Macbook air dies around 40-50% - Help!

Lets do a health check on your Battery

this wont hurt a bit

Open the Terminal app in your Utility folder

Copy the following and Paste it in to the Terminal window

ioreg -l -w 0 | grep Capacity 

the result will tell you (us) how your battery is doing.

Example of 1 year old battery looks like this

"MaxCapacity" = 6624

| | "CurrentCapacity" = 6444

| | "LegacyBatteryInfo" = {"Amperage"=574,"Flags"=7,"Capacity"=6624,"Current"=6444,"Voltage"=8558,"Cycle Count"=225}

| | "DesignCapacity" = 7150

OUCH

as you can see your Battery is clearly end of life.

Get a new one.

Also I forgot to say, your reported % is not accurate.

This comes from a chip inside your Mac that is responsible for management of the power in the Mac (including reporting the battery status to you).

That chip is called SMC (System Management Controller).

Some times it needs to be Reset to bring it back to normal.

Here is how you do that

1. Shut down the computer.
2. Plug in the MagSafe power adapter to a power source, connecting it to the Mac if its not already connected.
3. On the built-in keyboard, press the (left side) Shift-Control-Option keys and the power button at the same time.
4. Release all the keys and the power button at the same time.

While doing the crazy exercise watch the MagSafe light briefly flicker indicating you did it.

You % indicator should be accurate now.