I want to disable charging my iPad when it is connected to my MacBook Air.

You can't disable charging on the iPad. If it is connected, it will draw a charge. It doesn't charge quickly from your MacBook Air if I have my facts straight, but it will still draw a charge.

If you are not syncing to the MBA, disconnect the iPad and charge with the wall charger that's provided for that purpose.

I understand your wishes for the convenience factor, but there is no way to disable the charging only when the iPad is connected to your Mac.

Here is an Apple support article about charging the battery

http://support.apple.com/kb/HT4060

Here is a detailed Apple article about the iPad battery

http://www.apple.com/batteries/ipad.html

I have an iPad1, and it will also charge when connected to my MBA. I don't usually leave the two connected for much longer than it takes to sync it. If I know I'll leave them connected for longer than that, I simply plug in my MBA to the wall socket.

Sly

One possibility would be to use a USB hub. Many hubs will only supply the USB standard of 500mA, which is not enough to charge the iPad while it's on. I have my iPad connected to the built-in hubs on my Dell monitor and the iPad will show "not charging" with the display on. The iPad is trickle charging after the iPad goes to sleep (i.e.: iPad display is off) but once the iPad goes to sleep, it's charging at a much slower rate so would have much less impact on the Macbook Air's battery. So perhaps small non-powered hub would be close to what you want. I normally carry with me a really small 4 port hub with my Macbook Pro that doesn't take up much space and weight in my bag. (Similar to this one.)

I think I'll have to go with Asatoran's suggestion.Is the hub solution still valid for the latest iPads?

It's a pity that you just can't disable charging. I've got an Macbook Air 11" and an iPad 4. It still charges when the Macbook is close to empty and the iPad is 99%.

Just out of curiosity... Doesn't the USB standard allow the port to stop charging?

As Taia I'm using the iPad for development.