Power Macbook Air with Early 2008 Macbook Pro I have two batteries from my old 15" Macbook Pro Core 2 Duo (Early 2008). It's a non-unibody (discrete) model. Anyone know of any way to use these batteries to power my 2nd Gen Macbook Air? An external battery costs ~$300-$450 and it seems kind of unnecessary if there is an adapter or some hack job way to make the old batteries work.

Note: I'm not talking about replacing the macbook air batteries, I'm talking about using the macbook pro batteries as an external battery connected to the magsafe power port.
Much like the commercial versions you mentioned, what you want to do is possible (that there are commercial versions essentially proved this). 

You'll need to chop apart a MagSafe cable or figure out how to interact with Apple's airplane adapter. You'll also need to figure out how to interact with the batteries to get the correct voltage (and maybe do cell wear levelling). 

Also, I doubt the MacBook Pro batteries will last anywhere near as long or be as reliable as the commercial versions. 

(There was an original answer, but it made incorrect assumptions that were addressed in the comments. See the edit stack for the originals.)