Why my smartphone is charging very slow on USB ?

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:
  • 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 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
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!

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  5. Great post! I just finished 8 hours of testing 20 different power supplies and 20 different usb cables, and the results were saddening. Most cables connected to a drained nexus 5 wouldnt push more than .6-.8A with the OEM 1.2A adapter. Snagged one of my aukey qc2.0 cables and sure enough, 5.2v 1.17A. Almost all of my cables not branded qc 2.0 failed to supply 1.2A. The reason im here is because im trying to determine why my fast charger that came with my gs6 edge+ will supply 5/ 6/9 V @ 1.67A but when i stole this thin white "fast" cable from its adapter and plugged it into the nexus 5/nexus5 adapter, it behaved just like the other slow cables and only pushed 5V .7A. What in the world gives?
    So ive now confirmed with three separate OEM Samsung chargers...their cables seem to charge at 5v and .5-.8 A when plugged into nexus 7 adapters (1.5A), or nexus 5 charger (LG 1.2A). WHY?? Why does it seem that they are "all or nothing"? Why cant my cables perform at 1.0-1.5A? I bet others that have done this level of testing have also noticed this. Additionally, i grabbed a flimsy generic cable yesterday for 3 dollars and expected no more than the .7A performance i was getting, and i was right. Plugged into OEM Ipad air 1 adapter, asus tf700 (transformer infinity 2A 5v when NOT plugged into the tf700). 6 different chargers, all .6-.8a performance...until i plugged this flimsy, cheap cable into the aukey 15000 mah portable charger with its QC 2.0 adapter, and viola! 9v, 1.7A!!!
    1.HOW is this cheap cable performing identically to the samsung oem cable?
    2. More importantly, why are samsungs fast charging cables not able to handle 1A-1.5A when connected to those adapters? Their cables seem to be "all or nothing".

    I would say its having issues negotiating the 1.2A with the nexus 5, and therefore defaults to .5A (usb standard?) But my aukey cables handle every adapter i own and amazingly, supplies the amperage posted on the adapter.

    One final note: despite having multiple generic chargers that claim to be 5v 2A, there isnt a single cable that ive tested (out of 30) that has delivered 2A. Ive gotten as high as 18W delivered (12v, 1.5A, and 9v1.67A), but None of my devices actually charge at 2A, so i dont know if its the cables or my lack of test equipment.(my ipad air oem charger delivers the expected 2.4A 5V, but this is excluded as its obviously not USB)


  6. I think that some Samsung cables have an additional pin on the phone's end. This makes the cable to be "identified" by the phone.

    Most common, it's not the charger that is not able to push as specified (maybe 10% less), it is the phone that is "too shy" to suck at maximum current (considering a very good cable).

    The phone must have some guaranties that the cable and the charger will not be overloaded (risk of fire). Your cheap cable might just mimic the right signaling that the cable and charger are able to provide maximum power. Otherwise, the phone will limit itself to a "safe" current, that can be like 500mA (USB 2.0), 900mA (USB 3.0). For more than 1A, I'm expected phones to be very picky on the cables and chargers signaling. There are some cables that can digitally identify themselves as supporting such turbo charging like 2-3A.

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