If you are concerned that your portable Mac battery is providing less run-time than it did when you initially purchased the system (or the battery), you might want to check mAh reading.
mAh stands for milliamp-hours - essentially this is a measurement of how much energy is stored in the battery. The lower this number, the less energy will be available for your notebook while disconnected from external power.
You can check the mAh reading for your battery in a number of ways. The reading can be found in the Apple System Profiler (located in Applications/Utilities) under the "Power" sub-section. Look for the line that reads "Full Charge Capacity (mAh):"
Alternatively, you can get more detailed information using a utility like coconutBattery.
If your mAh reading is above 5000, your battery is in relatively good shape. Most units ship from the factory with mAh readings between 5500 and 6000.
If your mAh reading is below 5000, you may want to attempt a battery calibration.
Full instructions for battery calibration are contained in Knowledge Base document #86284, but the basic steps are as follows:
- Fully charge your unit's battery (the light on the power adapter will be green)
- After fully charging, keep power connected for a few hours.
- Disconnect the power adapter and allow your system to run from the battery.
- Keep the system on and awake until it goes to sleep automatically because the battery is running low.
- Turn the system off, or let it sleep for 5 hours.
- Re-connect the power adapter and allow the unit to fully charge.
If doing this fails to restore your mAh readings and your actual battery life, your battery may be losing charge due to natural aging. You may want to consider a battery replacement.