I have a USB battery which has a USB-C port on it.

When I connect it to my MacBook Pro (regardless of whether the MacBook Pro is running off its battery or via the mains), the Mac doesn't charge the battery, but instead uses the battery to power the Mac.

If the Mac is not on its own power supply, it will use the USB battery for charging (very slowly, but useful in a pinch).

In every case, the Mac tries to make power flow from the battery to the computer.

How do I reverse this? How do I make power flow from the Mac to the battery?

Apparently this is selectable, and other OSs provide an option.

  • Try to connect the cable first tp the Mac and then to the battery – Fez Vrasta Jan 23 '18 at 22:52
  • @FezVrasta - didn't make a difference, as expected – iAdjunct Feb 11 '18 at 22:58
  • The laws of Electricity. Current (A) will always flow from Higher Voltage (V) to Lower Voltage. – Ruskes Dec 28 '18 at 4:16
  • @Buscar웃 This does not take into consideration the very obvious fact that both sides can - and often do - have voltage regulators they can selectively turn on or off. If the answer to this question were that simple, this question would have been answered already. – iAdjunct Dec 29 '18 at 2:12

Method 1: Reverse/inverse the sides/plugs of the USB-C to USB-C cable. Might works or not.

Method 2.1: Use a USB-C to micro-USB(male) cable and a micro-USB(female) to USB-C(male) adapter.

Method 2.2: Use a USB-C to USB-A(female) adapter and a USB-A to USB-C cable.

Method 2 should always work. However you cannot use PD charge then.

Explain for method 1: I've heard C-C cables have sides to determine host / device. But I can't find the source. From my personal experiment, it depends on cables.

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  • nothing to do with cables or plugs. The laws of Electricity. Current (A) will always flow from Higher Voltage (V) to Lower Voltage. – Ruskes Dec 28 '18 at 4:18
  • @Buscar웃 Which side voltage higher is determined by eletronic protocol. – amdyes Dec 28 '18 at 9:13
  • You can not charge a 10 Volt battery with 8 Volt Battery. – Ruskes Dec 28 '18 at 16:55
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    @Buscar웃 Yes, you can, using a boost converter. I use boost converters regularly to operate 12V electronics in my car from the 5V USB sources because I despise having lots of the standard cigarette-lighter 12V plugs and splitters in my car. While it's true that current flows from a higher potential to a lower potential, it's also true that both sides of nearly every connection you use nowadays has active circuitry to control nearly every aspect of that interaction. – iAdjunct Dec 29 '18 at 2:15
  • @Buscar웃 Furthermore, using a USB-C power adapter (which is definitely 5V), you can power and charge the MacBook Pro's 13V battery. – iAdjunct Dec 29 '18 at 2:16

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