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A mobile phone I tested keeps charging at the same constant speed from 0% to 70%, which is 10 watts. It uses MediaTek PumpExpress (input power: 12V 0.83A, despite the wall charger is capable of 12V 1.5A).

Why does it not charge at e.g. 15 watts when at 0%?

Another phone charges at 9V 1.67A (15W) using Qualcomm fast charging. It maintains the exact same charging speed from 0% to 60%. I wonder, why it does not charge any faster while being at indicated 0% than while being at 60%.

Side note: I actually keep my batteries in the healthy percentage range.

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    \$\begingroup\$ Look at how a Li-Ion charger works : Constant Current, then Constant Voltage, then cutoff. Roughly 70% is the Constant Current phase - and a constant rate of increase of charge is the very definition of constant current. \$\endgroup\$ – Brian Drummond Jul 15 '18 at 11:07
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    \$\begingroup\$ Why do you think it should be faster at 0%? \$\endgroup\$ – pjc50 Jul 15 '18 at 12:26
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    \$\begingroup\$ Mobiles are responsible for their own battery choices and charge rates. Not the charger \$\endgroup\$ – Sunnyskyguy EE75 Jul 15 '18 at 12:44
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    \$\begingroup\$ As Bimpelrekkie says, no it should really charge slower below 30% and speed up where the battery is healthiest. But its a phone and its not designed with long life as a top priority. \$\endgroup\$ – Brian Drummond Jul 15 '18 at 12:54
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    \$\begingroup\$ It’s the classic compromise between impatient users in a rush tradeoff with lifespan \$\endgroup\$ – Sunnyskyguy EE75 Jul 15 '18 at 13:03
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Maintaining the exact same charging speed from 0% to 60% batterlevel is in my opinion a bad design choice. Yes it charges your battery faster but it also puts more stress on it. At least these phone slow down the charging above 60% to 70 % and that's a must.

It is generally recommended to only fast-charge Lithium based batteries (practically all phone batteries are Lithium based) at a charge level between 30% to 70 % (maybe 20 % to 80 % is ok-ish as well).

When a Lithium based battery is below 30% or above 70 % charge level fast-charging puts extra stress on the battery and this causes wear. So do that often and the battery will wear out sooner.

So a properly designed phone will only charge at maximum speed when the battery is not at a too low level and not almost full. If a phone does not do that then it is not treating the battery for optimal lifetime.

Source: Battery University and others.

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  • \$\begingroup\$ Only fast charge 30% to 70%? Not 0% to 70%? \$\endgroup\$ – neverMind9 Jul 15 '18 at 12:44
  • \$\begingroup\$ So you mean that instead of the charging speed being higher at 0%, it should be lower at 60%, and the initial charging speed at 0% capacity is already the maximum recommended speed? \$\endgroup\$ – neverMind9 Jul 15 '18 at 12:45
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    \$\begingroup\$ batteryuniversity.com/learn/article/… indicates that recommended method is to fast charge from 0%. However, overdischarged cells may recover if trickle charged to 2.5-3V, after which they can accept full charging current. \$\endgroup\$ – Cristobol Polychronopolis Jul 16 '18 at 12:40
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    \$\begingroup\$ @CristobolPolychronopolis Where did you read that? I do not see fast charging for Lithium based cells mentioned. What you might have readt is that they mean that normal charging (so at 1C, that's not fast charging) can start from 0% once the voltage has recovered enough. And again, I'm not saying (fast) charging cannot be done or should not be done at 0% charge level. It just stresses the battery more when you do. \$\endgroup\$ – Bimpelrekkie Jul 16 '18 at 12:50
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    \$\begingroup\$ @Bimpelrekkie, I was using "fast charge" to indicate the current limiting phase of a normal charge cycle. I wouldn't recommend charging over 1C at any point in the charge cycle without detailed communication with the battery manufacturer. \$\endgroup\$ – Cristobol Polychronopolis Jul 17 '18 at 12:13
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Constant charging speed from "conditional 0" to 60% is a simple matter of basic standard Li-Ion charging method - the charger is in CC - constant current mode. See more details here.

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