4

There is already a thread asking whether MagSafe1 chargers can be used interchangeably. However, having gone through all answers, and asked in the comments, I found that not a single person reported using a higher-wattage adapter on a lower-wattage MacBook successfully.

Ordinarily there should be no problem using a higher-wattage adapter, since the computer would only draw the amount of watts it needs (see the highest-rated answer there) -- but Apple thinks differently. They designed their higher-wattage adapters not only to supply more watts, but also to supply more volts to the computer. For example, the 45W Magsafe1 adapter supplies 14.5V, whereas the 85W Magsafe2 adapter supplies a whopping 20V (!). And in most cases, unless the port is specifically designed for the higher voltage, 3V over the spec is enough to fry the power port of the computer.

The accepted answer on that page quotes Apple as saying that we can use a higher wattage power adapter, but we cannot use one with less wattage without potential operating issues. However, this page has since been removed. I called Apple to ask about it, had to get transferred to a supervisor, and he hinted that Apple removed that statement because they did not wish to have liability to people whose power ports have burned up.

However, the one thing the supervisor did not answer is, how likely is it for a power port to burn up because a higher-watt charger burned up. I bought a couple of the 20V adapters without realizing they'd be at a different voltage, and I am afraid to hook them up to my 14.5V MacBook Air. However, the supervisor said that this is a question for the Engineering -- but Engineering support is not available for hardware which is no longer supported. Makes sense. But now to get the answer I have to ask the question here.

Has anyone heard of an 85W adapter successfully being used on a MacBook Air?

3 Answers 3

4

This is really already covered in the linked QA, so to simplify it right down as far as possible…

General rule: Voltage must match. If the Voltage & polarity matches, the Amperage can be a variable.
Higher than spec is better, because Amps are 'pulled' by the device not 'pushed' by the charger. This is 100% safe, 100% of the time.
Lower than spec, the device can potentially 'pull too hard' for the charger, causing overheat, whilst under-charging the device.

The definition of Watts is Volts times Amps.

A lot of this is obviated by USB-C, which is a 'smart' structure. It can negotiate its power requirement.

8
  • I understand the general rule, but de facto, a 3V difference in the voltage is acceptable for most cases. In the question I stated that the difference in the voltage is more than 3V. For this reason I asked this question. So this answer does not apply to my situation (other than to suggest that it is not okay because the voltage does not match).
    – Alex
    Oct 31, 2022 at 15:23
  • 3
    You can't just say "3v is OK". It just doesn't work like that. If it's been built that way, then fine, but if it hasn't, things go ftzz. There's every chance that any generic charger will present anything between 14 & 20v depending on the load, but measuring voltage under load is a nightmare at 'consumer with multimeter' level.
    – Tetsujin
    Oct 31, 2022 at 15:45
  • Right. Anything between 14V and 20V might be for Mac adapters (so 3V is OK in this case), but in general, I've seen even the 5V regulated power supplies provide up to 8V under small load, when they are close to end of life, and that's never burned any power ports in my experience. That's why I said that 3V is generally okay in all cases.
    – Alex
    Oct 31, 2022 at 15:50
  • p.s., I thought measuring voltage under load is easy enough to do, if you are willing to sacrifice the adapter you're measuring -- no?
    – Alex
    Oct 31, 2022 at 15:54
  • @Alex if your 5V adapter is supplying 8V, it's not regulated. Maybe the regulator is broken, idk. Oct 31, 2022 at 17:52
3

Yes, this is possible. The power port has been rated to withstand the higher voltage, so even though the apple.com page has been taken down, the rating remains.

However, using a higher-power adapter does lead to the battery charging more quickly, which puts more strain on the battery, leading it to wear out more quickly. Alex King experimented with using an 85W adapter on a MacBook Air, and after only 7 months and 174 cycles, the battery capacity went down to 63%.

In other words, the recommendation is not to do it unless there is a reason the original adapter is not powerful enough (as is the case here, because the battery is drawing more power than the adapter can provide).

5
  • "using a higher-power adapter does lead to the battery charging more quickly" - only if the device can pull that higher power. The evidence you link to is 14 years out of date and is merely anecdotal, not a rigorous scientific test. As such it is nothing more than hearsay.
    – Tetsujin
    Nov 2, 2022 at 8:34
  • Of course it's not a rigorous scientific test. But, he finds the concrete numbers; and, he also has a comparison when using a regular, 45W charger -- 95% capacity after a similar number of cycles. In other words, using the 85W adapter made the battery capacity go down about 7 times more quickly -- a significant difference, in my opinion.
    – Alex
    Nov 3, 2022 at 3:33
  • As far as being 14 years out of date -- that's when MagSafe1 adapters were around (these are precisely what this question asks about). In particular, the MacBooks that use those adapters still use those same, 14-year-old charging algorithms.
    – Alex
    Nov 3, 2022 at 3:36
  • Look up post hoc ergo propter hoc to see why this is a failed logical conclusion.
    – Tetsujin
    Nov 4, 2022 at 7:34
  • The idea is, that each battery discharge is a single "experiment", if we are to use the scientific terminology. In the sense that each discharge reduces the battery performance by a little bit. Under the mild assumption that in the linked article each battery discharge affected the battery capacity roughly the same, we would get 174 points of data in the "85W" group, and 191 in the "45W" group, rather than just one and one, pointing to the same phenomenon.
    – Alex
    Nov 9, 2022 at 16:34
0

As long as it is delivering it via USB-C, it is safe to use an adapter delivering higher wattage than the one coming with the MacBook according to the "Identify your Mac power adapter" Apple support webpage (italicization & bolding is by me):

If your Mac uses USB-C to charge, you can charge your Mac laptop with any USB-C power adapter or display. For the best charging experience, you should use a power adapter or display that provides at least the minimum wattage of the power adapter included with your MacBook Air, MacBook Pro, or MacBook.

2
  • 1
    I already said this information as a comment to Nohl's answer in the referenced thread. Note that my question is in particular regarding MagSafe, rather than about USB-C.
    – Alex
    Oct 31, 2022 at 4:03
  • 1
    OK, I missed that. You may find the "can I use an 85w magsafe 2 power supply on a Macbook air?" thread on another site and the links provided within helpful then.
    – Alper
    Oct 31, 2022 at 4:45

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .