MacBook – My MacBook Pro 13” works only with 85w charger

Most switching power supplies are most efficient when operating at 50% capacity. And they're definitely pretty inefficient at less than 20% capacity.

Since the MacBook draws less power than a MacBook Pro, I would suspect that an 85w power supply would work just fine, but the larger power supply for the MBP probably would draw more power from the wall socket than is necessary when connected to a MB because it is operating in a suboptimal capacity range.

Here's an example: Take for example the 45w power supply for the MacBook Air. Sure you could hook up the 85w power supply, but since the MBA might only draw 20 watts, the power supply is operating at less than 25% capacity. At this capacity, the power supply may only be 60% efficient (an educated guess), that means the power supply has to draw 33 watts of AC power just to supply the laptop with 20 watts of DC power.

Long story, short: It's a power conversion efficiency thing. I'd say buy the one that is designed for your laptop so that you save energy.

In the absence of a repair shop to run diagnostics, you might try isolating things in the following order:

1. Disconnect all external devices
2. Different power adapter
3. Remove RAM
4. Disconnect / remove the battery and then reset the SMC
5. Disconnect hard drive and optical drive

As you remove items, you remove causes of failure. The mac should still POST and beep if everything listed is taken away from the mac. Often, it's an odd part that is causing a failure of the system to charge or drawing too much power when these isolate the issue. In reality, it's likely the DC in board or main logic board, which means it's good you have warranty to cover that eventuality. Do use prudence in doing the above steps, you don't want to physically damage things and run the risk of warranty service being denied due to external damage or someone not technically capable doing work on your mac.

Even if you don't do all of the steps, you can rule out a few more things than you listed initially in the question. Also, Apple service providers will possibly be able to revert your firmware or at least confirm you have a working firmware install as well as be faster to isolate the cause than most consumers doing the steps listed above as they are set up to do this day in and day out.