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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.

neverMind9
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    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. –  Jul 15 '18 at 11:07
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    Why do you think it should be faster at 0%? – pjc50 Jul 15 '18 at 12:26
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    Mobiles are responsible for their own battery choices and charge rates. Not the charger – Tony Stewart EE75 Jul 15 '18 at 12:44
  • @TonyEErocketscientist I just meant, that the charger **offers a higher speed,** which the mobile phone could make use of if it wanted to. – neverMind9 Jul 15 '18 at 12:46
  • @pjc50 Because the battery is emptier at 0% than at 60%, so it should be charging faster at 0% than at 60%. – neverMind9 Jul 15 '18 at 12:47
  • @BrianDrummond Constant current, yes. So do you mean it is normal for a battery to have the exact same charging speed at 5% capacity as at 60%? – neverMind9 Jul 15 '18 at 12:48
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    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. –  Jul 15 '18 at 12:54
  • @BrianDrummond That's why some mobile phones have **user-replaceable** batteries. – neverMind9 Jul 15 '18 at 12:57
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    It’s the classic compromise between impatient users in a rush tradeoff with lifespan – Tony Stewart EE75 Jul 15 '18 at 13:03
  • How are you measuring the charge rate? The user-facing battery display may not be linear. – Russell Borogove Jul 15 '18 at 17:10

2 Answers2

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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.

Bimpelrekkie
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  • Only fast charge 30% to 70%? Not 0% to 70%? – neverMind9 Jul 15 '18 at 12:44
  • 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? – neverMind9 Jul 15 '18 at 12:45
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    http://batteryuniversity.com/learn/article/charging_lithium_ion_batteries 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. – Cristobol Polychronopolis Jul 16 '18 at 12:40
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    @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. – Bimpelrekkie Jul 16 '18 at 12:50
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    @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. – 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.

Ale..chenski
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