature reduces resistance and thereby power consumption. The irony of course is that this reduces heat output which enables the temperature to be kept lower. But minimal compared to the battery chemistry improvements at temperature.. – Stephen Nov 03 '23 at 01:27
  • @user58697 I have the opposite problem - if I face the screen away from my body then whenever I take my phone out while on a walk it has locked me out of the fingerprint reader (which is on the back) due to too many "failed attempts". – Michael Nov 03 '23 at 08:05
  • Android at least has doze mode: "If a user leaves a device unplugged and stationary for a period of time, with the screen off, the device enters Doze mode. In Doze mode, the system attempts to conserve battery by restricting apps' access to network and CPU-intensive services. It also prevents apps from accessing the network and defers their jobs, syncs, and standard alarms" – sbrudenell Nov 23 '23 at 11:31
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Effects of temperature on batteries are complex:

  • Higher temperatures will cause a higher self-discharge rate. However, for a battery in a phone which is turned on, self-discharge is negligible compared to what the phone draws on the battery.

  • Lower temperatures will reduce the capacity of a battery on a single discharge: if you take a battery which is full, it will take longer to reach the "empty" state if the temperature is higher (but still within operating conditions of course).

  • Higher temperatures will reduce the life span of the battery over multiple cycles: the battery's capacity reduces a little bit after each discharge-recharge cycle. This is more pronounced when the temperature rises, as can be seen in this graphic from this paper:

enter image description here

I'm not quire sure whether the temperature during charge or discharge is more relevant, though.

jcaron
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  • The y-axis offset makes it seem more dramatic. Isn't there a word for it? – Peter Mortensen Nov 02 '23 at 23:06
  • @PeterMortensen "manipulating the baseline?" or "Manipulating the axis" maybe. or a combination? https://venngage.com/blog/misleading-graphs/ https://www.lrs.org/2020/06/10/visualizing-data-manipulating-the-y-axis/ – WernerCD Nov 03 '23 at 04:24
  • If you’d start the graph at 0Ah the difference would be much harder to see. – Michael Nov 03 '23 at 11:51
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    @PeterMortensen It's worth noting that the industry standard is that when a battery goes down to 80% of the nominal capacity (the horizontal hashed line), it's considered defective. Even then, while the difference between 10°C and 25°C would probably be hardly noticeable, the one at 45°C would still exhibit a noticeable slop change (the 60°C would be quite obvious, but we're quite off from OP's question at that temperature). – jcaron Nov 03 '23 at 12:02
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The opposite is possible in some situations, even likely.

In many batteries, the internal resistance (Wikipedia for further reading) decreases with increasing temperature. This is effectively a resistor in series with the battery, causing a voltage drop. This means that to extract the same power from the battery - to carry out the same functions - more current needs to be drawn as less voltage is available. That's not true for all electronics, but phones use switching voltage converters, e.g. to drive the backlight (EE Times), for which it is true - to achieve the same brightness, more current is drawn from the battery.

But is this true for the batteries used in phones? Yes. Temperature, Overcharge and Short-Circuit Studies of Batteries used in Electric Vehicles, Łebkowski 2017 (via ResearchGate, where it's available in English) looks at (mainly larger) batteries of the same chemistry. Figure 7 shows that the internal resistance increases at low temperatures, for a battery comparable to a phone battery, by a factor of about 2.5 between body temperature and freezing.

So on a freezing cold day, with the phone active (screen on), you should get better battery life by keeping the phone warm. An illustrative case is when I use my phone as a bike computer on my handlebars. It definitely lasts longer on a warm day than a freezing one. You can't control the weather, but, if you're not using it for navigation, you might be able to put it in your pocket.

Chris H
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    It sounds like there is an *optimal* operating temperature (for this aspect)(?). What would it be? – Peter Mortensen Nov 02 '23 at 23:04
  • @PeterMortensen not really for this aspect in isolation - the internal resistance never increases again, but above about room temperature (dependingon the fine details of the battery construction) the decrease levels off. Other factors affect device performance at high temperatures, of course, and these may be detrimental to battery life, but I didn't find anything suitably general and citable. Plus I wanted to concentrate on the process with clear science behind it. – Chris H Nov 03 '23 at 06:36