Probably. This is actually a pretty common failure mode for a battery. Computers and other devices that indicate a battery percentage calculate that percentage by comparing the battery's voltage to a table of standard voltage values correlating to different levels of charge for that particular battery. When your battery is showing 90 percent it's being charged because the computer is plugged in, and the computer is being powered by line current from the electrical outlet.

As soon as you pull the plug this puts the battery under load, and its voltage drops very quickly, most likely due to high internal resistance in the battery. Your computer was fooled into thinking that the battery was 90 percent charged because the voltage attained a value that in a good battery would indicate 90 percent charge, but in this battery it doesn't.

Notebook computer batteries can fail fairly quickly if they're left charging constantly, as many are. Newer computers may have better charging control circuits to somewhat lessen the effects of this, but a good battery-saving practice is still to unplug a notebook computer when you're not using it rather than leaving it plugged in 24/7. (This is because, even with charging control circuits, the computer will keep the battery topped back up to 100 percent every time it drops a percent or two, creating repeated charge/discharge cycles that shorten a battery's life.) For a rarely used device, recharge the battery often enough to prevent it from sitting in a deeply-discharged state. Leaving a battery sit "dead" a long time can also damage it.

If you had the battery replaced by Apple, I'd take it back to them and see what they say.

If it was done by a third party, I'd say you're probably stuck where you are and it may be time to buy a new PC. You've gotten 7 years of service from it and that's longer than average.

Have you checked your Energy Saver preferences? I also have a 2009 Macbook Pro, and the original battery is still working fine. My laptop is set to turn off (i.e., sleep) after 5 minutes of inactivity whether it's plugged in or not. Good luck.

Hi kenjwy,

"Notebook computer batteries can fail fairly quickly if they're left charging constantly, as many are. Newer computers may have better charging control circuits to somewhat lessen the effects of this,"

What years' models are categories as Newer Computers?

I leave my 2014 Mac Pro, and the new Wintel laptop purchased a few weeks ago, plugged in at the desk all the time unless I take them with me .....should I be concerned?

Before making the trek to the Genius Bar or buying another MacBook Pro, you might want to try resetting the System Management Controller (SMC). It is some of the circuitry that manages batteries and power usage. It won't hurt to try it and, if it doesn't work, at least you've ruled that out as the problem.

Resetting the SMC