Macbook shuts down unexpectedly when battery reaches 20% and unplugged

Go to Applications/ Utilities/ System Profiler. Select "Power" from the menu on the left, and copy-paste the information about your battery here. That'll give us some idea.

Also, try resetting your SMC:
http://support.apple.com/kb/HT3964

Hi incomethax,

What does your battery information look like on the computer? It could just be coincidence.

About this Mac > More Info > Power >

Charge Information:
Charge remaining (mAh): 5487
Fully charged: Yes
Charging: No
Full charge capacity (mAh): 5540
Health Information:
Cycle count: 7
Condition: Normal
Battery Installed: Yes
Amperage (mA): -8
Voltage (mV): 12537

It'll look something like that.

Go to Apple Service Provider, asking technician to do a battery diagnostics test. I had a same issue with my MacBook Aluminum. Your battery need replacement. I hope you still have warranty or Apple Care.

I'm not so convinced your battery needs replacing yet. Please post battery info, so we can get a good look.

Doc

I am having the same problem, only mine shuts down at 80-90% ??

Here is my battery info...

Charge Information:
Charge remaining (mAh): 3330
Fully charged: Yes
Charging: No
Full charge capacity (mAh): 3465
Health Information:
Cycle count: 163
Condition: Good
Battery Installed: Yes
Amperage (mA): -1432
Voltage (mV): 11824

is it time for a new battery? Any help is much appreciated....THANK YOU!!!

First try resetting your SMC:
http://support.apple.com/kb/HT3964

If that doesn't work, and you're still under warranty, contact AppleCare, as your battery seems defective.

Hi Jimbob's Mom,

That Full charge capacity looks pretty low for a battery that's only got 163 and is about a year old (if that). If the SMC reset and a battery calibration doesn't resolve the issue, I'd agree with the above poster and contact AppleCare for a potential battery replacement.

"Check Battery" next to Condition is your sign--the battery is at the end of its useful life.

However, since you do not have all that many cycles (Apple expects you to get about 300), if you're still under warranty you can call AppleCare and you may be able to talk them into replacing your battery for free. Explain that this seems like an early catastrophic failure and that it should be covered. Particularly if your battery is less than a year old, you might have success with that.

Good luck.

Wow! I recently had this same EXACT problem and called Apple Care about it last week and said that they cannot do anything about it since my battery cycle is too high ( only 315) ! MacBook is the same model as yours. I finally brought mine in today at a local genius bar (Otay Ranch) and they replaced the battery even though My Mac's just a little over a month its 1 year warranty. I will now calibrate this battery and see if the same problem persists. When I called about this problem, the Apple Care rep says that he has never seen this problem before but now I just saw this and glad that I wasn't the only one, maybe its a faulty design on the first generation Unibody with removable batteries (Late 2008)

Same problem here, mine shuts down unexpectedly at about 35%. I'm at 270 cycles and surprised that I cannot expect more from the battery. 14 months without extended warranty - looks like i need to buy a new battery for myself.

Any pointers on how to keep the cycle count down with the next one? I didn't think it made any substantial difference to empty the battery between plugging it in but i guess that was a faulty assumption?

Calibrate regularly and use plugged in when possible.

Some reading for you:
http://www.apple.com/batteries/
http://www.apple.com/batteries/notebooks.html
http://support.apple.com/kb/HT1490

Thanks for the tips.

After four more cycles (274) battery status is now "Service Battery" - four cycles ago it was "Normal" but with problems as described above. Looks like something else than just the ordinary wear.

We're having the same trouble with my daughter's MacBook. Her cycle count is 355 and battery condition is GOOD but if it is not charging it will shut down with the battery charged as high as 68%. I'm baffled. The battery is 16 months old.

Did you try the SMC reset?

355 cycles is a lot; even if it says "Good," it may be time for a new battery.