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I am charging a MacBook Pro using a Thunderbolt 3 cable plugged into an iMac. This charging speed is exceptionally slow. The image shows 5+ hours to charge the laptop.

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I am using:

  • 2018 MacBook Pro 13" with Touch Bar
  • 2017 Retino 4K 21.5-inch iMac
  • Apple Thunderbolt 3 (USB-C) Cable (0.8m)

This cable costs $39 retail. At this price I am expecting better performance for charging than what I'm getting here. How could I diagnose this issue?

  • 13
    Why not just charge it with the charger it came with! – user3439894 Feb 12 at 19:36
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    I didn't want to schlep that to the office each day – William Entriken Feb 12 at 20:10
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    Then buy an extra charger and leave it at the office. – user3439894 Feb 12 at 20:18
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This cable costs $39 retail. At this price I am expecting better performance for charging than what I'm getting here. How could I diagnose this issue?

There's nothing to diagnose because everything is operating as it should.

The problem is not the cable (it's irrelevant, actually) nor the iMac as it's only capable of delivering up to 15W of power via the Thunderbolt 3 ports. From Apple's Support Document.

Your iMac also has two Thunderbolt 3 (USB-C) ports....Thunderbolt 3 also supports USB 3.1 Gen 2 connectivity at up to 10Gbps, and can supply 15W of power per port to external devices.

Emphasis mine

Your MacBook Pro requires a 61W charger so, in essence, you're using a charger that can only supply a quarter (25%) of what you're MacBook Pro requires. Therefore, the long charging time is accurate and to be expected.

Bottom line - use the charger that came with your MacBook Pro.

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    @WilliamEntriken no. You want each thunderbolt 3 port to have the capability to charge a laptop!? That’s such a bizarre edge case. Just buy a 60W charger. – Tim Feb 12 at 20:40
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    The power output of the iMac being 15W is about 84% of the required power for the iPad pro. It will charge "fairly quickly" but not at the maximum rate. Keep in mind that nowhere in the Tech Specs for the iMac does Apple say they're "charging ports." – Allan Feb 12 at 21:17
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    @WilliamEntriken, If you got your way, then others would gripe, "I wish my iMac was thinner and lighter." – Glen Yates Feb 12 at 21:51
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    @GlenYates: I looked up the specs. The iMac 5K has the highest power consumption of all iMacs, at 217W with full CPU load. The iMac has 6 USB ports (4 Type A, 2 Type C). Type A can deliver up to 7,5W using USB-BC. Type C can deliver up to 100W using USB-PD. So, the USB ports can deliver up to 230W in total, which is more than the entire iMac uses itself. This means, what the OP wants would require a power supply that can deliver twice the power, which roughly means it is twice as big and twice as heavy. The power supply is a major contributor to the iMac's weight after the display. – Jörg W Mittag Feb 13 at 2:20
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    … An iMac that can deliver full charging via its USB ports would not be heavier and thicker, it would be significantly heavier and thicker. (I ignored the problem that the power supply would likely also be warmer, for example. So, you also need more cooling.) – Jörg W Mittag Feb 13 at 2:23
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According to Apple the best way to charge your MacBook Pro is with the included AC adapter. They don't mention plugging into an iMac specifically, but based on your report of a long predicted charging time, I'm going to guess that iMacs don't put out as much energy over thunderbolt as the AC adapter.

According to this page on the Thunderbolt 3 Cable, it is definitely the iMac's power output, not the cable that is limiting the charging speed.

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