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Apple originally provided a 12W USB Power Adapter to supply faster charging for its Apple iPad's, but the larger iPad Pro models can charge faster  using USB Power Delivery based chargers like the Apple 29W USB-C Power Adapter. Here we look at the Apple 10.5-inch iPad Pro and original 12.9-inch iPad Pro to see how fast they charge with the Apple 29W USB-C Power Adapter compared to the 12W USB Power Adapter.

Using the Granite River Labs GRL-USB-PD-A1 test software, we can clearly observe the USB Power Delivery negotiations between these iPad Pro's and the Apple 29W charger.

First we direct plug in the Apple 29W USB-C Power Adapter to the 10.5-inch iPad Pro on a fully depleted battery using a Apple USB-C to Lightning Cable. The Apple charger is seen initially to declare 12 watts (5 volts @ 2.4 amps) and 29.6 watts (14.8 volts @ 2 amps) after a hard reset over USB Power Delivery.

The Apple 29W USB-C Power Adapter then accepts the 10.5-inch iPad Pro's request to pull an initial 12 watts (5 volts @ 2.4 amps) from the charger. 

We further observe over the negotiation, the Apple charger initiates a long list of Vendor Defined Messages which include Discover Identity, SVIDs, Discover and Enter Modes, and Unstructured VDM, to which the 10.5-inch iPad Pro complies accordingly.

After some time only we see the 10.5-inch iPad Pro finally requests 29.6 watts (14.8 volts @ 2 amps) which it goes on to establish this power contract  with the Apple 29W USB-C Power Adapter.

Starting from a 0% dead battery state when plugged in, the 10.5-inch iPad Pro pulls around 12.5 watts (5 volts @ 2.5 amps) in the first 140 seconds. As the iPad starts booting up, USB Power Delivery negotiation occurs as described above and 29.6 watts power contract established the voltage rises up quickly to 14.8 volts while the current pulled decreases slightly to 2 amps, thus achieving around 29.6 watts (14.8 volts @ 2 amps) of charging. 

This confirms that the 10.5-inch iPad Pro charges well with the Apple 29W USB-C Power Adapter over USB Power Delivery at 0% dead battery.

Similarly, we see how well the Apple 29W USB-C Power Adapter charges the original 12.9-inch iPad Pro from 0% battery charge when connected over the Apple USB-C to Lightning Cable.

Again as shown in the above power delivery protocol negotiations with the 10.5-inch iPad Pro, after a hard reset, the Apple 29W USB-C Power Adapter also starts off USB Power Delivery communications by offering 12 watts (5 volts @ 2.4 amps) and 29.6 watts (14.8 volts @ 2 amps) to the 12.9-inch iPad Pro.

Also as seen earlier with the 10.5-inch iPad Pro, the 12.9-inch iPad Pro initially requests 12 watts (5 volts @ 2.4 amps) and gets accepted by the Apple charger.

Both the Apple charger and 12.9-inch iPad Pro then continuously go through a series of Vendor Defined Messages, as similarly observed during above negotiations with the 10.5-inch iPad Pro.

As expected, the 12.9-inch iPad Pro also eventually establishes 29.6 watts (14.8 volts @ 2 amps) power contract with the Apple 29W USB-C Power Adapter.

Looking at the charging voltage and current between the 12.9-inch iPad Pro and the Apple 29W USB-C Power Adapter, we see similar charging behavior and power levels as what we have just seen when using the same Apple charger with the 10.5-inch iPad Pro.

Like the 10.5-inch iPad Pro, the 12.9-inch iPad Pro also starts drawing around 12.5 watts (5 volts @ 2.5 amps) when the iPad's charge level is at 0% on a dead battery. When 14.7 volts is reached after 110 seconds, the iPad pulls lower current at about 2 amps during power on to eventually charge at around 29.4  watts (14.7 volts @ 2 amps). 

As seen from the above USB Power Delivery transactions, the Apple 29W USB-C Power Adapter behaves consistently when negotiating and supplying power to both the 12.9-inch iPad Pro and 10.5-inch iPad Pro while at 0% dead battery state. By successfully delivering 29W charging power, the Apple 29W USB-C Power Adapter can charge these iPads at a much higher rate than using the 12W USB Power Adapter that comes with the iPad's through USB Power Delivery.

Now let's take a closer look at how these iPad Pro's charge with the Apple 12W USB Power Adapter which doesn't support USB Power Delivery, but rather uses Apple's proprietary D+/D- based mechanism to provide 12W over a USB Type-A connector (over the 7.5 watts allowed from USB Battery Charging 1.2 spec).

To determine how much power the Apple 12W USB Power Adapter actually provides to the 10.5-inch iPad Pro and 12.9-inch iPad Pro, we also  connect the charger to each iPad using a USB Type-A to Lightning cable and observe charging through the Granite River Labs GRL-USB-PD-A1 test software.   

Note that no USB Power Delivery protocol negotiation is expected since the Apple 12W USB Power Adapter doesn't support USB Power Delivery fast charging over USB Type-C. 

While being charged from a 0% dead battery, the 10.5-inch iPad Pro can be seen to draw around 12.2 watts (5.1 volts @ 2.4 amps) from the Apple 12W USB Power Adapter. 

While charging the 12.9-inch iPad Pro at the same 0% dead battery state and using the same Apple 12W USB Power Adapter, we also observe the 12.9-inch iPad Pro pulling the same 12.2 watts (5.1 volts @ 2.4 amps) as the 10.5-inch iPad Pro when charging begins.

Here we have shown how the Apple 29W USB-C Power Adapter provides 29W of faster charging compared to the 12W provided by the Apple 12W USB Power Adapter to the 10.5-inch iPad Pro and 12.9-inch iPad Pro. Given how long these iPad Pro's can take to charge, it's well worth it to use the faster Apple 29W USB-C Power Adapter.