Converting Lenovo 20v 90w power supply to replace Apple 20v 85w MagSafe 2 power supply

Recently, the 85w MagSafe 2 PSU for my Macbook Pro 15″ died in a horrible child/pet/natural disaster, needless to say I wanted to replace it. And seeing as how I am a cheap bastard, and an official replacement runs ~$80, I decided to look for an alternative online. But to my horror, every site I went to looking for a reputable replacement warned that almost all replacements sold outside of an official Apple channel, including units sold as OEM, are likely dangerous knockoffs.

Pulling out my magnifying glass, yes I’m cheap _and_ old, I was pleasantly surprised to see that the Apple PSU provided a single 20v rail at a maximum of 85w. Great I just needed to find a clean stable 20v PSU and convert it. But where was I going to find a PSU that wasn’t just a cheap knock off of a different manufacturers design? Looking around I found that the Lenovo T4## series of laptops use 20v PSUs and that many OEM units can be had for as little as US$10.

Let us take a moment to bow our head to remember, and thank, all of the defunct startups driven by VC input to use thinkpads, inevitably flooding the market with good quality used equipment when they closed up shop.

But I was in luck, I had a Lenovo PSU in my parts bin. A buddy of mine gave me a 20v 90w PSU that he didn’t need after his Lenovo committed suicide by jumping off of the roof of a moving car. I didn’t ask questions, as I thought it would be insensitive to press him during his time of loss, so I tossed it with the rest of my misc. power bricks.

For those that haven’t used Thinkpads, they have a reputation as well built and reliable business machines. Not the fastest, thinnest, or flashiest systems. But they are workhorses and the PSUs that come with them are no exception. The PSUs are not only really well engineered electronically, but they can physically take a beating thanks to their tough shells. All in all they are actually a little smaller than a comparable Apple PSU by volume and, at least in my experience, don’t seem to get quite as warm while in use.

Size1 1/8″ x 3 1/8″ x 3 1/8″1 1/8″ x 1 7/8″ x 4 7/8″
Volume~11 cubic inches~10.3 cubic inches
Max Power85w90w
NotesGets quite warm under full loadGets moderately warm under full load

Sourcing the replacement MagSafe 2 cable end was much cheaper than I expected. About US$8 on Amazon and it was, thank you Prime, in my mailbox two days later.

Now I am sure I don’t have to tell you this but I will. Don’t try this at home unless you know what you are doing. Do not try disassembling a PSU while it is plugged in. Do not touch the PCB with your bare hands, there are caps that can and will shock you. Do not solder/desolder without proper ventilation. Do not eat raw chick or electronics.

First things first, I had to disassemble the brick. Since I didn’t care about the Lenovo laptop connector it was pretty easy to do.

I started by stretching the rubber cable protector away from the body of the PSU using a pair of needle nosed pliers. I just grabbed the protector about an eighth of an inch from the shell and rolled/twisted the rubber away from the body. This loosened up the rubber just enough for the next step.

Next I worked a slotted screw driver in between the rubber and the shell, with a little twist of the screwdriver the shell started to split at the seam. There was some damage to the seam, but not enough to prevent the shell from aligning properly when put back together.

Here is where I went ahead and just clipped the cable to get it out of my way. Keep working your way around the shell by twisting the screw driver and then sliding down the seam until you can’t slide any more, then twist again.

The last side is always the hardest. But just kind of play with it and find the best angle to get some leverage.

The guts of the unit aren’t attached to the shell in any way, unlike the white glue/paste inside the apple PSUs, and should come out of the split shell with minimal effort.

I have seen two types of PSU designs from this series. This one is my favorite because the positive and negative leads are easily accessible without removing any of the shielding or insulating tape. The second version requires removing some shielding. Not hard but you might have to desolder one of the tabs that hold the shielding to the back of the PCB.

There is a lot of solder on these connections, but it is pretty low temp stuff and easy to remove.

Here is the cable I found on Amazon, comparing it to the cable from the Apple PSU it is obvious that the shielding on the ground wire is a little thinner. Probably not a huge deal, but I am sure you can find a replacement cable to the original spec if you hunt through different options online. I have been using my replacement PSU for about a week now and haven’t noticed any problems with the cable heating up and, even with frequent movement, no shorts or visible cable fatigue.

Next I soldered the new cable to the appropriate pads on the PSU. This one has ground on the outer corner and the other pad is positive.

I snapped the shell back around the unit and sealed it up with some kapton tape. If I don’t have any problems after a couple of weeks I will probably button it up with some epoxy.

Here is the new PSU plugged in and powering my laptop. I let it sit and charge my laptop from 22% to full, took a little more than an hour and a half, and then I took this picture with the pleasant green light.

And here you can see that the OS really does recognize and accept the PSU as its own.

In the end, if you have access to a recent Lenovo 20v PSU and need a solid, reasonably priced replacement for your original Apple PSU the conversion process is fairly simple and the results are amazingly stable.