Why does my MagSafe power connector only work one way round?

Here's my theory. The magsafe connector, head-on, looks like this: [**.**] What I write as "*" looks like an electrical contact. It really only needs to connect two things (the "+" and "-", possibly ground too). To allow you to plug it in either way, it must be designed symmetrically. So for example it could be wired so that the two external contacts are connected to "-", and the two internal ones to "+".

Now, contacts can get damaged over time. What might have happened is that, say, the magsafe's rightmost contact does not work anymore. So you have something like [**.*.]. By sheer bad luck, it might be that the corresponding contact on the laptop side also does not work anymore.

So when you plug it one way, the leftmost outer contact works and does the job. When you plug it the other way, each side has a broken contact and so the circuit cannot complete.

Here's a pictorial version:

           orientation 1      orientation 2
magsafe    [**.*.]            [.*.**]
computer   [**.*.]            [**.*.]
(result)   works!             does not work

edit: checked the actual wiring, and it is indeed symmetrical as I describe.

MagSafe Connector Pinout

The MagSafe connector is symmetrical. There are two ground pins, and two 16.5V.

Image Source : wikipedia

Only the center pin and one of each other type of pin must be connected for the laptop to charge.

To prove this, I put non conductive tape over different combinations of positive and negative pins. As long as the center pin, and one ground and one positive pin made contact, my laptop successfully charged.

Connectivity Test

Still worked in both orientations without these pins

Still worked in both orientations without right most pins

As soon as I covered two matching pins (i.e... 1 and 5 or 2 and 4) the laptop failed to charge.

The Pins

Effectively the pins are mechanically identical to electronic pogo pins. They typically consist of gold plated, hollow brass tube with springs inside.

Image Credit: wikipedia

Over time those springs can wear out. It is common for pogo pins to be rated for 10,000 cycles.

High end pogo pins are gold plated on all surfaces inside and out Cheap / knockoff pogo pins often are only gold plated on the outside, allowing for corrosion to enter the internal components.

If you read reviews of unofficial Apple Magsafe chargers sold on Amazon, you will find a surprising percentage of reviews mention that the pins get stuck.

Solution

The cause of the MagSafe charger to only work when in one orientation, is that 3 of the essential pins are not touching the laptop.

There are two reasons the pins do not make contact.

1. Corrosion

2. Insufficient pressure from spring.

I have found that when my MacBook doesn't charge, gently running a clean pencil eraser over the pins and the laptop connector will usually solve the problem for a few weeks/months. This addresses both corrosion and exercises the pins.

I find that new chargers from Apple usually last 2 years before they start exhibiting this behavior.

Additional Information

EEVBlog explanation of pogo pins
Pogo pin durability
Pogo pin corrosion

Especially if you are experiencing other issues, you should consider resetting the System Management Controller. On my Macbook Pro, I had issues with the MagSafe adapter suddenly working in only one direction, in addition to my home WiFi password being forgotten and sporadic shutdowns. Following the steps in that link has so far fixed the MagSafe issue (and hopefully the others...).