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Battery overcharing
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Buriedcode:
With all these anedotes flying about its worth remembering that each scenario has a lot of factors that aren't mentioned: 

1) Device current draw - we really don't know what your phone/tablet/laptop is doing.  Often phones will update and perform scans whilst idle but on charge, raising their temperature and power consumption. Simply leaving a device on charge doesn't mean its sleeping, or that its charging for that matter.  And it may charge to 100%, then update, using up a fair chunk of battery life and start charging again quite quickly once the charge level has dropped below 80%. Or... it may do none of those things.

2) Enviromental - ambient temp? humidity?

3) The obvious one - usage.  It's very easy to say things like "heavy use" or "constant use" but both terms are subjective.  I've sometimes used my phone for frantically googling for part availability, on site, with poor converage and drained my battery down within an hour.   Other times doing the same thing next to a WiFi hotspot and barely dropped 10% in an hour.  Actual usage correlates with power consuption but its not an easy comparison, so judging battery life vs usage has so many variables you can't really be certain.

I'm fairly sure the takeaway for PlainName is:  Don't worry.  Even the cheapest modern devices handle the charging just fine, and any gains to be made by micromanaging the SoC and charge terminations will most likely be marginal.  Unless you're living somewhere with an ambient temp constantly above 30C, or you're using a wireless charging pad (that heat up a fair bit) in high ambient temps, your battery will be fine.
Buriedcode:

--- Quote from: PlainName on December 27, 2023, 02:28:32 pm ---
--- Quote ---I guess that a good charge controller is supposed to switch off after the saturation phase anyway.

--- End quote ---

Well that's the issue here - we are guessing, and no-one seems to know why some manufacturers (who, after all, would probably prefer not to make out that their product might be a bit iffy) state that the phone shouldn't be left on extended charge. If the charge controller, however that's implemented, could just consider the battery done and turn it all off, there would be no need for such warnings, would there? Indeed, does Apple have similar warnings despite apparently being able to control the charge from software?

--- End quote ---

As has been pointed out a few times - documentation meant for consumers is often boilerplate, generic, and has parts left over from previous documentation that may or may not be applicable to your device.
The leaflet that came with my phone mentions not to leave it on charge in four places.  And I am sure Apple, Samsung, Google will have similar statements in their manuals to cover themselves. 

It is far easier to use up extra ink and printing space on documentation than the tiny risk of potential future law suits from not explicitly and repeatedly stating instructions to minimise the small possibility of failure-leading-to-injury. 

A random example: Some snow cleats I have for boots, on the instructions mention "The use of this product does not prevent injury from falling in snowy/icey conditions, take extra care when walking in such conditions as this product does not prevent injury" - see the repetition?

On the off-chance I'm wrong: I would have thought that the review from your device would mention battery issues - if the device mismanaged the charging it would be obvious fairly early on in the devices life.
tooki:

--- Quote from: Halcyon on December 26, 2023, 11:19:44 pm ---
--- Quote from: tooki on December 26, 2023, 01:00:45 pm ---I once replaced the degraded battery in an iPhone 6 (my obsolete backup phone) with a new battery (replaced at an Apple Store, so zero chance of counterfeit battery) and after a year of being left plugged in, the battery swelled.

--- End quote ---

I think there needs to be further explaination here. The iPhone 6 series were riddled with problems. Premature failure of the LCD/digitiser, to battery issues (as you have found out). The iPhone 6 were just a garbage phone with a very high failure rate. Not sure why, but when I was doing digital forensics examinations on an iPhone 6 that wouldn't boot, first thing that got removed was the battery and we powered them up using external power.

--- End quote ---
I don’t think that this “further explanation” is either reasonable or relevant.

For one, the other issues you mention are unrelated and thus totally irrelevant, and I don’t actually believe it had a significantly high failure rate in the long run*. Second, the same device (not another unit of the same model, the same exact unit) had no trouble with its battery when used “normally”, i.e. plugged in overnight but unplugged during the day.

If you don’t even know why you were removing the battery and powering it externally, I hardly consider that a useful anecdote (never mind a data point).

The point was this: the same phone, with the battery used normally, did not result in swelling. But leaving it plugged in 24/7 did, with a brand-new, first-party battery.


*There are few public statistics on failure rates on iPhone models before and after the 6, and what statistics do exist are incomplete and aren’t adjusted for life cycle. (E.g. they compare the iPhone 6 two years in vs the iPhone 7 shortly after launch. So the iPhone 6 looks bad, but it’s unclear whether it actually was that much worse over its lifetime.) IMHO what matters is the failure rate after 3-4 years of use, excluding consumables and accidental damage. Also, I’m not sure if the stats include battery replacements or not: for sure, the iPhone 6 (and all other models in the same basic enclosure design, i.e. 6S, 7, 8, SE (2nd), and SE (3rd)) all have IMHO undersized batteries that get absolutely hammered in normal use, but it’s still arguable to call this wear and tear of a consumable item, not failure in the context of reliability stats.
Siwastaja:

--- Quote from: Buriedcode on December 27, 2023, 02:41:06 pm ---if the device mismanaged the charging it would be obvious fairly early on in the devices life.

--- End quote ---

Which is also the practical, common sense message to those who don't believe in "theoretical" knowledge - if it was normal for a phone battery to completely die by sitting two months plugged in, we would be seeing a lot of complaints, and N would be in millions, not 1.
tooki:

--- Quote from: gf on December 27, 2023, 12:14:13 pm ---Has anyone really evidence that a smart phone battery will last longer than roughly 500 charge/discharge cycles (or say 1-2 years of daily discharge/recharge) if it is not kept plugged-in after the battery has reached 100% SOC? I guess that a good charge controller is supposed to switch off after the saturation phase anyway.

--- End quote ---
Only tangentially relevant, but some LiIon batteries do give multiple cycle life specs depending on usage (for example, the Samsung INR18650-32E datasheet gives a cycle life of 500 cycles when discharged at 3.2A, but just 300 cycles when discharged at 6.4A). But of course we don’t have access to the raw battery specs in a smartphone, just the sanitized/dumbed-down specs given with the phone itself. :/
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