You accidentally left your micro USB charger back at the hotel... 500 miles away. Now it's time to buy a new one. You pick up a cheapie from your nearest electronics retailer and hey, what gives? What used to take 3 hours to charge now takes 12.
Turns out not all chargers are created equal, even if they look similar. Here are some quick buying guidelines that'll save you time and ensure you're using a charger that's optimized for your device's power requirements.
First, some background. Charging power is based on three things: power (P, measured in Watts), current (I, measured in amps or milliamps), and voltage (V, measured in volts). The amount of power is determined by the equation P = IV. In other words, power is the product of current multiplied by voltage. Because larger devices like tablets have substantially bigger batteries than smartphones, chargers designed for the former tend to deliver energy at a higher rate (a higher current).
For example, consider these charging scenarios for the Retina iPad mini. You could use a Lightning connector plugged into a computer (via USB), an iPhone charger connected to a wall socket, or an iPad charger connected to a wall socket. A PC USB charger delivers 2.5 Watts of power (5 volts at 500 mA). An iPhone charger delivers 5 Watts (5 volts at 1000 mA). A Retina iPad mini charger delivers 10 watts (5.1 volts at 2100 mA).
While all of these will charge your iPad, using the USB connected to a PC will charge your Retina mini four times slower than if you used the iPad charger it came with. Conversely, if you use a tablet charger for your smartphone, it'd charge up faster than normal (Note: Some devices like the iPhone will only draw up to 1A of current no matter the charger). If you play mix-and-match with these types of chargers like this, don't worry -- you're not going to blow up your phone or anything crazy like that. And the myth that charging your device at a faster rate will reduce the life of your device's battery is false. For some older devices, the higher specced charger just won't work at all, while newer devices will just charge faster.
Ultimately, it's really the amperage that determines how fast a charger will supply power to your device. If you want quicker charging, look for a wall or car charger that delivers 2100 mA of current at 5 volts (or whatever voltage the device you're trying to charge is specced at).
If you're grabbing a new charger off the shelf, there are a couple other things you should consider. One of them is the logo that identifies compliance with international standards. These can be faked. The CE mark is a popular one, and as someone in this forum thread unfortunately discovered, his substandard charger sported a fake CE mark. The C and E in the actual logo should each be approximately a half circle, and if you continued the circle of each letter fully, the two circles would just link together in the middle.
If a charger has incorrect capitalization for the current and power output it's supposed to deliver (think "MA" instead of "mA," for milliamps), that could be a sign that the charger isn't up to snuff too. Having no manufacturer label on the device could also be a red flag.
Updated 2:30 PM ET to clarify iPhone charging
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