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Aim: I would like to charge my MacBook Pro (MBP) form a 12V lead-acid battery. However there are no official chargers from Apple for that purpose. There exist quite some alternative products on the internet. For example this one simply comes with a MagSafe adapter and lets you choose 18V. However I am afraid that such a solution might hurt my battery.

Does anybody know how exactly the MBP battery is charged? Does it require a particular charge line? Is it OK to simply connect 18 Volts and see what happens?

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If you connect 18 V to the power/ground pins, your Mac will be powered off the 12 V battery, but it won't charge: https://apple.stackexchange.com/a/132332/26231

To charge, the laptop needs to identify the charger by communicating over the signal pin in the middle. http://www.righto.com/2013/06/teardown-and-exploration-of-magsafe.html

Solution: Sacrifice an apple charger by cutting the cable and connecting the correct pins to your DC/DC converter. I recommend using the 45 W ones so your DC power supply doesn't have to be very big.

  • Thanks! Will the MacBook limit power consumption to 45W in your example or draw whatever it can? – n1000 Jul 4 '14 at 5:03
  • Yeah, if you cut the cable from a 45 W charger, the Mac will read the chip in the connector and identify the charger – Kevin Chen Jul 4 '14 at 18:07
  • ok. finally did it. got an efficient dc/dc converter and a new magsafe cable from ebay. works. – n1000 Oct 23 '14 at 7:18
  • Hey @n1000 thanks for the great question and comment. Please may I have suggestions on which DC/DC converter to buy? – nslntmnx Aug 9 '15 at 11:40
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    @nslntmnx Mine is similar to this one. But I guess any DC/DC converter with enough power / cooling will do. However, I encountered a problem, which seems related to 18.5V MBPs. – n1000 Aug 9 '15 at 13:47
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Use a DC to AC inverter. You simply attach the alligator clips to the correct battery terminals, turn the device on and plug your cord into the inverter. It's that simple. Amazon sells a 400 watt inverter for $25, that's all you'll need. http://www.amazon.com/gp/product/B001RNOHBC/ref=oh_details_o01_s00_i00?ie=UTF8&psc=1

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    The system should be as energy-efficient as possible. Doing DC-AC-DC involves quite some losses and is therefore unsuitable for my situation / question. – n1000 Jun 8 '14 at 20:23
  • Inverters are fairly close to 100% efficient these days, which is why 500 W inverters only have tiny power supplies. – Kevin Chen Jun 22 '14 at 13:59

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