A Myriad of USB Chargers
With the popularity of microUSB based charging on latest smart phones, tablets, and even some 2-in-1 laptops/tablets, I have basically quite some chargers around in home. These chargers normally have a USB A female socket and use a USB to microUSB cable, to charge the devices that has a microUSB female port. I just simply call them USB chargers.
Most of these USB chargers can be characterized with the rated output voltage and current. Below are some that I have:
Charger Voltage (V)
Charger Current (A)
|LG Optimus V
|Motorola DEFY XT
|Samsung Galaxy Rush
|Moto X 2nd Generation
|Hisense SERO 7 Pro
|Asus Transformer Book T100
As you can see, they have similar output voltages. Most of them have 5.0V rating. HP Touchpad has the highest output voltage, 5.3V. Their output currents can vary a lot though.
All the manufacturers warn that only their approved chargers can be used to charge the devices, to avoid damaging the device or charger. Of course this should be followed if possible. But I think we all have the chances that we want to just charge it with any convenient charger that we can grab close by. Can we do that?
USB Charging Basics
The USB interface has 4 wires/pins, Vcc(i.e., Vbus, +5V), D-, D+, Ground. Although USB was mainly designed for data communication, the presence of Vcc and Ground allows supplying power from one device to another device. The USB standard only allows the A-type socket to provide power. That’s why all these charges have a USB-A socket.
Any A-type socket may be ready to provide power, so it is dangerous to connect two A-type sockets using a USB A to A cable. That’s why most USB cables have only one A-type end; the other end is either B-type or micro-B type.
The device that receives power from USB normally have either USB-B type or microUSB-B type connector. In fact, ITU and other standard bodies endorse micro-B USB as data exchange and charging interface for the phones (and tablets). There are exceptions, such as Apple products that use the proprietary Lightning port, but they also provide microUSB to Lightning adaptors.
Connectivity aside, the actual charging involves drawing power in a way such that both the charger and the device agree. Unfortunately, this is very messy in the USB world. You can check The Basics of USB Battery Charging: A Survival Guide for details, but this is a short summary.
- The dumbest charger/device just provides/draws power without any negotiation. The charger just leaves D+/D- float (not connected). The device may use D+/D- for data communication, but when in charging it does not check or makes no use of the two wires. Basically, it just uses USB as two wires and that’s it. This is not compliant to any USB standard. I suspect most cheap USB chargers are still like this.
- USB 1.0/2.0 Power Specs. The initial USB 1.0/2.0 specs define two power supplying modes: 5V 100mA (Low Power) and 5V 500mA (High Power). A device can draw 100mA once connected to a USB power source; then it establishes USB digital data communication with the source through D+/D-, and use a digital protocol to negotiate with the source. It may be allowed to draw 500mA, or instructed to keep in the Low Power mode.
- USB BC 1.1. The USB 1.0/2.0 Power Specs do not provide enough power to more power hungry devices, so a new spec, BC 1.1 comes to help. In BC 1.1, the device needs to check D+/D- (and sometimes additionally perform digital negotiation) to draw power from the source. There are three types of USB power sources:
- Dedicated Charging Port (DCP). DCP internally shorts D+ and D- with a resistor of 0-200 Ohm. The device can detect the short and discover the source is a DCP. No digital communication is necessary. A DCP can supply up to 1.5A. DCP is still relatively dumb, and a lot of wall plug or car chargers belong to this category. A DCP does not perform data communication, i.e., is purely a power source.
- Charging Downstream Port (CDP). A device needs to negotiate with CDP using hardware handshake through manipulating and monitoring D+/D- levels, however, no digital communication negotiation is needed. CDP can supply up to 1.5A. CDP powering and USB data communication can co-exist. Newer PCs and laptops have USB ports as CDPs to charge other devices.
- Standard Downstream Port (SDP). SDP is basically the USB 1.0/2.0 Power Specs port (100mA or 500mA). BC 1.1 requires both D+ and D- to be grounded with 15k-25k Ohm resistor, and the device can detect that. From there, USB 1.0/2.0 Power Spec negotiation through digital communication is performed to determine whether possible to get to High Power mode. SDP powering and USB data communication can co-exist. Older PCs and laptops have USB ports as SDPs to charge other devices.
- Manufacturer specific protocols
- Apple chargers. Apple chargers pull D+/D- to certain levels, and Apple devices detect the levels to determine how much current to pull from the charger. See iCharging: The mysteries of Apple device charging for details.
- Qualcomm Quick Charge. Qualcomm Quick Charge is open USB charging technology, see below.
A lot of USB wall chargers are dumbest chargers. The have arbitrary amperage ratings and non-5V voltage ratings, therefore are not conformant to the USB standards.
In general, higher charger output voltage means more capable in charging devices. For example, if I try to charge HP Touchpad with SERO 7 charger, most likely it would not charge anything, because the output voltage is just too low out of the charger. One the other hand, my HP Touchpad charger is the most powerful one and charges all the devices in my home. I was even successful in reviving almost dead battery of LG Optimus and breaking its boot loop as described in a previous post.
The output current dictates how fast the charger charges the device. For example, with the same output voltage, a 2.0A charger can charge 2 times as fast as a 1.0A charger. Normally more powerful devices have batteries of higher capacities, and use chargers with higher current ratings, so they do not need long time to recharge.
Asus Transformer Book T100 and BatteryBar
A lot of devices are also dumbest devices per USB charging. It draws power from the charger as long as it deems safe, without complying to the USB standards.
For some devices that try to be conformant, they may appear as picky ones, because they only draw a safe amount of current if they cannot determine the source type when the source is the dumbest charger.
The Asus Transformer Book T100 is 2-in-1 laptop/tablet that is “notorious” for picking chargers. Basically, its accompanying charger is slow in recharging the tablet. People try to use some other chargers with higher currents (and similar voltages), but they often find that it is charged even slower, drawing merely 500mA. Some people come up with mods to short D+ and D- in a dumbest charger, therefore per BC 1.1 DCP, T100 can draw up to 1.5A. Some other people report that not only T100 may be picky on chargers, but also picky on USB cables.
I am not sure if it’s really the different charging protocol, or just the voltage loss is too much on long USB cables of inferior quality (thinner metal wires therefore bigger resistance). And I do not want to modify my chargers. Luckily there is an application called BatteryBar in Windows, which can report the charging rate in real-time. BatteryBar Pro is $8, but I find the free BatteryBar Basic version is good enough for my purpose. It is only for Windows though.
Once installed, BatteryBar displays a toolbar on Windows bar. If you click it, it shows more information. While charging, the most important information is the wattage. For Asus T100 and HP Touchpad charger, I do find that a 6-feet-long USB cable would yield only about 2500mW (i.e., 500mA at 5V) charging, but a 3-feet-long USB cable would give about 5000mW (1000mA) charging. That’s a difference between 12+ hours and 6 hours for a full recharge. Asus T100 and Aukey Turbo Charger (see below) work at about 5000mW as well.
Qualcomm Quick Charge
Qualcomm’s Quick Charge technology provides another way to charge the devices faster by boosting the line voltage.
Qualcomm Quick Charge uses the same conventional USB A to microUSB-B cable. However, if both the charger and device are Quick Charge compliant, they can negotiate about faster charging. Quick Charge 1.0 allows up to 10W (5V/2A) charging. Quick Charge 2.0 can use various voltages up to 12V, at 3A current, the charging speed is 36W.
Similar to Apple charging protocols, Qualcomm Quick Charge uses the D+/D- levels to negotiate the charging speed. Below is an illustration (image courtesy Qualcomm Quick Charge 2.0 protocol specification and support).
Devices using latest Snapdragon SoCs support Quick Charge 2.0, including phones and tablets. Other devices may support Quick Charge as well. You can check here for certified devices.
You can also find Quick Charge certified chargers here. Quick Charge 2.0 certified chargers can output different voltages. If you check the fine text on the charger, you will not see one voltage like 5V, but instead you will see 5V, 9V and 12V with respective current ratings. The key to charge faster is that the charger outputs high voltage with high current, and the device’s charging circuit would convert it to low voltage with even higher current to charge the battery.
As long as both charger and device conform to Quick Charge, they can come from different vendors. This should make you worry less when exchanging charges for your Quick Charge enabled device.
Qualcomm Quick Charge is backward compatible with conventional USB charging.
- If the charger is conventional, Quick Charge device behaves like a conventional device, and charging is slow.
- If the charger is Quick Charge certified but the device is conventional, charging falls back to 5V and is slow. Your device will not be fried.
- If both charger and device are Quick Charge enabled, they negotiate at a high power charging and charging is fast.
- Quick Charge 2.0 is also backward compatible with Quick Charge 1.0.
AndroidAuthority has a nice article explaining Quick Charge. The FAQ of Qualcomm Quick Charge can be found here.
The devices keep being more powerful and requiring larger battery capacity, and technologies like Quick Charge will be more popular. In fact, both Moto X 2nd Generation and Asus Transformer Book T100 supports Quick Charge 2.0, per device list. I just bought a Quick Charge certified Aukey charger off Amazon for a little over $10. This charger is able to output 12V/1.5A, i.e., 18W, which is promising.
In one occasion, the Aukey Turbo Charger charged Moto X2 from 5% to 42% in 20 minutes (both the phone and charger got hot). In the next 25 minutes, it continued to charge to 72%. This seems to be pretty good. It however yields only about 5000mW on Asus T100 according to BatteryBar.