Apple's iPad - The AnandTech Review

USB Charging

There's been a lot of discussion about the iPad's ability (or lack thereof) to charge from most USB power sources. Even among accessories, the compatibility situation ranges from relatively to completely uncertain.

Let's start from the beginning though - it isn't like you're going to be unable to charge the thing. The iPad ships with a beefy 10.71 watt (5.1 volt, 2.1 amp) USB power supply and a heartier-than-normal feeling USB dock cable. Given the device's relatively long battery life (as we've already demonstrated), it isn't like charging is going to be something you'll do more than once a day. But Apple has left the charging situation uncharacteristically vague.

We tested charging with a number of dock connector charger accessories we had, and found the state of things confusing. Let's start with the good; Apple's ultracompact USB power adapter, the one shipped with the iPhone 3G and 3GS, does charge the iPad. If you've still got any laying around, the older, larger iPod chargers that have USB charging circuitry also work. Interestingly, both of these power adapters are only rated at 5 watts.

Support for charging from accessories from 3rd parties, however, varies wildly: devices that support firewire-only charging definitely won't work - the iPad lacks that circuitry the same as iPhones and iPod Touches. Other adapters that lack Apple-specific charging circuitry also don't work. Third party battery packs also won't charge the iPad.

Car chargers are a mixed bag - Griffin's PowerJolt SE car charger, sold at the Apple Store, doesn't charge the iPad at all. However, Belkin's Micro Auto Charger (rated at 5 watts), also sold at the Apple Store, does charge. Interestingly, the Griffin has a hard connected dock connector, while the Belkin has a USB termination and aftermarket dock cable - it's likely that the iPad favors USB-based chargers rated at or above 5 watts.

The iPad is picky about its power - it's something of a connoisseur

On the desktop side, charging is even more of a mixed bag. If you've got a PC, you're probably out of luck unless motherboard supports the USB charging specification. Remember that USB 2.0 by default supports 500 mA of current at 5 volts for a maximum of 2.5 watts. From what we've seen so far, at least 5 watts is required for the iPad to charge. Newer USB ports that support the battery charging specification can supply up to 1.5 amps at 5 volts, for a maximum of 7.5 watts. From what we've seen, it's likely that 5 watts is the requirement for USB charging to work properly. Apple's iPad charging knowledge base document mentions that a "high-power USB port" is required for charging, but neglects to enumerate what computers support the standard, or what standard they're calling high-power. Apple notes that "many recent Mac computers" support the high-power USB port charging standard, and that "most PCs or older Mac computers" will charge the iPad when it's in sleep mode.

We tested with a MacBook Pro 2,1 (pre-unibody Core 2 Duo), Mac Pro 4,1 (Nehalem), Latitude XT, and desktop with an EVGA X58 SLI - none of them charged the iPad over USB while powered on or in standby. It's a step backwards that the iPad discharges while syncing, and keep discharging if you leave it plugged into a non-supported USB port. Remember to plug it back into its special beefy power supply, or you'll likely be left without a working iPad.

The iPad a Super Head Unit is Not

Unfortunately, it looks like the dock connector circuitry on the iPad does not support car head units. We tested with a JVC KD-PDR40 and an Alpine iDA-X001 (which Alpine touts was designed in part with Apple to emulate the iPod interface), and both resulted in the following notification on the iPad:

Bad news for people with aftermarket head units

The Alpine iDA-X001 showed the following:

While the JVC KD-PDR40 constantly reported "reading" as its status:

It seems that, for whatever reason, the iPod playback hardware and software playback paths are not present on the iPad. This is disappointing, especially considering the number of car enthusiasts hoping to use the iPad as a super-sized head unit. In fact, much of the initial speculation was that vehicle integration would be part of the iPad's secret sauce. Based on these results, that obviously was never the intention.