Parts required
Instructions
Using your cable Tips #3 and #4 (both included with your universal DC power adapter) will mate with the MagSafe cable you made. Tip #3 supplies 19.5 V while tip #4 supplies 16.5 V. Either will work, but using tip #3 may result in greater than desired power consumption. Use tip #4 if your battery is more than 71% charged; that mimics the behavior of the Apple power adapter. The battery in a MacBook Pro can be charged from empty with tip #4 even while the computer is being used; it just takes a lot longer. Use tip #4 if you have a MacBook (not a -Pro). The LED in the MagSafe connector will behave a little differently when your cable is plugged into a universal power adapter. The LED will continue to indicate red for charging and green for charge maintenance. When you unplug the MagSafe connector, however, it will stay illuminated in the same color. With the Apple AC power adapter, the LED turns off immediately after unplugging the MagSafe connector. Using your Apple power adapter Put the DC power plug on the Apple MagSafe AC power adapter (on the cable you cut earlier). Connect the center conductor to the center pin. Now you can continue to use your MagSafe cord with your existing AC adapter when you have an AC outlet available.
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The modification I suggest above does not fully work with the Apple Airline MagSafe Adapter. Apple warns that the adapter powers but does not charge the computer's battery. Their warning is correct. Previously, I had thought that the charging issue was purely a consequence of the low voltage from airline in-seat power connections (15 V, see below), but it turns out that there is no charging regardless of the voltage supplied to the cable. Credit goes to Kieran Hervold for discovering this and reporting it to me. In past versions of this page, I suggested modifying a cheaper Airline MagSafe Adapter following the steps above, or even using a cigar lighter socket to achieve the same result without cutting any of your fancy Apple cables. Unfortunately, that was bad advice. I apologize to all those who pursued this solution and found unsatisfactory results. That said, some people may want to make a “universal” MagSafe cord that doesn't need to charge the computer's battery. With that in mind, the figure below shows an adapter cord that you could assemble without cutting your fancy Apple cables. Wire everything center positive. Again, with the following adapter, the battery will not charge. 
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The MagSafe connector has a symmetric pinout:
The LED is somewhat of a mystery to me. It illuminates green if the voltage between V- and V+ is greater than about 16 V. The Apple power adapter drops to about 6.25 V if it has no load, so the LED turns off. The universal adapters supply their selected voltage regardless of load, so the LED will stay illuminated even if the computer is not connected.  The MagSafe LED turns red if the computer “knows” that it is charging. Therefore, I suspect that pin 3 (the center pin) of the MagSafe connector controls whether the LED is green or red. With an ohmmeter, pin 3 registers about 426 kΩ to the V- pins and no circuit with the V+ pins. Also worth noting is that, with a universal adapter, the LED remains illuminated in the same color when you unplug the MagSafe connector from the computer.There is no harm in putting a 2-contact connector in the middle of the Apple MagSafe adapter's cable. As you can see in the photo to the left, there are only two conductors (that's a string sticking out to the left). There is no additional signaling that will be broken by the modification. Kieran Hervold has disassembled a MagSafe connector. Inside is a little integrated circuit (a “chip”) that I suspect controls the LED color. I also suspect that the chip inside the Apple Airline MagSafe Adapter is able to signal the computer to shut off the battery charging circuit. |
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65 W adapter As far as I know, the 65 W adapter simply runs at 16.5 V at all times that the computer is connected. 85 W adapter The 85 W adapter varies its voltage depending on its load. At maximum load—say, when charging a discharged battery—it supplies up to 18.5 V. At minimum load, it supplies 16.1 V to 16.5 V. I monitored a charge-up from a battery at 37% to 100% with the screen at full brightness, a low CPU load, and the 802.11g turned on. Below 70% charge, the Apple adapter supplied between 17.0 V and 18.1 V. At 71% charge, the Apple adapter abruptly dropped down to 16.5 V. For the rest of the charging time, the voltage ranged from 16.1 V to 16.4 V. |
| The universal adapter power tips have sense resistors in them that tells the adapter what voltage to supply. I have measured values of sense resistors and their resulting voltages for some of tips. |
Some airlines have installed in-seat power connectors in some of sections (usually only business and first classes) of their aircraft. This connection is called an EmPower receptacle, and it is a product of Astronics Advanced Electronics Systems. It supplies 15 VDC to power notebook computers directly; the idea is that it could power a notebook computer directly without the need for its bulky AC adapter (exactly as Apple's MagSafe Airline cable does). Many computers require more than 15 VDC, however, and consequently travelers usually end up carrying an additional DC-DC power adapter or a universal AC/DC-DC adapter like the Targus/Kensington/iGo. Because of this situation, many airlines are now installing 120 VAC receptacles.Do not use tip #3 in an airplane. The EmPower connector, pictured at right, has a 5 A (or 75 W) circuit breaker. Many modern notebook computers (including the MacBook Pro) can exceed that load capacity. Use tip #4 with MacBook Pro systems in order to keep power consumption below 75 W. Photo credit: Astronic's website. Additional info: Wikipedia. |
| The standard 12 VDC sockets that are in almost every car are called cigar lighter receptacles, and their design is governed by standard J563 of the Society of Automotive Engineers. The receptacle inner diameter may be between 20.93 mm (0.824 in) and 21.01 mm (0.827 in), while the plug outer diameter may be between 20.73 mm (0.816 in) and 20.88 mm (0.822 in). The standard also states a maximum current rating of 8 A. As of 15 July 2007, Wikipedia has an excellent overview of SAE J563. The standard itself is a one-page document; it is almost certainly at your local engineering library. |
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In the time that this page has been on the web, I have received dozens of grateful e-mails from people who have needed 12 V power for their MacBooks. The reasons have varied from typical (car or airplane) to exotic (solar homes and sailboat trips around the world). I appreciate the e-mails, but I also apologize for the long delays that sometimes precede my responses. I keep myself pretty busy, and this stuff is not my my “day job.” With that said, feel free to contact me. I especially am interested to hear about any “neat” places or uses that my 12 V setup opens up to you and your computer. And if you have any questions, please e-mail me—if something is unclear, hopefully I can offer some assistance. If you’re looking for more information, I do have a few extra notes that I've collected about the various topics covered in this page. And if you any additional information, I'd love to read it. With your permission, I'll probably include it in future versions of this page. I have one more comment that does not seem to fit anywhere else on this page. Another website offers the modification I described above for a fee. If you are not comfortable with soldering, consider purchasing that service from them. I will not provide a link to that site because that site sells instructions on how to perform the modification. That strikes me as a little exploitive—information of this type should be free. Hence I have taken the time to put together this webpage. I have not seen the other site's instructions, but I am sure it contains the exact same steps I described above. |
