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Why does it so long for an Lithium-ion battery to be charged from 98% to 100%?

It is not even comparable to the effort I exerce to pump my bike tires from 95 psi to (the minimally usable) 100 psi. There the effort is a few times more than that needed to pump from 35 psi to 40 psi. With Apple's Lithium ion batteries, it takes at least an order of magnitude more to charge from 98-100% as it takes to charge from 8-10%.

Does the analogy hold? Is it actually chemically harder to charge? Is it detrimental to charge any faster (for heat, perhaps)? Is the internal charger simply delaying the charge to reduce the duration it will stay fully charged (because that itself is detrimental)?

The difference in rate-of-charging may be particularly noticeable this time because I was using a 20 W charger.

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    $\begingroup$ Well, in some sense, chemistry is involved in everything, from psychology to cosmology, but it does not mean chemists are experts for everything. $\endgroup$
    – Poutnik
    Nov 23 '20 at 15:58
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    $\begingroup$ @Poutnik I'd have to dig out the reference, but chemists and other people who work in areas that are more inter-disciplinary tend to be better at understanding more things. $\endgroup$
    – Zhe
    Nov 23 '20 at 16:05
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    $\begingroup$ @Zhe I do not disagree, but people not involved in chemistry often stretch this assumed knowledge a lot, taking chemists almost as magicians who control the mystery of knowledge of everything. :-) $\endgroup$
    – Poutnik
    Nov 23 '20 at 16:08
  • $\begingroup$ What does it mean (to Apple engineers) that the battery is 98% charged? How do they measure it? $\endgroup$ Nov 23 '20 at 18:16
  • $\begingroup$ @KarstenTheis The output voltage probably correlates well with the amount of charging. $\endgroup$
    – Zhe
    Nov 23 '20 at 18:26
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The short answer is: The ending stage uses much smaller charging current, so it lasts longer. TCharging of any cell in the contant voltage mode, not limited to lithium cells, leads to asymptotically decreasing of the charging current and progressively slowing down charging proces.

The reason behind is the charging voltage must not cross the maximal allowed voltage to avoid detrimental or even dangerous side chemical processes. These processes may include :

  • forming metallic lithium on surface of graphite-lithium intercalate $\ce{C6Li_x}$
  • reaction of lithium with the dialkylcarbonate solvent
  • irreversible collapse of $\ce{Li_xCoO2}$ structure, forming $\ce{CoO2}$ and $\ce{Li2O}$, if too many of lithium ions leave the compound.
  • forming gases from side reactions, like alkanes and ethers.
  • a rupture of the cell
  • ignition of the volatile flammable solvent, powered by metallic lithium.

This observed slow end of charging phenomena is not iPhone specific, but applies to all devices using Li-ion/Li-Po cell technology. It is matter of the lithium cell protection-circuit design and the device charging control software/firmware design.

The usual Li-ion/Li-Po charging schema is

  1. Optional Trickle charge ( the link terminology ) by low current to revive deeply discharged cell.
  2. Charging by a constant current until the battery maximal voltage threshold is reached. This happens typically when 70-80% of the actual cell capacity is reached, the lower value for older cells with greater internal resistancy.
  3. Charging switches to the constant voltage charging, with the charging current asymptotically decreasing toward zero. Charging is then usually stopped when the currect drops below the chosen minimal charging current threshold. This mode provide lower charge increase per time, as the average current is lower.
  4. Some charging algorithms use as the last step the "topping" charging mode of staying at the maximal voltage for some time, to push into the cell the maximal charge ever possible, with expense of the cell life-time. This topping significantly prolongs the total charging time, as the average charging current is minimal here.

I have answered from my memory,but there is plenty of related online resources, like e.g. powerelectronics - charging-liion-batteries-for-maximum-run-times

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  • $\begingroup$ That all makes sense, but what is your level of confidence in your answer? When you write "chemistry is involved in everything, from psychology to cosmology, but it does not mean chemists are experts for everything.", I get the impression that you're taking a stab at the answer, but are not really sure. Can you clarify either way? $\endgroup$
    – Sam
    Nov 24 '20 at 19:09
  • $\begingroup$ @Sam I have not fully got your last sentence before the ending question. I have not answered from the chemistry point of view, as chemistry has little to do with that ( It has, but indirectly, via the critical cell maximum voltage). I have answered from point of view of many years of experience in lithium cell maintenance. All chargers charge them asymptotically, so it is clear that the ending amount of charge takes a lot longer than the same but starting amount of charge. – $\endgroup$
    – Poutnik
    Nov 24 '20 at 19:24
  • $\begingroup$ By other words, I am quite confident in that, what is based on quite a lot of reading and experience. I think if you searched about the topic little bit before asking, you would reach the similar conclusion and would not even ask. It is quite widely spread public knowledge about CC/CV charging of lithium cells. $\endgroup$
    – Poutnik
    Nov 24 '20 at 20:35
  • $\begingroup$ If the question is junk, let's not sully the site with noise. Please delete your answer, so that I can delete my question. Thanks. $\endgroup$
    – Sam
    Nov 25 '20 at 0:35
  • $\begingroup$ No need to delete, there are much worse questions here. $\endgroup$
    – Poutnik
    Nov 25 '20 at 6:24

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