Li-ion batteries EOL?

Yes - capacity drops. IME, it's a long gradual process over most of the life, but at end of life the rate of decline accelerates considerably.

What's the battery meter? Often, this is based on previous behaviour of the battery, and it can be very wrong when rate of decline accelerates.

When I swapped a new battery into my Samsung S4, the battery meter showed a similar rapid decline that the old one had done, but later you can see the phone changed its mind and the remaining charge is shown as increasing when it's not on charge, presumably as it realised the voltage was not dropping at the rate it had got used to with the previous battery. After a charge/discharge cycle, it was showing more accurate remaining charge graphs again, having learned the new battery characteristics.

yes.

sometimes

They lose capacity and peak power delivery. In short they behave like SMALLER batteries.

I've just had the only one that has failed of mine go from normal time between recharges to very little time at all on a laptop.

Well on a sample of one they seem to chaarge and look good but gradually seem to have less time on load before things go pear shaped. I have a feeling its probably one cell much as many rechable packs to, that get reverse charged by the others when they die first making the death come sooner. Brian

Phone so a single 3.7 V cell.

From being "normal" maybe two weeks ago this one has decided it will just sometimes die, extremely quickly, without warning. It's sort of related to load, flash photo caused it to die this afternoon, turning on and using the WiFi for 20 mins can trigger it as well.

The battery failure is sudden, no warnings, and the phone reboots or tries to. There isn't quite enough juice left for the boot to complete so it ends up in a reboot loop. B-( The phone may give a rather belated "critical" battery warning if a boot gets far enough.

My phone with its old battery behaves strangely. It will still make a call for as long as it used to, but it uses the battery up in less than 24 hours when idling. So the capacity is good when under high load, but poor when under low load?!

So same result as mine in that regard.

Don't really get to vary the load much with a laptop so I can't test that.

Don't get that with a laptop, it just shuts down completely.

It will usually attempt to start again if you hit the power button but doesn't usually stay up for long tho it does boot.

The laptop just turns itself off, no real warning like you get with phones.

I haven't tried opening the battery up to check the individual cells. This is a double the capacity battery that I got for that laptop years ago now which did work very well for a long time and still does give a few minutes before it shuts down, and still works fine on the charger and I haven't looked very hard for the original battery yet, and don't want to open this one until I have found it. Its very unlikely to be a bad joint or something easily fixable so its more just interesting to see what has happened to it to get that sudden death rather than just the more commonly seen gradual loss of time between charges. Likely one cell has just died.

Eventually it reports charged soon after being connected or the safety fuse bricks the battery completely if it gets too hot.

Are you sure you haven't got something running in the background on the phone that is wasting power? Fancy smart phones barely last 24 hours heavy use anyway. Better ones last almost a week of light use.

I find I lose significant battery life if the signal is marginal which means I take the phone out of my shirt pocket when at my desk. Adds about 30% to the standby battery life by giving it a better signal.

There is almost no mobile signal where I live.

It's not a fancy one. It's about the earliest phone which will go on the internet. It cannot render anything but the simplest of websites. I think it runs on some Java OS - virtually no apps will work on it, although I managed to get an Ebay and a Facebook one.

That last line is it. I have 1 to 2 bars of signal in my house. I can always make a call in 99% of my town, but I often miss a couple of words out of the sentence spoken to me.

Now 6 months ago, I had my battery lasting only an hour or two, and the phone was reporting no signal at all. I phoned Virgin Mobile, who told me there were no cell outages in my area, and that the dying battery is caused by it repeatedly looking for a signal (presumably it transmits a lot). Turned out that SIM cards die after about 10-15 years (which mine was) and they sent a free replacement.

Hmm. we need a way to distinguish 'discharges quickly' from 'has reached its end of life'