As far as I could understand, the charging speed is measured in amps. Like, if I use Ampere, it will tell that it charges in 2.5 amps, or 1.5 amps. I hope I got this right. Amps is the only measurement I should look at when it comes to charging speed, right?

Also, I'm really confused about this. Here is the writing on my charger.

Output: 5v=2.5A/9v=2a ... 12.5v=1.5a

Power = Amps X Voltage. So, when delivering...

2.5 amps = consumes 12.5 watts
2 amps = consumes 18 watts
1.5 amps = consumes 18.75 watts

I've looked at other phone chargers and haven't seen anything like this. They usually have only one formula; like 5v = 1a

I think the reason behind these 3 different formulas is due to the quick charge technology. I've heard that this technology usually "decides" how fast to charge at a given temperature and battery level. Is that the case?

Also, since electricity bills are measured by how many watts you use, why does it use more watts when charging slower and use less watts while charging faster? Why does it up the voltage while delivering less amps thus causing more electricity consumption?

This really doesn't make any sense to me so if someone could explain this formula, I would be grateful. (I know that phone chargers don't consume that much elecricity - money wise. It's just that I'm trying to understand how electricity works but the more I try to understand, the more I get confused.)

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    \$\begingroup\$ I start to suspect that majority of confusions and multitude of questions of this sort originates in wrong terminology adopted in the marketplace. The "charger" in question is not really a battery charger, it is a "AC-DC Adapter", or simple power supply, with several output options. So any mentioning of "output currents" on the nameplate of the adapter has nothing to do with actual value of current used to charge the actual battery inside a phone. The battery is charged by a different, embedded charger, which maintains proper charging protocol, and obeys limitations of available input power. \$\endgroup\$ – Ale..chenski Sep 13 '18 at 4:09

First, these are OUTPUT parameters. The charger doesn't consume the stated power, it can POTENTIALLY DELIVER the stated parameters. A device connected to the charger will determine how much to take depending on its design, negotiated mode, and battery charge status (nearly fully charged batteries will take less and less current as they are charged up). This charging process has nothing to do with stated parameters, they just define the limit for what a device can take as INPUT.

Second, charging "speed" is not measured by input amperes. The charging speed is measured by internal output of an actual Li-Ion charger inside the smartphone. The input power gets CONVERTED by internal charger into battery charging current with nearly 95% efficiency and in inverse proportion to the input voltage. So the 12.5 voltage mode with 1.5 A maximum source gets converted into 4.2 V charging voltage (typical for Li-Ion battery), or 3:1 conversion making potentially about 4 - 4.5 A of battery charging current (1.5 A x 3), which could deliver a very "fast charge" if the battery can hold it. So it is not charging slower with 1.5A input, it is charging faster (at 4+ A) and thus consumes more power overall.

Third, your charger has three possible modes of operation. The switch to the modes is negotiated between the device and charger upon connect, using one of available "fast charge" protocols, PD or QC or else, you are right here. Except that it is not based on temperature nor battery level, the mode is negotiated once and stays. Overall power delivery in each mode has nothing to do with each other, they are standard operating conditions and are INDEPENDENT (nearly). So your question is based on few false premises.


No, amps is not the only thing that matters. Voltage does too. As your own math shows, power is voltage times amps. The higher power is used to fast charge your phone. Slower charging uses the lower power. Ever since they introduced multiple voltage and amp combinations for quick charging, the amperage of a usb charger is no longer the sole number determining it's charging speed.

The 5V x 2.5A = 12.5W is (now) the standard slow charger (compared to The original usb 0.5A and 1A standards). The 12.5V x 1.5A = 18.75W is the faster charger.

And yes, there is some negotiation between the phones battery charging circuit and the wall power supply quick charge or other fast charging technology. It can drop down if or when it needs to due to battery feedback.

  • \$\begingroup\$ Okay... So, the more watts delivered, the faster charging - you say. Alright. But then why do apps like Ampere and Gsam only show the amperage levels? It usually goes between 1.5 and 2.5. Why don't they show watts if that's the correct speed measure for charging? \$\endgroup\$ – Burak Okuroğlu Sep 13 '18 at 5:02
  • \$\begingroup\$ @BurakOkuroğlu the higher adaptive fast charging voltages are to improve power delivery to the device without changing the standard usb cable impedance. The device itself will determine both the voltage supplied to it as well as the current used to charge the battery using a current controlled voltage converter. There will be additional conversion losses in this stage but it allows the device to be quite versatile in terms of the power supply it can function with. Load regulation on usb chargers can be awful. \$\endgroup\$ – K H Sep 13 '18 at 5:14
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    \$\begingroup\$ @burak mostly because they haven't been updated for fast charging, or because the system/android doesn't always allow apps to get this information. Even when they do, they aren't always accurate because they don't account for actual charging rates. They will guess it's x Amps based on the usb standard but don't include usage drain (you can use your device while charging and that can drain your battery faster than it's charging) or efficiency in power conversion. The apps are not perfect for various reasons. \$\endgroup\$ – Passerby Sep 13 '18 at 5:45
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    \$\begingroup\$ @burak I dabble in android development and hacking. It's a mess. Some manufacturers do use things that export this info to the system, some don't. Some apps or phones require root access to get accurate low level information \$\endgroup\$ – Passerby Sep 13 '18 at 5:47
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    \$\begingroup\$ That's the battery internal voltage. \$\endgroup\$ – Passerby Sep 13 '18 at 12:31

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