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I recently found a great appreciation for the long battery life of my newly acquired mac book pro and was thinking how I can scale that up even more (since I travel and work on site a lot), by building DIY "Power Bank" for the mac. So I have some very basic knowledge of electrical engineering (built a few small home Raspberry Pi Projects), and now just need some guidance with taking on this project. I'll keep it to a strict question-answer format for the things I am unclear about.

The plan is to take a 12V 7ah Lead Acid Battery and build the nessecary transformer/step-up module to get a USB-C output delivering 20.3 V @ 3A or 9V @3A (both work for my mac).

To get from the 12V to 20.3V, I'd thought I'd use this step-up module. Will this be able to do the job? Also, how do I specify on the step-up module the exact voltage that I want (In & Out).

Then I read here that one needs a pull-down resitor in order for the USB-C connection to be recognised and enable VBus, the post suggests that you "use a regular USB A male cable with an A to C adapter..." to achieve this - that makes sense, but how to I get power from the step-up module to the regular USB head?

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closed as too broad by Chris Stratton, RoyC, Warren Hill, Finbarr, Brian Carlton May 3 at 2:40

Please edit the question to limit it to a specific problem with enough detail to identify an adequate answer. Avoid asking multiple distinct questions at once. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.

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    \$\begingroup\$ How will you mange the USB handshaking to allow 20 V charging? \$\endgroup\$ – winny Apr 29 at 13:18
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    \$\begingroup\$ What you do not know yet is that the MacBook will "talk" to the power adapter and it expects a specific "answer". This is called a "handshake". This is done using specific chips and is definitely not something a beginner can do successfully. So I suggest that you buy a car adapter charger specifically designed to charge a MacBook over USB-C instead of trying to build your own because I already know now that it is not going to work the way you want it to work. Very likely the MB will simply refuse to charge its battery. \$\endgroup\$ – Bimpelrekkie Apr 29 at 13:22
  • \$\begingroup\$ @Bimpelrekkie Thanks, that makes sense, yes. And I will indeed not be able to build such a PCB on my own. I'll look into and get back to this post, update with an answer If I find enough info. \$\endgroup\$ – Marnus Steyn Apr 29 at 13:32
  • \$\begingroup\$ @Bimpelrekkie +1 on your comment. I appreciate your useful answers and input - and puzzle over why you are so helpful in these cases and yet discourage others with close votes when questions could be improved or left to others to answer. Life is complex :-). \$\endgroup\$ – Russell McMahon Apr 29 at 14:17
  • \$\begingroup\$ @Bimpelrekkie please make your comment into an answer. \$\endgroup\$ – TimWescott Apr 29 at 15:01
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What you do not know yet is that the MacBook will "talk" to the power adapter and it expects a specific "answer".

This is called a "handshake". This is done using specific chips and is definitely not something a beginner can do successfully.

So I suggest that you buy a car adapter charger specifically designed to charge a MacBook from a car's 12 V system, you should be able to use your 12 V battery for that as well.

Instead of trying to build your own because I already know now that it is not going to work the way you want it to work. Very likely the MacBook will simply refuse to charge its battery. It's an Apple product and it might even expect an Apple charger. I mean, a generic USB charger might not even work.

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  • \$\begingroup\$ I think it's worth pointing out that this is standard behaviour for USB-C, not something specific to Apple. A MacBook with USB-C should charge from any 'USB-C with Power Delivery' charger - see here for example. \$\endgroup\$ – nekomatic Apr 30 at 13:04

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