Why Old Laptop Battery Discharge Fast from Full to Half

The rapid initial battery drain followed by a slower decrease in later percentages on a MacBook is usually due to a combination of lithium-ion battery chemistry, ageing, and how the software estimates charge remaining.

Why This Rate Change Occurs

• Battery Calibration and Age: Over time, the battery’s capacity diminishes and its ability to hold charge becomes uneven. The macOS battery percentage is calculated based on voltage levels, internal resistance, and charge cycles, not just a direct measure of electrons.
• Voltage vs. Capacity Curve: Lithium-ion batteries do not discharge linearly. The voltage drops quickly at higher charge levels (the “top” of the battery), then stabilizes, and finally drops again near zero. The operating system tries to map this nonlinear behavior into a percentage, but inaccuracies are especially noticeable for older batteries.
• Battery Wear & Capacity Loss: In older MacBooks, the cells become less efficient and their chemical reactions can lead to inaccurate readings. Rapid decrease at high percentages indicates degraded capacity, and slow decline near zero is a sign the system is trying to protect the battery from deep discharge.
• macOS Predictions: The software estimates, sometimes optimistically, how much energy is left based on prior usage patterns — which can make the top percentages “fall” faster, while lower percentages hang on longer as background processes slow down.