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I’m looking to understand how to test and evaluate chargers so this is a bit technical a problem. Since MagSafe is proprietary, I can’t consult a USB power distribution specification to know how things are designed to work.

What is the voltage output of an Apple 85W MagSafe 2 model A1424 without a load?

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  • Feel free to edit in some of the specifics - how you are going to measure this might help others greatly. Knowledge is power and if you can caution people not to use power to start a fire or damage gear, even better.
    – bmike
    Commented Aug 24, 2019 at 19:22

3 Answers 3

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I believe these are smart chargers, and initially only supply about 3 volts in order to power a power management chip inside the device to be charged. Once powered by the 3 volts, the power management chip inside the device then communicates with the charger verifying that the device is compatible to be charged, and if that fact is verified, only then will the charger output full charge voltage.

Additional info provided by @MarcWilson.

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  • Hi @Hitek. Thats what I read on internet too about sending a small volt at beginning and then the full. But whats strange is that even the small voltage sometimes is 0.2V and only one time it was 3V . I measured between Ground and Power according to this picture static.righto.com/images/magsafe/1-magsafe_connector_labels.png . Was wondering if someone knew the correct "no load voltage" for the model A1424. Thank you
    – AlberAls
    Commented Aug 23, 2019 at 9:55
  • @AlberAls - From an engineering perspective, the charger has to provide power to the target device in order for communication to happen, in the case of the target device's battery being completely dead. In such a situation, if the charger didn't supply power, then the dead device could never be charged again. If you are not getting any significant voltage(BTW 0.2V is NOT enough) from the charger at times, then the charger may very well be faulty. It's rather hard to say specifically, though, as I don't have a Magsafe 2 charger, and Apple's standards are highly guarded secrets.
    – Hitek
    Commented Aug 23, 2019 at 10:21
  • @AlberAls - Also, if you are getting 3 Volts sometimes and not others, you should hold a meter on the connector while having a second party flex the cable around, to make sure it isn't just an intermittent connection due to a bad cable that could easily be repaired. Another possibility is the fact that some chargers for other brand products actually send the standby voltage over the data(center in your case) pin rather than the power pins, so you might check that also.
    – Hitek
    Commented Aug 23, 2019 at 10:25
  • @nekomatic - Sorry I don't have the reputation to comment directly to your answer. You are correct that the charger only produces it's full output voltage when it senses the resistance across the power lines, but from what I understand, the device will not introduce this resistance until after it has been verified that the charger is valid during the previous negotiation....
    – Hitek
    Commented Aug 23, 2019 at 21:31
  • 1
    This is how they work. righto.com/2015/11/macbook-charger-teardown-surprising.html Commented Aug 24, 2019 at 2:07
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Based on the answers to this question the supply doesn't communicate with a power management chip - it simply looks for a resistance of approximately 40 kΩ between its output and ground, and if this is detected then the full output voltage is enabled.

Also based on the above answers, if you often measure 0.2 V rather than 3 V then an intermittent fault in the cable seems most likely.

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I'm measuring the outputs on a 45W MagSafe 2 adapter, and measuring 0.6V (without load) that trickles down to 0.1 V at 0.1 V/min, then resets back to 0.6 V and cycles down again.

I think this already means the adapter itself has a problem, as many references indicate I should read at least 3 V (no-load).

It does not charge or provide any sort of LED on the MagSafe connector when attached to the laptop, so I'm starting to eliminate parts.

The output cable showed continuity for each of the voltage and ground wires, nothing seems to indicate the cable is frayed.

I was reading about 0.2 V on the MagSafe connector between + and -, and was still going with the assumption its output was OK based on what people were writing above. So I wanted to eliminate the output cable as source of the problem altogether, before I consider the adapter itself the problem.

I cracked the adapter case open, easy enough, to look around.

I clipped off the output cable + and - inside the adapter so I can easily re-solder and keep using the strain relief built into the cable, as it was all in good condition, and re-measured the cut wires on the adapter output itself, that's where I read the 0.6 -> 0 -> 0.6 V.

Nothing looked burnt inside the adapter, nothing smelled burnt, so I'm tempted to test if the adapter can ramp up properly if I short it to 40 KΩ.

I then did one more test on the separated output cable, and measured continuity between its + and - and it was essentially shorted.

At this point I feel confident the MagSafe connector circuitry is hosed, and investing $10 on a replacement cable may be OK to revive this adapter.

If anyone can confirm if the 0.6 V looks wrong and I likely have a damaged adapter, I'll save myself the $10 in a new cable.

Hope any of the above helped anyone troubleshooting these adapters, I may follow this up with the results once I bring in a new output cable otherwise.

Edit 4/30/23: Before I soldered on the new cable I shorted the +/- outp with a 40K ohm 2W resistor while measuring the voltage and the adapter came up to the 14/15V it was rated to on its label.

Soldered on the replacement new $8 cable and all's working as expected.

Like someone else here or in another site said, the cable is expected to remain the weak link and probably fail again in a few years, given the one I found is not just thinner, but more plasticky, lacks the rubbery flexibility of the original. While the adapter main body and electronics seems solidly designed and should last through a few cable replacements at least, so don't glue back the adapter halves too strongly, unless you have to, as you will be cracking them open again to replace the cable again.

-MW

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