Will Your Phone Let You Quickly Top up the Battery?
Conducting some limited testing to find out if an iPhone will quick charge the last 10% of battery percentage if plugged in to quickly reach full charge. The answer is that the iPhone doesn't care.
If a phone is plugged in at 25%, 60%, or 90% battery, will it follow the same charge profile to 100%, or try to charge at a different wattage to optimize for the scenario? Will it detect that you're trying to quickly charge the final 10% and provide more energy?
I have always assumed that keeping the charger and ambient temperature the same, it would just resume along the same path to 100%. However, with how complex battery charging is, nothing can be guaranteed.
We still don't know definitively, but I have a few tests below to show that for the iPhone 17 Pro, when the phone is plugged in, it will 'resume' the set charge profile based on the state of charge.
Testing
We used the iPhone 17 Pro and Apple 40W Dynamic Power Adapter for this test, measuring at the USB Type-C port with the Infineon CY4500-EPR. We used our standard test settings and procedures detailed here and here, with the exception of the following. Instead of completely discharging the iPhone, it was only discharged to the desired state, and then turned off, allowing it to cool before conducting the test with the phone powered off.
The reported battery percentage of iPhones(and most battery powered devices) is essentially a good guess at the total energy stored in the battery. Cycling the power can cause the reported battery percentage to change(the one displayed to the user), so the tests weren't started at exactly the 0%, 25%, 50%, 75%, and 90% points, but thankfully it isn't particularly critical to this exploration.
Results
Below are a few variations of a graph, all with the same data. As indicated in each graph, the presentation varies depending on whether a rolling average was applied, and whether the traces are aligned when the device stopped charging, or at the transition to constant voltage charging(vertical dashed white line).
{{slide}}Charge Curve Transition Aligned - Device Off, Rolling Average{{/slide}} {{slide}}Charge Curve Transition Aligned Aligned - Device Off{{/slide}} {{slide}}Charge Curve End Aligned - Device Off, Rolling Average {{/slide}} {{slide}}Charge Curve End Aligned - Device Off{{/slide}}
The first graph is likely the most illustrative, with the rolling average applied and the traces aligned at the transition from constant current to constant voltage charging(around 60 minutes). We can see that the full charge maintains roughly 30 W until around 50% charge, with the 25% charge joining and mirroring it. Then the wattage gradually decreases until the device reaches around 75-80%, where it charges at less than 10 W for the rest of the duration, tapering down quite gradually. Excusing some variance expected for any battery test, all of the curves are remarkably similar.
The Last 10%
My hypothesis was that phones may act a little differently when plugged in at a higher state of charge, to sacrifice a little bit of battery health longevity to quickly top up your charge. However, the results above are convincing that the iPhone 17 Pro will follow a relatively consistent charge curve no matter the battery percentage when it is plugged in. There’s no convincing it to quickly charge that final 10% before you head out for a long day.
We don’t have the data to say this for all devices, but I haven’t seen any conflicting results for other charge curves. Let us know if there are other devices that behave strangely. We might be able to test them out and try to answer your questions!
