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I have just upgraded from a Macbook Air (Late 2012) to a Macbook Retina (Late 2013). Now I have a 45W and a 60W charger, both MagSafe 2.

Using iStat Menus and Battery Health for monitoring voltages and currents, I can see that when charging my Macbook Retina with the 60W adapter, it really does draw nearly 60W from it. Using the 45W one, it draws nearly 45W. It never draws more power than the charger can deliver (I also would be very surprised if Apple would let that happen). I know that some people disagree and say this is dangerous, but for the sake of the argument let's just assume that it works like this.

Now, Battery Health shows that my Macbook is actually charging with around 30W when using the 45W adapter (light Macbook usage, leaving around 15W for CPU, SSD and everything else) and around 45W when using the 60W charger. That means in both cases my battery is charging fine, only with different speeds.

Researching the internet, it seems a fact for Li-Ion batteries that:

  1. High temperatures decrease battery longevity
  2. Fast charging/discharging decreases battery longevity

So my assertion is:

Knowing all this information, using the 45W adapter instead of the 60W one should actually help battery longevity.


Note:

Apple officially says [1]:

Although you should always use the proper wattage adapter for your Apple notebook, you can use an adapter of a higher wattage without issue.

They say nothing about using an adapter of a lower wattage. Which I am interpreting as: Using a lower powered adapter can work, but you might experience issues such as slow charging or no charging at all (under high macbook load). Of course Apple won't officially say that this is ok (otherwise people will complain), but if this really were a serious hazard, then they wouldn't have all the chargers designed to use the same plug.

But like I said, this is not what I want to discuss. What I would like to know is if my assertion is correct under the assumption that 45W chargers can be used for the MBPr.

  • I wouldn't deliver less power than the device actually requires. It can just harm more than it can do good. – Rob Jun 21 '14 at 9:42
  • If the Macbook requires 60W and the charger can deliver 45W, it will draw the remaining 15W from the battery. The charger will never deliver more power than it was designed for (as you can confirm monitoring the voltages and currents). – michaelk Jun 21 '14 at 9:45
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Since the Temperature is a major factor in Battery life degradation, slower charging is preferred.

However, if you are charging while consuming /(using your computer) the charger might not provide enough to actually charge the battery and it will get depleted.

In that case using the lower power charging is only recommended when computer is off.

Source 1:

Source 2:

  • Thanks for your feedback, that's exactly what I am thinking. I would only use the charger when the Macbook is turned off or under light load (as then there still seems to be a 30W buffer). – michaelk Jun 21 '14 at 10:23

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