I wanted to power my iBook G4 for a long period of time away from an outlet. The idea was to be able to use the laptop during the ~13 hr flight from Sydney, Aus to Los Angeles in coach where there are no power jacks (getting the whole thing through airport security going to the United States was really... fun.....
This is actually pretty simple. The idea here is to have a large battery pack, a regulator to keep the voltage constant at 24V (should be able to handle at least 1.5A continuously), and a male 2.5mm stereo headphone connector (same as the typical cell phone hands-free connector), which fits the proprietary Apple power connector (it just won't have the metal shield). Obviously, the pack and the regulator can be anything of your preference -- this is just how I did it. Also, this page is not talking about the new magnetic connector that the MacBooks have. This is about the connector that was used in the Dual USB iBooks, TiBook, and pretty much all other Apple laptop models from then up until the MacBooks.
Click to enlarge. This is the funky stereo 2.5mm to dual stereo 3.5mm adaptor I used because RadioShack was out of all the 2.5mm connectors.
Now, the Australian airport security guys, from experience, are a lot more managable than the LA trash, so I decided to use alkaline D batteries and just throw them all away after landing. The pack had 20 D-cell alkaline batteries in series, providing nearly 36V when fresh (under load). A heatsinked LM317 regulated the voltage to 24V. Using a multimeter I found that usually my iBook drew 600-700mA with no disc in the optical drive and a full internal battery (not charging). The current fluttered about as the hard drive and cpu activity went up and down, and charging brought the current to over an amp. So typically a 12" iBook G4 will consume 15 to 20 watts. Obviously the LM317 design loses a lot of energy to heat, since the LM317 is a linear regulator and just turns the excess energy into heat, so if you build this, use a VERY VERY BIG HEATSINK!!! I could have used a more efficient regulator, but it was cheap, and it worked, and served its purpose. Oh yea, and it did last the whole flight, with power to spare :)
If you are replicating this, just build the thing below and it should work, piece of cake, though you might want to put a switch in there between the batteries and the circuit, so you can switch it on and off. To build the pack of D's, I simply took a 4-cell D battery holder that had each cell next to each other and cut it in half down the middle with the band saw at school (shop class was the only thing my Australian public school seemed to be good at). I then mounted the two halves with screws on a piece of wood 5 D cell lengths apart. 4 x 5 = 20 D cells. After carefully putting all the D-cells in the pack I put a piece of cardboard over them (to keep them in place) and duct taped it on (see photo and diagram).
Click to enlarge. The photo on the left shows the current in amps, and the right shows the 20 D cells, the regulator (and enclosure), and multimeter. Oh yeah, and a 10Base-T ethernet hub, but that's not important ;)
I never looked into whether or not you can provide less than 24V to the laptop and still be within operating limits. If you can, then forget the regulator and just use 13 or 14 cells in series. Just BE CAREFUL: DO NOT GO ABOVE 24V!!! As always with these kinds of things, I AM NOT RESPONSIBLE FOR YOU BLOWING UP YOUR LAPTOP AND/OR CAUSING PERSONAL INJURY TO YOURSELF OR YOUR STUFF.
BTW here's an Apple technical article that goes into some depth about the Apple power connector: http://developer.apple.com/qa/qa2001/qa1266.html
Questions/Comments may be sent to adwiens AT gmail DOT com.