If you've recently taken possession of a shiny new iPad, you may have noticed an odd message when connecting the tablet to some computers or USB chargers: the words Not charging in the iPad's status bar. Or maybe you've plugged a third-generation iPad into Apple's official charger while you're using the tablet, and later noticed that it doesn't seem to have charged at all. Or perhaps you've left the new iPad to charge and it seems to be taking longer than with your older iPad.
Why? Compared to iPhones and iPods, the iPad simply has more-demanding charging requirements--and for the latest iPad, those demands are even more challenging. Some USB ports, especially those on older computers and many USB hubs, don't provide enough power to charge an iPad when the screen is on. And with the third-generation iPad, even Apple's own charger can struggle at times. What this means is that whether or not a particular USB port or charger can charge the iPad's battery--and how quickly--depends on which iPad you have and how it's being used.
Here's a quick summary of options and information, based on our experiences, Apple's support site, and information provided to Macworld by Apple:
Fastest charging: iPad charger For the fastest charging of any iPad, use the iPad's included 10-Watt USB Power Adapter or a third-party charger certified for fast-charging an iPad. This will fully charge a first- or second-generation iPad in a few hours, even if you're using the iPad at the same time; the third-generation iPad will take a bit longer, as explained below.
Slower charging: high-power USB port When connected to a high-power USB port--such as one on a recent Mac, Apple's USB Power Adapter for iPhone, or third-party chargers certified for the iPhone--a first- or second-generation iPad will charge, even during use, but more slowly. (Some third-party USB hubs provide higher-power USB ports, but many don't; similarly, the USB ports on many Windows PCs don't provide this additional power.)
Slowest charging: lower-power USB port When your iPad is connected to a lower-power USB port--those on older Macs, many Windows PCs, and many USB hubs (powered or unpowered)--the battery is charged only while the iPad is asleep. If a first- or second-generation iPad is awake and in use, its battery essentially holds its current charge level. What's confusing here is that the message Not Charging appears in the menu bar when the iPad is awake, which might lead you to assume that the offending USB port can never charge your iPad. But rest assured that once you put the iPad to sleep, the battery will charge.
The third-generation iPad
The newest iPad has even heftier charging requirements than its predecessors, for a couple reasons. First, it has considerably more battery capacity than the first two iPads--42.5 watt-hours, compared to 25 watt-hours--so even at the fastest rate of charging, the third-generation iPad takes noticeably longer to fully charge. (And, in fact, according to testing by DisplayMate, the new iPad's battery doesn't reach a full charge until about an hour after its battery meter displays 100%.) So no matter how you charge, it will take longer to fully charge a third-generation iPad than either previous iPad.
Second, though every iPad model charges more slowly if you're using it while charging, this slowdown is much more noticeable on the third-generation iPad because the new iPad's electronics--its screen, processor, and the like--require more overall power than the previous models' components.
What this means is that the first three charging guidelines above apply differently to the latest iPad. For starters, some users report that even when using Apple's 10-Watt power adapter, if you're doing processor- and graphics-intensive tasks such as playing a demanding video game, and you've got the iPad's brightness set to maximum and Wi-Fi or cellular data enabled, it may seem as though the battery isn't charging at all. During other tasks, the tablet might instead charge very slowly. Using a high-power USB port, a third-generation iPad will charge slowly when asleep, but generally not when in use--in my testing, a 2010 iMac's USB port held the battery level steady while I watched video at medium brightness. And when connected to a lower-power USB port, the iPad will charge--very slowly--only when asleep; when in use, a third-generation iPad will likely use more power than it gets, resulting in a gradual decline of battery level (although at a slower rate than if the iPad wasn't connected to power at all).
How you use it matters
Because the iPad's screen is such a huge part of the tablet's power drain--especially on the third-generation iPad--your own experiences will vary depending on the your preferred brightness level. Most of the observations above were based on screens set to roughly 50- or 60-percent brightness. If you set an iPad's screen to full brightness, it will take longer to charge while in use than if it's set to medium brightness. In fact, with a third-generation iPad set to maximum brightness, the battery level may even decline when connected to a high-power USB port. Similarly, actively using Wi-Fi or a cellular-data connection will lengthen the charging time, as will playing a demanding game or streaming high-definition video. On the other hand, if you've got your screen brightness set to the lowest level and you aren't accessing a network, your in-use charging time will be shorter.
The takeaway here is that if your iPad's battery seems to take a long time to charge--or, for owners of a third-generation iPad, even longer than before--there's nothing wrong. The tablet is just hungry (or hungrier) for power. But by keeping an eye on how you charge the iPad, and what you're doing while you're charging it, you help it charge more quickly.