Powering your (Apple) laptop in the wild

I know, summer in Europe is almost over so what use is this information now? Next summer, when you might want to power your laptop in your car, tent, camper, RV, boot, spaceship will come faster as you think. I couldn't find a comprehensive guide online how to do it without an ac inverter (loud, inefficient). So here we are.

Disclaimer

1) Keep backups (yes multiple) of your data.

2) Most Stuff on this site will void your warranty and has the potential to break things, or even yourself (this includes death). I might have just been lucky. Think for yourself!

3) All the instructions and code are provided "as is", please follow/use them at your own risk, I will not be liable for anything what so ever.

4) When fuzzing with 12V car systems keep in mind that their batteries they can provide huge currents. So if you short them they turn cables into vapour really fast. Disconnect the battery first. Do not override fuses, they are supposed to blow before other cables do.

Let's talk about how it's supposed to be first.

Both Magsafe and Magsafe 2 connectors are,except for their dimensions, almost the same and you can adapt Magsafe to Magsafe 2.

The magnetic Magsafe connectors have 5 pins. The center pin is a 1-wire bus between your laptop and the microcontroller in the connector. It controls the LEDs in the connector and contains an ID, which tells your mac what type of charger is connected.
The outermost pins are both GND. The remaining two pins are for positive charging voltage. So the connector s symmetric and can be connected either way.

The cable between power supply and magsafe connector is a coaxial cable. The core conductor is positive. The "shield" negative.

When measuring the pins of a magsafe connector you might notice the power supply only providing a low voltage (3-6V). This is because the PSU turns the voltage off when not connected. This might be to save energy or to prevent accidental sparks when something touches the semi protected contacts.

When it senses a load of about 40k Ohm it turns on the full Voltage. That voltage depends on the wattage and version of your power supply. The voltage ranges from 14,5V to 18,5V on Magsafe 1 and up to 20V on Magsafe 2.

What I did:

I cut off the cable with the magsafe connector from broken power supplies I obtained and added my favourite connector to the open end. Now I have two of those cables, one from a 85W and one from a 60W Magsafe 1 version from 200X.
After hooking them up to my Labsupply and testing them thoroughly,
it seems they just differ in voltage while supplying round about 4A max. So 60W/3,75A = 16V and 85W/19,5V = 4,3A. They do seem to make MacBooks behave a bit different. With the 60W version tested MacBooks cap amperage at about 3,8A. With the 85W plug they draw up to 5A.

The more important questions:

Can I power my Macbook from my car/camper? short answer No, but almost;-)

All devices I tested start recognising "a charger" when they sense 13,8V. That means you can't power your laptop directly from a lead-acid car battery because it's open-circuit voltage is about 12,6V - 11,8V.
But charging voltage for lead-acid batteries is at least 13,8V. Your alternator should supply about 14,2V to 14,8V, so your connected MacBook will likely charge. You should measure the voltage in your 12V car socket. If the open circuit voltage is above 13,8V but your Laptop isn't charging it might be that you are loosing too much voltage on the way from your car battery to the socket. That includes all cables, connectors and fuses.
It would be nice if all cables were ideal, by having zero resistance, but they sadly don't. Thats one reason why voltage drops under load.

Example:
Voltage at battery 14,2V
Cable from battery to socket: 3m (10 feet) 1 mm^2 (18AWG)
current drawn: 4A

resulting drop 0,5V or 3,53%
so you'll only get 13,7V under load and your Laptop doesn't charge.

What can I do?

You could install thicker cables in your car so charging will work while the motor is running. (not recommended)

Or you need a step up converter, also called "Universal 12V Laptop power supply", you can buy online and add a matching connector for your cut off Magsafe cable. The come with cigarette lighter plugs and decent casing. Most of them will also step down from 24V used in commercial trucks.

You should take care the Universal power supply you buy is rated for the same, or more, power (Watt) than the original power brick you got your Magsafe connector from. Also the Voltage should be around 18V (15V to 20V) is fine.

But beware there is one cavecat. Most likely your new newly bougt Universal PSU will supply power to your magsafe connector as soon as you plug it in the cigarette lighter. So take care that noting is shorting the pins in the Magsafe connector before plugging it in.

I used a generic chinese "600W DC-DC boost step up converter module 100W", I found on eBay. It works really great while being 95% efficient.
There are plenty of them. Lookout for ones with a heat sink that isn't too small.
Don't believe everything advertised! That 600W in no typo, but probably absolute maximum rating of the uses circuit (in: 60V * 10A = 600W), but we are fare away from that.
The only case I found my Macbook drawing the full power is while charging a empty battery, running all cores full load, maximum brightness and supplying something with 20W on each (2) USB port.
All other times it ranges around 15-40W of original 85W.
My chinese converter barely got warm at any given time.

Picture of my solution

Alternatives

You can also use any power source that provides 2-4A @ 14-20V like:

  • 18V cordless tool batteries, works great (Makita, Ryobi,...)
  • Solar panels (though I wouldn't recommend connecting them directly, but it works)
  • make your own emergency powerpack out of alkaline cells
  • ...

Further reading: