So we’ve all heard that the iPad Pro is slow to charge, but I’ve been doing some research on the actual numbers and have had some interesting findings. I’m the developer of a battery/charging information app on the App Store called Battery Health and so I am often looking at the innards of iOS devices in terms of batteries and power management. This research is based on live stats reported by the internal charger device driver on the iPad Pro. iOS devices ship with an internal charger chip (that's inside the device, NOT the power adapter that ships with it) that is configured to charge the battery at certain rates, depending on several factors. Battery charging speeds are typically measured by a unit called C, which is basically a fraction of the total capacity of the battery. For example, a 1000mAh battery that charges at 0.5C indicates that we’re charging it with a 500mA current, or half of its capacity. The higher the charging current, the faster the battery gets charged. For most iOS devices, the base charging rate for the constant current stage (the early, fast stage in the charging process) is around 0.55-0.65C. For example, The iPhone 6 Plus has a 2855mAh battery and it charges at a maximum current of 1730mA, or about 0.60C. The iPhone 6S Plus has a slightly smaller 2725mAh battery, and its maximum charging current is 1790mA, or about 0.65C, so it charges a bit faster than the 6 Plus. Some Android phones will go much faster, up to .80C or even 1C. The catch is that faster charging shortens the overall life of the battery, so quite honestly Apple is playing it very safe with these kind of numbers, which is why their batteries tend to last so long. Now, the iPad Pro has a gigantic 10,088mAh (38.3W) battery, but its maximum input current at the USB port is only 2400mA (12W, same as all other recent iPads). When you charge an iPad Pro, its charger is configured at 0.55C, same as many other iOS devices. The problem is that with a 10,088mAh battery, that comes to 5560mA, or 28W. This means that the iPad Pro technically needs a 28W charger to charge at optimal speed — at the INTENDED speed. The 12W charger it ships with is only supplying 40% of the current required to charge it at maximum speed. What’s more alarming is the fact that if you push the Pro hard (full screen brightness, LTE video streaming, etc.), its current draw sometimes actually reaches 2400mA! This means that you could be plugged in with the 12W Apple brick while using it, and the battery won’t actually get charged. Note that as best as I can tell, the device is configured for a maximum input current of 2400mA at the Lightening port (again, that’s what the device is reporting), so be wary of any 3rd party chargers that claim to charge it faster than the stock charger. I’ve not seen anything like that, but it seems like a likely scam… For now, the bottom line is, make sure to charge your iPad Pro while it is in standby with the screen turned off. It will barely charge while being used, even with the 12W charger. UPDATE: Apple has now released a USB-C to Lightning cable that allows the use of the 29W MacBook charger with the iPad Pro.