Why is my smartphone charging very slow on USB ?

Better read:

• Data Consistency in Distributed System - CAP, PACELC
  Morality series - a heptalogy

This blog is mainly focused on my Philosophical and theoretical ideas.

From time to time I have practical posts that are too few to deserve a blog by themselves. I still hope my other work (like above) is even more valuable than such practical article as below.

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Sometimes the phone is connected to a charger, it reports "charging", however the battery level increases painfully slow, or it could even actually decrease. What is the solution?

On short:

• Update 2020: prefer original chargers. Even using QC 3.0 charger, some Samsung phones (like A51) will charge faster with original charger - when the display is ON. With display OFF, QC 2.0/3.0 chargers might still be as fast as original chargers.
• For phones, prefer the 1A ("phone") charger port to the 2A ("tablet") charging port. The 2A port is likely to charge slower than the 1A port (see below why).
• Use the original cable if possible. If not possible, USB charging cables that comes with cheap 1-2A chargers are usually better than more expensive "data and charging" cables. Prefer small cables (like 1m) or charging cables explicitly rated (ideally 2.5A or more).
• Use dedicated chargers, not USB. Power wall or car chargers rated 800mA or more are usually way better than PC/Laptop USB ports. Prefer USB3 (blue) ports over USB2 (black). Some other USB colors (like orange/red) could be better than USB3 sometimes (or not).
• Some devices might require an original power adapter (for example older Apple devices)
• Update 2017: try using a charger rated Qualcomm Quick Charge 3.0 (or at least 2.0) if your device supports it. The device might negotiate 9V or 12V instead of 5V, resulting in 2-4 times faster charge until 80%. Also, PowerIQ and VoltageBoost might help if the device does not support QC.
  
  
  

Update 2020: Some Samsung phones (like Galaxy A51) will not quick charge when display is ON.

• With display OFF, there is no difference between the original Samsung charger and a QC 3.0 charger: 9V/1.3A.

• With display ON the quick charge gets deactivated on the Samsung A51 phone. In this case, A51 will only draw 5V/0.75A from an Anker QC 3.0 charger, while it would charge way faster from the original Samsung charger: 5V/1.3A. There is no problem with the Anker charger, it can easily provide up to 5V/1.8A to other Samsung phones. Somehow the Samsung Galaxy A51 phone would not correctly negotiate with a non-Samsung charger.

Update 2020: The app "OPO Charging Current" does not work anymore for many Samsung phones. You can try other "charging current" applications like "Ampere". Note that current is usually taken from battery, so quick charge (higher voltage) will be translated to a higher current (Amperes) on the battery - as battery must always receive its nominal voltage. Also, you will not see the current that is used by the phone while charging (it can be 200-500mA). These days you can buy a cheap enough "USB Current meter tester" that can be directly attached on the cable. Ideally you should prefer an USB cable with soldered tester as an additional plug can itself affect the charging speed.

Update 2016: For a faster feedback in your tests, try to install a phone application that reports the current (charging rate). A very light application that I found on Android is "OPO Charging Current". On my Android 5.1.1 (Nexus5) you have to divide by 1000 to obtain mA, but it just works. You can choose any other current monitoring application you prefer that works on your phone. Search for "charging current" for example.

Power charger too powerful?
First, for the people that are wandering: the phone or tablet will not blow up if the power charger is rated for 1A or 2A (Amperes) while the phone is rated for only 0.8A (800mA).
The phone or tablet will not draw more than he needs. He could drain less, however, resulting in that slow charging. In some situations, the phone could draw less than normal even if the charger is able to provide more.

This is true for any USB powered device and any power source that can provide higher current than device needs. The important thing is the voltage, that should be 5V for any standard micro-USB charger. On the other hand, if the power source is rated less than the device, we can see slow or no charging. But this is not the only possible cause for slow charging.

Why is the charging so complicated?
There are 3 parts that must be met for a fast charging:

1. The cable should be able to carry that much current (the resistance should be low)
2. The power source should be able to provide enough current to the device  (like 800mA, 1A, 1.5A, 2A, etc)
3. The device should be aware that it is allowed to request more than 500mA from that USB port (the device is polite)

Not all USB charging cables are created equal

I developed a simple methodology for testing my dozens of micro-USB cables, regarding charging. I classified the cables in 3 categories:

• very good, that could draw up to 2A to a tablet
• good, that could draw up to 1-1.5A
• bad, that could draw 0.5A or less

• The bad. The "bad category" contains cables that comes with very cheap devices and many "data cables" and most of the long cables (like more than 1 meter). Using this cables, the phone shows "charging", however the device is actually discharging.

• The good. Original power cables from "not so power hungry" devices (like an ebook reader) falls in the "good" category usually. They are usually good enough for charging a smartphone, however they are not charging a tablet very fast.

• The very good. The original Nexus 5 cable was the clear winner among my cables. If was able to provide the best charging current to a tablet. The phone does not need 2A to charge, however the cable is able to well carry such current. Very close comes cables delivered with decent wall chargers. Even if the charger is rated on 1A, the cable was able to carry around 2A for a tablet.

Cable rating (ad-hoc) methodology
While instrumenting the USB cables to measure the current is a little complicate or expensive, I bought a cheap wall power meter, that is able to measure the power consumption from the wall, in Watts. It is not really accurate, but it is enough for this purpose.

Good cable

I connected a reasonable (but not very expensive) 1+2A USB wall power charger in it, then connected the phone in the 1A port of the USB charger. This allows me to measure how much power is consumed from the power wall. This is not exactly what the phone is receiving, as the USB charges has some power loss - visible in the slightly heating of the charger. However, the power consumption from the power wall should be proportional with the one sent to the phone.

Most of the phones have an integrated power meter (accessible by an app). My Nexus 5 does not have one, I could not find the charging current application for Nexus 5 at the time (see above), so I had to use the wall power meter. You may also find gadgets that are able to measure the current directly on the USB cable, choose what is easier for you.

Normally, based on the original charger, my phone should consume a maximum of 1A. At 5V, this should mean 5W(watts). The reading from the power wall is around 6W with a "good" or "very good" cable, so the additional 1W should be the power loss of the charger. The phone does not seem to retrieve more than 1A from the charger even if I run an intensive CPU/GPU benchmark while charging on the 2A port.The situation if different for a "no name" tablet I have, that goes up to 2A with the same port and cable - but only when charging and running heavy load.

When I connected the tablet to the 2A port of the USB charger and started a CPU benchmark, the power meter was showing a maximum of 11W. This should translate to around 2A (10W) actual current delivered through USB. Only 3 cables from couple of dozens were able to deliver such current. For the other cables, the current was way lower, many of them showing 0W at power wall, that means less than 0.2A, that is the measured error of the power meter. In this time, the charging symbol was still active, while the battery was actually discharging.

The results are very "reproducible". I first suspected the connectors, I tried to move them to improve the contact. No, the good cables are good cables, the bad cables are bad, no matter how you fit them.

The cause for bad USB charging cables
The only explanation I found is that bad cables has a higher impedance (resistance to current). This is caused by very thin copper section combined with the length of the cable. Long cables (over 1meter) are less likely to charge well, even if they have good wires inside. However, even very short cables (20cm) and small adapters can be very bad, dramatically reducing the delivered current.

Why 1A charger socket is sometimes better than 2A?

Well, I said that the phone will draw as much current as he needs. This is still true, but it has some exceptions.

Historically, the USB 2.0 used to provide a maximum current of 500mA (0.5A). When a device draws a bigger current, the port used to protect itself. Once I had an USB extender that was laying on the floor, and I stepped on it with the chair. The Windows reported that an USB power surge was detected. The port was actually disabled in order to prevent damaging the USB controller. If you connect a power hungry device to such a 500mA (only) port, the port would enter in protection and you would not be able even to exchange data over that USB. This is the reason why decent devices will not draw more than 500mA if the charger does not signal them somehow that it is able to provide more.

An micro-USB phone charger is signaling that it could issue more than 500mA by shorting the 2 data cables (D+ and D-) by a resistance of less than 200 ohms). Only if there is such signaling, the phone will retrieve more than 500mA. With some old or very bad chargers, it could happen that the charger does not signal this, so the phone will draw only 500mA, that is less than what the phone consumes (when used as GPS/PNA for example).

A 2A tablet charger seems to signal it's ability is a slightly different way, with a higher resistance. From my limited experience it seems that some phones are not able to interpret this signal, so it will fall back to a lower charging current (probably 500mA). So, if you are unsure, avoid using USB charging ports rated 1.5-2.5A or "tablet", because it could result in an actual lower recharge rate. Better use the 1A ports, that is usually accompanied by a smartphone symbol. At least my phone does not charge well on the 2.1A charger port while the tables works perfect there.

Needless to say, chargers rated 500mA will not charge a smartphone, especially if it is on. You need at least 800mA to provide only the current consumed by the phone, when used as GPS for example, and some additional current to charge the battery.

Computer/Laptop USB ports

Normal/black USB 2.0 ports are usually limited to the 500mA. Even worse, they should only provide 100mA when the cable is plugged. Only after digital configuration of the device they are required to send 500mA. I think most of them will provide up to 500mA anyway.

For USB 3.0 ports (the blue ones) the limit is 900mA, but only after configuration. However, there are dedicated charging ports, usually colored in yellow/orange/red or has a sign like lighting or battery, that could power a device with higher current. These ports could remain on even on sleep or shutdown. However, the current is normally limited to a maximum of 1.5A for ports that are capable of data, in order to avoid interferences.

An intelligent device like a smartphone or tablet should be able to configure the USB port to send at least 500mA (USB 2.0) or 900mA (USB 3.0). For more current, it depends if the port implements the "charging port" specifications or a non-standard feature that makes it able to send more current, sometimes even for non-digital devices. However, you cannot bet on this, it is more likely that it will charge slower than a standalone USB power charger.

Special cases for USB power adapters

Some Apple devices (usually old ones) requires a non-standard signaling in order to charge at high current. Such signaling could be applying a certain voltage (like 2V) on the data cables.

Newer devices on USB 3.1 could negotiate a higher current, and even 12V instead of 5V, using a digital protocol over the power wires. Hopefully they will still obey the old protocol to charge on simple power adapters.

USB 2.0 uses 4 wires. The 2 connectors on the extremes are for power (5V), and the middle ones are for data (half duplex). The micro-USB connector from smartphones (OTG enabled) has a 5'th pin, named "ID" that is used to signal if the port will send power to a guest device or it is receiving power (default). In order for the smartphone to act as a host device and send power, the OTG cable must connect a small resistance between this 'ID' wire and GND.

Update: fast chargers.
I tested a "fast charger" that came with Asus Zenfone 2 (Intel CPU version). This can provide 2A at 5V and also 2A at 9V. Of course, the phone needs to do a special digital negotiation in order for the charger to start sending 9V. It seems that it still provides only 5V to my Nexus 5, however it can provide up to 1.5A to it, so that Nexus showed for the first time "Charging rapidly" in the unlock screen.

The fast charging (1.5A) of the Asus charger depends on both the charger and the cable:

• My old 1A charger port with my old Nexus5 cable charges at 0.8A
• My old 1A charger port with the Asus cable increases the charging rate from 1A to 1.3A. The phone still prints "charging" (not fast). It's kind of strange that the 1A port send 1.2A, probably it is because the charger is prepared for 2.1A (it has also a "tablet" port)
• Using the "tablet" port of the same charger (2.1A) with Asus cable charges only at 0.5A (phone prints "charging slowly"). This is similar with my cable. See above the issue with the tablet ports
• My cable with the Asus charger is showing 1A, so my original Nexus5 cable is not that good for fast charging, however it works better anyway with the Asus charger
• Asus charger + Asus cable charges at 1.5A !!! In fact it prints 1.4A, but the phone consumes the other 0.1A. I suspect that the cable has both lower resistance and a digital tag that permits to negotiate higher current without risking to meld a "noname" cable.
• For the original Asus phone, this fast charger can go up to 18Watts (9V * 2A), depending on battery temperature and battery level (it does not charge so fast over 70%)

Hope it helps :)

References: http://en.m.wikipedia.org/wiki/USB#Charging_ports

Disclaimer: my electrical experience is limited to hobby, use this information with care and without any warranty. Anyway, I hope it will be helpful. If you have clarifications, corrections, please leave a comment!