Tim Moynihan, Wired Jul 17, 2014 You plugged in your phone or tablet hours ago, and the battery has recharged about 10 percent. Or maybe the needle didnt even move at all. The connection between the cable and the device seems fine, the chargers plugged in, and the battery indicator says your device is charging. What went wrong? Its likely to be one of three problems (or a combination of them): You could be using the wrong kind of power outlet, the wrong kind of wall plug, or the wrong kind of USB cable. In each case, the slow charging has to do with amps, the rate at which electrons are flowing into your gadget. Most mobile devices these days have 5-volt chargers, but they will still charge your device at different speeds depending on those amps. Smartphones usually come with 1-amp chargers. Tablets and larger phones usually come with 2-amp chargers. (This is why tablet chargers are also known as 10-watt chargers, as Amps × Volts = Watts. So, effectively, 1-amp chargers are 5W chargers and 2-amp chargers are 10W chargers.) If youre charging directly from the USB port on your computer, the amperage may be as low as 0.5 amps. Assuming the same voltage, the difference between a 0.5-amp, a 1-amp, and a 2-amp charger is like the difference between a one-lane, a two-lane, and a four-lane highway: At each step, twice as much charge is being delivered in the same amount of time. The amperage of the charger that comes in the box with your device largely depends on the capacity of its battery. Because a lower-capacity battery will recharge more quickly, most phones come with a 1-amp charger in the box. But any device that ends with -ablet has a higher-capacity battery, so big smartphones, iPads and other tablets will come with a 2-amp charger to speed up the charge. There are two cases where its easy to tell whether your amperage is insufficient for your device. First, avoid charging big gadgets from a powered USB 2.0 port on your computer; if its a USB 2.0 port, youll likely get between 0.5 and 1 amp. (USB 3.0 supports up to 5 amps when charging, so you should be OK with USB 3.0 port charging.) The same slower performance applies to many USB hubs, which may charge gadgets very slowly because they run at low amperage. Second, just because the wall wart fits, it doesnt mean its appropriate for your devices charging needs. The best example here is using the smaller (1-amp/5W) iPhone wall plug to recharge an iPad (which charges more quickly with a 2-amp/10W charger). In most cases, 1-amp AC adapters are physically smaller than 2-amp AC adapters, so you can usually judge how many amps youre going to get based on the size of the adapter. If youre unsure, the amperage is also printed on all wall plugs albeit in the smallest, most eyesight-challenging font imaginable. The third possible cause: Its your choice of cable. This is trickier to identify, because all USB cables generally look the same. On the inside, however, they may support more or less amperage, and that important bit of information is never listed on the USB cable itself. At this point, you probably have an ever-growing collection of USB cables from various gadgets in a drawer somewhere. Its hard to keep track of which cables go with which gadgets, but it really does make a difference, even if all the cables look identical. One solution is to label or mark all your cables as soon as you get them. But what happens if a cable or two goes missing? If you want to replace a cable to charge your tablet without paying a premium for a manufacturer-supplied replacement, your best bet is to look for a 28/24 AWG (American Wire Gauge) USB cable. This is a wider gauge than the 28/28 cables supplied with 1-amp chargers, and more of the cables wires are dedicated to transferring a charge. These cables are easy to find at all the major outlets (Monoprice, Amazon, Newegg) starting at less than $2. Dont worry about any problems using a 2-amp charger with a device designed to be used with a 1-amp charger. In some cases, it may speed up charging of the smaller device. In other cases, the smaller device will limit its power-sipping to 1 amp. In general, if you want to take only a single cable on vacation to recharge all your mobile gear, the best bet is to bring along a 2-amp wall wart with a 28/24 cable. It should work fine with phones and tablets alike; just make sure you double-check that you packed it before leaving the house.