Does the MagSafe charger only work with certain power supplies?

RAVPower Ladegeraet

Who measures measures crap – the old saying still applies in the USB-C age.

Aaron from Zollotech has shared a video on YouTube that proves that the Apple MagSafe charger only works well with Apple’s 20 watt charger and reaches the maximum 15 watt charging power. With other USB-C power supplies the performance should be far below that.

We noticed that for the measurement simply a USB-C meter with a small display is connected in between and it is supposed to prove that different power supplies of Anker and Aukey mentioned in the video do not work well with the MagSafe charger.

We can also kill ourselves with these USB-C measuring devices and our experience to the devices is the following: They measure what they measure, i.e. the actual voltage and current. But the problem happens first. Because after the Power Delivery Standard the power supply and the connected device have to negotiate which voltage and current is suitable for both, some data has to be exchanged first. And during this process these USB-C meters disturb very often. So they introduce an error in the measurement setup. The consequence is that the negotiation for the current flow cannot be done correctly and therefore the devices play it safe and choose a slower charging protocol. So if the measured current is lower than it should be theoretically, our experience is that the USB-C Meter is the cause of the problems.

The USB-C Meter itself is the problem

It is very easy to check this: just measure with a power meter that you plug into the socket before the power supply, how many watts are actually drawn. Do the measurement once only with power supply and cable or end device and once with an interposed USB-C meter. If the data deviate, this is a sign that the USB-C meter prevents a correct negotiation of the connection. There is still a current flow, but mostly a much lower one, because the devices cannot communicate well with each other. Conclusion: Measurements with these inexpensive USB-C meters give partly good clues but very often lead to a problem in the measuring section.

Also theoretically this can easily be checked: The MagSafe Charger needs for its 15 Watt charging power a power supply with 9 Volt and 2.22 Ampere. That is 20 watts, which is the same amount as Apple’s small USB-C power supply. The difference of 5 watts is due to the fact that wireless charging is associated with losses: To charge with 15 watts, you have to use 20 watts – five watts are wasted as heat. So we now know that we need a power supply that delivers at least 2.22 amperes at 9 volts. And that also explains, as mentioned in the video, that Apple’s old 18 watt version lets the wireless charger deliver only 13 watts – because the five watts of loss are still there. The technical data show that the 18 watt variant only supplies 9 volts and exactly 2 amperes – and that’s simply not possible. In this case, however, the negotiation of the connection between the two devices worked out well and the maximum current flow is established.

Apple usually adheres to the specifications

Apple’s 96 watt power supply, on the other hand, should only output 10 watts. This would have to be counter-tested with the above mentioned method. Since the power supply is able to deliver even 3 Ampere at 9 Volt, we strongly recommend the USB-C Meter, which causes the problems here. The same is true for the PowerPort Atom PD 1, which is able to deliver 3 Ampere at 9 Volt, as well as Aukey’s 65 Watt adapter. In our opinion the power supplies will deliver full power without the meter malfunctioning.

The USB-C devices we know from Apple strictly follow the specifications printed on the power supplies. Sometimes a voltage level is missing, but those that are offered work perfectly. Therefore it is very surprising that even Apple’s own 96 Watt power supply is not supposed to have got the MagSafe adapter working with 15 Watt. However: it can of course be the case. As you can read here, the MagSafe adapter does not even work correctly with all iPhones. So it’s best to try it yourself if the charger works – you can’t rely on it at the moment.

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