Why Phone Charging Slows After 80%: The CC-CV Curve • The Science 360

Apple says an iPhone 17 reaches 50 percent in about 20 minutes on a 40 W adapter. Nobody advertises how long the trip to 100 percent takes, and there is a reason. In the charging model I built for this post, a 4,500 mAh phone cell charging at that rate gets to 50 percent in 20 minutes, 80 percent in 33 minutes, and needs another half hour to finish. Slow the charger down to a quarter of the current and the last stretch, from 80 percent to full, takes almost exactly as long.

That last fact is the clue. The slowdown is not mainly your phone being cautious with its settings. It is what happens to any lithium-ion cell charged against a fixed voltage ceiling, and once you see why, the 80 percent number, battery wear and "optimized charging" all fall into place.

Quick answerLithium-ion batteries charge at constant current until the cell reaches its maximum voltage (about 4.2 to 4.45 V), usually around 60 to 80 percent. The charger then holds that voltage, and current falls as the battery's own voltage rises toward it. This protects the cathode from overcharge and the anode from lithium plating.

Infographic: Why does charging slow after 80%? A drawing shows a phone at 80% beside a sketch of charging current, flat and then decaying. In the post's model of a 4,500 mAh cell, 80% to full takes about 32 minutes at 1C, 1.5C or 2C, because in the constant-voltage stage the battery sets the current, with a taper time constant of about 10 minutes. A bar chart shows 0 to 80% taking 96 minutes at 0.5C, 48 at 1C and 26 at 2C, while 80% to full takes 37, 32 and 32. Tiles: stopping near 80% gives about 60% more lifetime energy; near 0 °C the anode reaches −80 mV, a plating risk. — Tap the image to view it full size. View the accessible, editable SVG.

What is CC-CV charging?

Almost every lithium-ion charger uses two stages, which Battery University describes as a constant-current stage followed by a constant-voltage "saturation" stage.

Constant current (CC). The charger pushes a fixed current, as high as the cell and the phone's thermal design allow. The cell voltage climbs steadily as it fills.
Constant voltage (CV). When the cell voltage hits its limit, the charger stops raising it and holds it there. From then on the battery decides how much current flows, and the answer keeps getting smaller. Charging ends when the current falls to about 3 to 5 percent of the cell's amp-hour rating.

Figure 1 shows the whole thing for a modelled phone cell at 1.5C (6.75 A into a 4,500 mAh cell, roughly 28 W, which is what a modern fast charger delivers once conversion losses are taken out).

Simulated CC-CV charge of a 4500 mAh phone cell at 1.5C. Current is flat at maximum for 28 minutes while state of charge rises linearly to 69 percent and cell voltage rises from about 3.7 to 4.4 volts. Then voltage stays at 4.4 volts and current decays smoothly toward zero while state of charge curves up slowly to 99 percent at 65 minutes. — Figure 1. Simulated CC-CV charge at 1.5C with a 4.40 V limit. The switch to constant voltage happens at 69 percent after 28 minutes. The model is a resting-voltage curve for a cobalt-oxide/graphite cell plus a 50 mΩ internal resistance; real cells add slower diffusion effects, so treat the times as illustrative.

Nothing in that curve is a software decision. Once the voltage is pinned, the current has to fall, for the reason in the next section.

Why does the current fall in the constant-voltage stage?

Treat the cell as its resting voltage $V_{oc}$ (which depends on how full it is) in series with a small internal resistance $R$ . Whatever voltage the charger applies, the current is set by the difference:

I = \frac{V_{\mathrm{charger}} - V_{oc}}{R}

During CC the charger raises its voltage to keep $I$ fixed. During CV the charger voltage is fixed at the limit, so the current is simply the gap divided by $R$ . As the cell fills, $V_{oc}$ rises toward the limit and the gap shrinks. Here is the worked version for our model cell (4.40 V limit, R = 50 mΩ, capacity 4.5 Ah):

When does CC end?

\begin{aligned}I_{CC} &= 1.5 \times 4.5\ \mathrm{Ah} = 6.75\ \mathrm{A} \\ I R &= 6.75\ \mathrm{A} \times 0.050\ \Omega = 0.34\ \mathrm{V} \\ V_{oc} &= 4.40 - 0.34 = 4.06\ \mathrm{V}\end{aligned}

CV begins at that resting voltage, when the cell is about 69% full.

Current during CV

\begin{aligned}I_{80\%} &= \frac{4.40 - 4.16}{0.050} = 4.8\ \mathrm{A}\ (1.07C) \\ I_{90\%} &= \frac{4.40 - 4.27}{0.050} = 2.6\ \mathrm{A}\ (0.58C) \\ I_{95\%} &= \frac{4.40 - 4.33}{0.050} = 1.4\ \mathrm{A}\ (0.31C)\end{aligned}

Resting voltages: about 4.16 V at 80%, 4.27 V at 90% and 4.33 V at 95%.

The taper time constant near full

Slope of $V_{oc}$ near the top ≈ 0.13 V per 10% of charge:

\begin{aligned}\frac{dV_{oc}}{dQ} &= \frac{0.13\ \mathrm{V}}{0.45\ \mathrm{Ah}} = 0.29\ \mathrm{V/Ah} \\ &= 8.0\times10^{-5}\ \mathrm{V/C} \\ \tau &= \frac{R}{dV_{oc}/dQ} = \frac{0.050}{8.0\times10^{-5}} \\ &\approx 620\ \mathrm{s} \approx \mathbf{10\ min}\end{aligned}

That τ is the same idea as the time constant of a capacitor charging through a resistor: the battery behaves like a very large capacitor near the top of its range, and the current decays roughly exponentially, falling by a factor of e (to 37 percent) every 10 minutes or so. Each percent therefore takes longer than the one before. That is why the last few percent drag even on a charger with power to spare.

Notice what sets the 80 percent "knee". It is the voltage the internal resistance eats, $IR$ . A bigger current wastes more voltage across $R$ , so the cell hits the limit sooner and spends longer in the taper. That leads to the most counterintuitive result in this post.

The misconception: a faster charger speeds up the whole charge

Most explanations say that fast charging "slows down near the end to protect the battery", which implies a slow charger would just do the same thing slower. Run the same model at four currents and look at Figure 2.

State of charge versus time for CC-CV charging at 0.5C, 1C, 1.5C and 2C. The fast curves rise steeply but switch to the taper earlier: at 56 percent for 2C, 69 percent for 1.5C, 81 percent for 1C and 92 percent for 0.5C. All curves approach 100 percent with similar shapes; total times are about 59, 65, 80 and 133 minutes. — Figure 2. Same cell, same 4.40 V limit, four charge currents. Quadrupling the current from 0.5C to 2C cuts the time to 50 percent from 60 to 15 minutes, but the time to finish only falls from 133 to 59 minutes. The dots show where each charge switches to constant voltage.

Time for each part of the charge (model)
0.5C: 0 to 80% in 96 min; 80% to done in 37 min
1C: 0 to 80% in 48 min; 80% to done in 32 min
1.5C: 0 to 80% in 33 min; 80% to done in 32 min
2C: 0 to 80% in 26 min; 80% to done in 32 min

The first 80 percent scales almost perfectly with current. The last 20 percent barely changes, because in the CV stage the current is set by the battery, not the charger. Battery University makes the same point from measurements: a higher charge current fills a cell to about 70 percent quickly but lengthens the saturation stage, so it does not hasten the full charge by much. This is why phone makers advertise "50 percent in 20 minutes" and never "100 percent in 60".

Why not just push full current all the way to 100%?

Because two different parts of the cell would be damaged, and the voltage limit protects both.

The cathode and electrolyte. The voltage limit exists because a cobalt-oxide cathode becomes unstable when too much lithium is pulled out of it, and the electrolyte starts oxidising at high potentials. Battery University gives 4.20 V ± 50 mV as the classic limit for cobalt-blend cells, with high-capacity designs going to 4.30 V or higher; current phone cells typically run higher still, around 4.4 V or more, using cathode coatings and additives to cope. If the charger kept pushing constant current past the limit, the terminal voltage, and with it the stress on the cathode, would keep climbing.

The anode, and lithium plating. This is the part most articles skip. While charging, lithium ions have to slide into the gaps between graphite layers in the anode. The graphite's potential relative to lithium metal is only about 0.1 V when the cell is nearly full, and charging current pushes it lower by an overpotential that grows with current. If it goes below 0 V versus lithium, it becomes easier for the ions to deposit as metallic lithium on the surface than to slip inside. Research groups such as the Cui group at Stanford have measured graphite potentials dropping below 0 V at higher charging rates, and NREL frames the whole challenge of very fast charging as staying ahead of plating onset. Plated lithium is permanently lost capacity, and in bad cases it grows into needles that can short the cell.

Figure 3 makes that budget visible with a simple model: the graphite's resting potential (from a widely used fit by Doyle and colleagues) divided by an assumed overpotential per unit of charge rate.

Maximum charge rate before the graphite anode reaches zero volts versus lithium, against state of charge. At 25 degrees the limit falls from above 4C at low charge to about 2.5C at 80 percent and below 2C at full. Near 0 degrees the limit is about 1.1C at 40 percent and 0.8C at 80 percent, below a 1.5C fast-charge line almost everywhere. — Figure 3. Illustrative plating budget. Graphite potential from the Doyle et al. (1996) open-circuit fit; the overpotentials of 40 mV per C at 25 °C and 120 mV per C near 0 °C are round-number assumptions for a phone cell, not measurements. The shape, a budget that shrinks as the cell fills and collapses in the cold, is the robust part. Recent work notes that 0 V is a guide rather than a sharp threshold.

Plating margin at 80% charge, 1.5C

Graphite potential at rest ≈ 100 mV versus Li.

\begin{aligned}\eta_{25^\circ\mathrm{C}} &= 1.5 \times 40 = 60\ \mathrm{mV} \to +40\ \mathrm{mV} \\ \eta_{0^\circ\mathrm{C}} &= 1.5 \times 120 = 180\ \mathrm{mV} \to -80\ \mathrm{mV}\end{aligned}

The arrow gives the anode potential. At 25 °C it stays safe; near 0 °C it goes below lithium, a plating risk.

This is why phones refuse to fast charge when cold, why Apple notes that fast charging might not work when the phone is too cold or too hot, and why Battery University's plainest rule is not to charge at freezing temperatures. It is also why modern phones often step the current down in several stages before the voltage limit is even reached: the safe current shrinks as the anode fills.

How much does stopping at 80% actually help the battery?

A lot, and the reason is voltage rather than percent. Battery University's table for cobalt-based cells charged to different voltage limits (Figure 4) shows the trade.

Discharge cycles and stored energy versus charge voltage limit for cobalt-based lithium-ion cells. At 4.30 volts, 150 to 250 cycles and 110 to 115 percent energy; at 4.20 volts, 300 to 500 cycles and 100 percent; at 4.06 volts, 600 to 1000 cycles and 81 percent; at 3.92 volts, 1200 to 2000 cycles and 65 percent. — Figure 4. Cycle life and stored energy versus full-charge voltage, plotted from Battery University's BU-808 table for cobalt-based cells. Their rule of thumb: each 0.10 V lower roughly doubles cycle life. Phone cells designed for 4.4 V or more shift the whole picture to the right, but the slope is the lesson.

Reading the table

Full charge at 4.20 V: 300 to 500 cycles, 100% energy. Stop at 4.06 V (about 81% energy): 600 to 1,000 cycles. Using range midpoints, lifetime energy delivered in full-charge equivalents:

\frac{0.81 \times 800}{1.00 \times 400} = \frac{648}{400} \approx 1.6

About 60% more.

So stopping at roughly 80 percent costs you a fifth of each day's capacity and roughly doubles the number of cycles. Time matters too: a cell parked at full voltage ages even when nobody is using it, which is why the features below focus on how long the phone sits at 100 percent.

What do optimized and adaptive charging actually do?

They exploit the curve above. Apple's Optimized Battery Charging uses on-device learning of your routine to "delay charging past 80% in certain situations", showing "Charging On Hold" while it waits, then finishes shortly before you usually unplug. Google's Adaptive Charging on Pixel does the same overnight, and many Android phones offer a fixed cap at around 80 to 85 percent. Some phones also pause when warm.

Physically, the phone is choosing to spend the night at a resting voltage near 4.1 V rather than at the limit. The charging current, the plating budget and the final capacity are unchanged; only the hours spent at the top are cut. If your phone seems to "stop at 80", check those settings before blaming the charger.

What this means for you: if you are in a hurry, 20 minutes on a fast charger gets you most of the useful charge, and the last 20 percent will take about half an hour on almost any charger. Fast charging in the low and middle range is not the main wear mechanism. Heat, cold-weather fast charging and long hours at 100 percent are.

FAQ

Is it bad to charge my phone to 100%?

Not dangerous, but it ages the battery faster than stopping lower. Wear depends strongly on how high the cell voltage goes and how long it stays there. Reaching 100% occasionally is fine; sitting at 100% for many hours every night is what charge limits and optimized charging try to avoid.

Does fast charging damage the battery?

Fast charging at moderate temperature, in the low and middle state of charge range, causes little extra wear, which is why phones only charge fast there. The risky combination is high current at high state of charge or in the cold, which can plate metallic lithium on the anode. Charge controllers are designed to avoid exactly that.

Why does my phone stop charging at 80%?

Usually because a battery protection feature is on, such as Optimized Battery Charging on iPhone or Adaptive Charging on Pixel, which pauses near 80% and finishes before you normally unplug. Heat can also pause charging. A fixed charge limit setting will hold it at 80% permanently.

Why does the last 1% take so long to charge?

In the constant-voltage stage the charging current is proportional to the gap between the charger's voltage limit and the battery's own voltage. As the battery fills, that gap shrinks, so the current falls roughly exponentially and each percent takes longer than the last.

Sources

Apple Support, "Fast charge your iPhone" (iPhone 17: up to 50% in about 20 minutes with a 40 W adapter; fast charging may not work when too cold or hot): support.apple.com/en-us/102574
Apple Support, "About Optimized Battery Charging" (delays charging past 80%, "Charging On Hold"): support.apple.com/en-us/102338
Battery University, BU-409 "Charging Lithium-ion" (CC then saturation stage, 4.20 V ± 50 mV, termination at 3 to 5% of rated current, higher current lengthens saturation, do not charge at freezing): batteryuniversity.com/article/bu-409-charging-lithium-ion
Battery University, BU-808 "How to Prolong Lithium-based Batteries" (table of cycles versus charge voltage; 0.10 V lower roughly doubles cycle life): batteryuniversity.com/article/bu-808-how-to-prolong-lithium-based-batteries
Hansen and Cui et al., PNAS 2020 (graphite potentials below 0 V versus Li at higher charging rates; temperature and plating): web.stanford.edu/group/cui_group/papers/Hansen_Cui_PNAS_2020.pdf
NREL, fast-charging pouch cell study (plating onset as the limit to 80% in 10 minutes): nrel.gov/docs/fy23osti/86256.pdf
Gao et al., Joule 2021 (lithium intercalation versus plating on graphite particles): mit.edu/~bazant/papers/pdf/Gao_2021_Joule_Li_metal_graphite.pdf
Review on lithium plating onset criteria (0 V criterion alone is inadequate), Journal of Materials Chemistry A, 2024: pubs.rsc.org/en/content/articlepdf/2024/ta/d4ta00244j
Doyle, Newman, Gozdz, Schmutz and Tarascon, "Comparison of Modeling Predictions with Experimental Data from Plastic Lithium Ion Cells", J. Electrochem. Soc. 143, 1890 (1996), source of the graphite open-circuit potential fit: doi.org/10.1149/1.1836921

Method note: Figures 1 to 3 come from a deliberately simple model (resting voltage curve plus one resistance, 4.40 V limit, termination at 0.05C). Real phones use multi-step current profiles, temperature-dependent limits and cells with diffusion delays, so exact minutes differ by phone. The relationships, not the minutes, are the point.

Header image: NASA (image iss064e041250), regraded for this article.

What is CC-CV charging?

Why does the current fall in the constant-voltage stage?

The misconception: a faster charger speeds up the whole charge

Why not just push full current all the way to 100%?

How much does stopping at 80% actually help the battery?

What do optimized and adaptive charging actually do?

FAQ

Is it bad to charge my phone to 100%?

Does fast charging damage the battery?

Why does my phone stop charging at 80%?

Why does the last 1% take so long to charge?

Related on The Science 360

Sources