Talk to me about smartphone batteries

[MikeFromLFE]

I've had an otherwise unused smartphone in service as a bedside clock/radio/podcast machine for the last 18 months or so. When it wouldn't for into my homemade mount yesterday I found it had an interestingly bloated battery. This is apparently an Extremely Bad Thing - and it's now safely in a concrete bunker awaiting the battery recycling collection.
This phone - a dreadful Doogee X5 Max Pro - had been plugged into a charger for around 23 hours a day every day for the last 18 months. I assume that keeping it on constant charge is what has caused the battery to fail.

My questions are around this assumption (constantly being on charge is bad) - is this a common/predictable cause of failure?
Or, was it likely to be a symptom of cheap-and-nasty design & production to keep the phone cost to a minimum?
And - if this assumption  is correct, then why do Amazon encourage Fire tablet users to do exactly this to take advantage of the 'Show Mode'  - are tablet batteries somehow a different sort of magic?

[rusky]

I've had 3 phones with puffed batteries,  2 were iPhones and all 3 were used normally. I don't think leaving devices on charge makes a difference as my S6 that's permanently on charge and is 4 years old is fine.

Sent from my SM-T820 using Tapatalk

[Kim]

The charge controller won't over-charge the battery.  It will keep it full, which isn't the ideal state for long-term storage (60% or so is optimal), but is probably better than repeatedly cycling it.

My phone (MotoG 2n gen) was bought in early 2015 and has spent the majority of its life connected to external power.  It's borderline usable with a lean install of LineageOS but steadfastly refuses to die.  My (Galaxy Tab S2) tablet was bought in summer 2016 and has spent even more time on charge.  It's also fine.

TBH I work on the principle that in the absence of physical damage (especially during manufacturing) the batteries on modern devices will outlast their usefulness as a general-purpose computing platform.  You may be able to give them a suitable retirement as a clock/radio or ebook reader.  Barakta has an ZTE Blade (remember those?) with a knackered battery (lasts a few hours without any networking) and a screen that hasn't been quite right since it went for a bath, but it's still fine for ebooks (the aspect ratio seems to work well for her dodgy vision and hands).  Things have definitely improved since the heyday of Nokia.

[ian]

Batteries are always finite life components, people like me wait for them die before upgrading, but that's because I'm a last-adopter. My first Macbook finally swelled up and I had Dell that burst into flames (or rather, toxic smoke). I presume these days with batteries built into slimline forms, quality is higher, no recent device has suffered (and there's a lot of litigation around). Even a c2011 Macbook Air is still getting almost two hours of its heavily used battery (though it does occasionally remind me that the battery needs replacing – which ain't going to happen, sorry). I think most manufacturers these days are focused on getting the most hours out of a single charge, there's no real incentive for long-lifetime batteries, given they want and expect you to upgrade before that becomes an issue.

[Kim]

Yeah, software bloat will generally get them before battery failure will.  That consumers will generally accept non-replaceable batteries demonstrates that premature battery failure is no longer a common problem.

[ian]

Interestingly, people are hanging onto devices for longer but battery life seems to hold out. It was standard for the mothership Dell batteries to die after a couple of years (not always spectacularly, fortunately). Blackberries (remember them) also tended towards suicidal batteries, though at least they were easily and cheaply replaced.

My iPhone 6 is good for a day plus of normal use and I can't, tbh, think of a reason to update it. Things like face-ID might be cool, but really opening it with a fingerprint isn't a labour, all the apps I use still run plenty fast and I can do everything I need. I'll probably only upgrade when it doesn't hold a day's charge, Apple stop providing the updates, or I drop it in a puddle.