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I have a campervan, and was wondering if I can charge my macbook pro late 2012 60W (A1425) with a homemade 24W usb-magsafe charger. I am asking this question, because I am planning to build a charger that can charge using the cigarette lighter.

Is it better to have a higher current / wattage capacity - 75W charger for safe operations of MacBook Pro?

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  • As you see - the definition of home made charger is causing some issues with one of the answers. Do you have a specific charger design in mind or is this a “theoretical” question before you get into what might actually be involved to do the project.
    – bmike ♦
    Commented Jul 29, 2019 at 22:08

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The rating is a maximum power value, and your MBP will draw only what it needs. Your MBP needs up to 60W in order to work at full power AND recharge the battery. Using a 24W charger will only charge the battery if the MBP is using less than 24W to run.

However, because 24 is much less than 60, it's likely that this power source will be insufficient to power the MBP and recharge the battery. If the MBP is sleeping, you might charge the battery very slowly overnight. If in use, you might charge the battery very slowly, or even just make the battery discharge more slowly than normal. I've used 45W power supplies with 60W Macs and they have charged slowly while in use; but when the Mac works hard, the battery level falls, though slower than without the supply.

I'm prepared to say that using a lower-wattage supply will not harm your battery. Whether the actual device you make is safe will depend on you.

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  • Mind if I make a pretty major edit? Please roll back if you don’t like what I added and I’ll make a new answer.
    – bmike ♦
    Commented Jul 29, 2019 at 21:23
  • @bmike It was a lovely answer, but not what I wanted to say.
    – benwiggy
    Commented Jul 30, 2019 at 8:41
  • @benwiggy thanks for your feedback! very complete. And if we take it the other way around: If I have a 75W magsafe to cigarette-lighter charger and plug it to my MBP, will it harm my computer that's supposed to take 60W?
    – Steph
    Commented Jul 30, 2019 at 9:32
  • @Steph No. See my first sentence. The computer draws the power it needs from the available supply. The supply doesn't push all its power into the Mac.
    – benwiggy
    Commented Jul 30, 2019 at 12:34
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It is safe to use a lower power charger than the factory supplied charger but NOT safe to use a higher one. The biggest problem you may have is that your laptop may use more power than it is receiving even when on charge so you need to let it charge without use if using a significantly lower power charge like you will receive from your cigarette lighter. It may not even be enough power to charge when not in use.

Good luck though.

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    And you are sure the homemade charger will complete the necessary handshaking?
    – Solar Mike
    Commented Jul 29, 2019 at 17:40
  • @Chris Bothwell I think you're incorrect. A charger's rating in watts describes only the amount of power the charger is capable of delivering. An 85w MacBook charger, if plugged into a fully charged MacBook, won't deliver 85w of power — it will only provide what the computer asks of it, only the very small amount of current necessary to keep the internal battery fully charged. There's nothing electrically wrong with using a charger with a smaller rating, save that (as you point out) the computer battery may run at a deficit if the computer's being used.
    – DavidRecallsMonica
    Commented Jul 29, 2019 at 18:19
  • This answer could be interpreted two ways - one is mostly wrong as a safe charger is safe if it negotiates a slower USB C PD - proper gear won’t over or under change as they are intelligent. One is very right - if you blindly apply max power without proper fusing / control chips and signaling, you will damage your cables and Mac hardware. I’m not going to vote either way until an edit makes it clear what Chris is saying here.
    – bmike ♦
    Commented Jul 29, 2019 at 21:30

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