USB Chargers, misconceptions and explanations

I stumbled few times in news entries from websites as big and well informed as Lifehacker and as posting a comment there has only limited effect, I decided to write a small essay about it. I do not really know where to start so I’ll go on an FaQ style, that will as well be easier to update with new discoveries.

Arctic Home Charger 4500

Q: I have an iPad, with an input rating of 2.1A, I just bought a charger that delivers 4500mA, will I burn my iPad?
A: Not at all! your iPad will be the one dictating the charging current that will flow from the charger to the iPad. The charger rating is actually the maximum it can do, so it can without any issue be a lot higher than the one of your device. Actually, it HAS TO be that way, indeed, if it is smaller, you will then risk to blow it while plugging your iPad as this one will be asking too much from the charger. It will be like asking somebody to work more than he can.
Bottom line: The device dictates what is the charging rate, you’ll need a charger capable of AT LEAST this value.

Q: My phone charger is a 2000mA, if I buy that new 4500mA, will it charge my phone faster?
A: Well, based on the above, you’ll think so, but again, that is not the case. Your phone has its own charging rate, it is often not so easy to find out that rate, a pretty safe approach would be to calculate it. For that, take the battery capacity (information can be found in this wonderful database), from there, take the time for a full charge with the original charger (use the original charger as it will most likely be the fastest as manufacturer optimized it). Then do the maths.
For instance, my Xperia Z charges its 2330mAh in about 3h, therefore the charging rate is about 2330/3 = 777mA, add 20% for the different losses in the charging circuitry, you get 932mA, any charger delivering 1A or more will be fine but a higher value will not impact the charging time.
Bottom line: Output of the charger needs to be at least big enough to cover the device needs but bigger output does not improve the charging time.

Q: Well, this is nice, but why does my new charger, which is fully capable, does charge my phone a lot slower than the original one.
A: Well, a charger can be capable but not delivering the full potential.Why? For that, I’ll need to set some technical background. A USB connector, in its v2.0 form, has 4 pins, 2 of them are the power line, 2 of them are the data lines, those used to transmit data in the case of a harddrive or other USB device. On a charger those 2 lines are not used (yes, no data to transfer!!). USB 2.0 specification just ask to ties them together.
Well, they are not exactly “not used”…
With the latest generation of phones and especially with tablets, the batteries considerably grew in size, the 2330mAh battery of my phone would then take 5-6 hours to charge with the current allowed by USB 2.0 specification, the iPad 3 battery would requires 24 hours or such…, devices not being able to go above this 0.5A limit otherwise risking to blow a USB port somewhere (maybe on your PC…). This long charging time is a no go for most customers, therefore, manufacturers (starting with Apple) bring in the “smart charging ports”.
What’s that Smart Charging stuff? What happens is basically that the manufacturer instructed to check what is happening on the data pins to define the charging rate. Apple defined for instance that the iPad will go full throttle if it detects 2.80V on one line and 2.05V line on the other, otherwise, it will go in slow charge.
Basically, this way a charger can tell what it is capable of and the device will then pull whatever is more appropriate (i.e. as much as charger allows them, without going above their own maximum charging rate)
Bottom Line : A charger has its own limitation but has as well to be configured in a way that the device will understand for that latter to pull the maximum current.

Q: My charger is fast charging my iPad, but not my Galaxy Tab, why?
A: Well, the Smart Charging system is a wonderful idea, but all manufacturer have their own version. Some are compatible, some are not. My Sony phone will charge on the Apple charger just fine, but a Blackberry phone or a Samsung Tab won’t. The incompatibility can be of different sort :

  • Charge at lower rate
  • Not charge at all

The most common being the first.
Bottom Line : each manufacturer has its own fast charge mode.

Q: So, can I find a charger that fast charge all my device?
A: Many charger will claim that, but most can’t do it for sure. As said before, there are many fast charge config, some not being compatible with others, therefore, except having one port for each config, there won’t be a 100% compatibility. Well, almost… some companies such as Texas Instrument have developed intelligent charging ICs that will browse through the different config until it finds the best one, but that will only be useful on the config existing at the time the IC in the charger has been developed. Any new config won’t be covered, and as demonstrated, manufacturers can be creative…
Bottom Line : a 100% compatible charger is a rare thing.

Q: My Asus Transformer won’t charge on my USB charger, what is happening?
A: Well, here again, creativity is the source. To charge faster, Asus decided not to go with higher current but with higher voltage, in this case 15V, for that, they developed a special charger with its matching cable. It is using a USB port but it is NOT a USB charger.

I hope this will help clear out some misconception in the USB charging and help some of you guys. Do not hesitate to link this anywhere else, I’ll be editing this very same post with my new discoveries.

well,I can’t end without linking Ken Shirriff’s blog who taught me a lot