i used an old Nokia 5185i for ten years, had the original plus two other stock type batteries bought from flea markets where they had baskets just full of the phones/parts (they didn't make the battery anymore and i wanted to keep using it until i got my first Smartphone, at the time i was resistant to smart devices of any kind) so i do know that they will usually last longer than you think, but i think most Li-Ion batteries have a 'brain' of some kind that tells the charger to stop charging when they reach a designated charge cycle number (100, 300? etc) and this is usually a year or two now. the old batteries in that Nokia were NiMH, and never did last long when new, but i stopped using that phone and upgraded when i barely got a day of standby anymore. sad that they don't last as long as they used to, but i could be wrong. my old Coby Kyros tablet is at least two years old, non-removable battery and still holds a 5-7 hour charge, which it did out of box.
one of the first laptops i owned that had Li-Ion battery packs was a Latitude CPi, and it threw a error code on the charge display (back when the pack had LED readouts if you removed it and hit a button, they would also flash codes of certain LEDs to indicate faults such as high temp and such) when it reached the end of life, and would not only refuse to charge, but wouldn't let you use what was left. it was known as the infamous Dell 1-3-5 error since the code would flash the first, third and last LEDs in that little readout when it was on charge cycle 300
best way to extend a Li-Ion battery lifetime to its max is never let it drop below 50% if possible. charging at a low SOC (20% or less) not only harms them, but also counts as 1 charge cycle. the lifetime if they are brain equipped tops out at 100 to 300 depending on the make/model. budget phones/tablets have it set low. i always charge each night, when there is well over 75% left. this way the brain is fooled into assuming it's still on the last known cycle, and does not register a count. i don't take chances, with battery tech getting as smart as our phones, i don't like to risk it. they may not all have this little electronic gizmo in there but i don't want to find out the hard way.
