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Charging the OnePlus One smartphone
Since I could not find any specifications, I conducted a test. I measured one charging cycle from 0 to 100%, so I have an estimate now. I first discharged the battery to 0%, until the phone powered down. Then I connected the phone to the original power adapter and occasionally noted the charge percentage and the clock.
The first result is that the phone reached 50% after 45 minutes, which is normal for a smartphone, perhaps even a bit on the fast side.
Assuming that the displayed percentage is correct, that the charging up to 50% is linear, and that the charging efficiency is 100%, this calculates a charge current of 2.07 A. Given that the charge efficiency of Li-Ion cells is 97% to 99%, the actual charge current should calculate a tad higher, let's say 2.1 A.
This coincides very nicely with the specifications of the charger, which states a maximal output current of 2100 mA.
Of course this calculation is not necessarily precise. The 50% figure may be inaccurate or the cell may have a capacity that differs from the official 3.1 Ah specification. Still it provides a fair estimate of the initial charge current. Also I had powered up the phone while charging and even used it for very few minutes, so my results are realistic. If you charge the phone while it is powered down, charging may be slightly faster, but not by much, as the sleeping phone uses very little power.
Since you cannot charge a Li-Ion cell at maximal current until it is full, the current eventually tapers, and it takes longer than 90 minutes to reach 100%. In my experiment I approximately reached 50% in 45 minutes, 90% in almost 90 minutes, 95% after 105 minutes, and 100% after a bit more than 2 h.
A good question is what happens when I connect the phone to a charger that cannot deliver 2.1 A. (See also the first comment below.) Unlike the digital signalling between phone and charger of the iPhone, all Android phones I know have no signalling between phone and charger, except that the data lines are shorted with a low resistance to signal a high-current (AC) charger to the phone, compliant with the Common External Power Supply (EPS) standard, also known as EN 62684:2010 and IEC 62684:2011. This leads to the phone displaying "Charging (AC)" in Settings, About phone, Status, Battery status. If you connect the phone to a computer's USB port, this will display "Charging (USB)", and the phone will only draw a maximum of 0.5 A, complying with the limitations of USB data ports. This incurs at least a threefold charging time.
This means that no signalling takes place. As far as I can tell, using a charger that cannot deliver 2.1 A could have any of the following effects:
I don't think very highly of cheap chargers and am pretty much certain that using a 1 A or 1.5 A charger will at least lead to long or extremely long charging times, if not to damaging the charger. It will hardly lead to damage to the phone though.
Therefore my recommendation with weak chargers is to either avoid them altogether or test them carefully, but note that I refuse any responsibility for such tests. You are on your own. I think you should carefully observe two things during the test:
By the way, the high charging current has another implication: You have to use a low-resistance USB cable. At 2 A a total cable resistance (both wires) of 0.4 Ω (0.2 Ω in either wire) will reduce the voltage at the phone by 0.8 V and will likely reduce the charging speed, as the cell needs to be charged to some 4.2 V to be fully charged, as the output voltage of the charger is 5 V. Longer cables are more prone to high resistance, but I have seen short cables with far too thin wires.
If you don't have measuring equipment, the simplest way is to charge the phone when its charge is below 30% and see if you can charge it 10% higher in 9 min or 20% higher in 18 min. If yes, the cable is fine.
Recommended Micro-USB charging-only cables are made by portapow and sold, among others, through Amazon.
And here is a hint for the neurotic technophiliacs among us. If you want to extend the life of your Li-Ion battery, don't charge it fully (unless you really need it full, of course). If you only charged it to 50% or 60%, it would live almost forever, through thousands of charge-discharge cycles. Only the higher voltages stress the battery and reduce its usable charge-discharge cycles to below 500.
But please don't become too neurotic about this. A new battery does not cost all that much. I'm only saying, if you are charging and are certain that the current charge level will last until you hit the next charging opportunity, just pull the plug and be happy. On the other hand there is nothing wrong with charging your phone full every night and buying a new battery after a few years.
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