Why does my iPhone battery drop fast below 20%?

When an iPhone seems to drop fast below 20%, the most common reason is that the battery is aging and can no longer deliver stable voltage under load as well as it used to.

Here’s what’s usually happening:

1. Low battery is less forgiving
   Near the bottom of the charge range, battery percentage can seem to fall faster because there’s less usable charge left and the phone has less buffer for sudden power demands.

2. Battery wear causes voltage sag
   As batteries age, their internal resistance rises. When the phone suddenly needs more power — for example, opening an app, using the camera, playing video, or getting weak cellular signal — the voltage can dip briefly. That can make the battery percentage drop quickly or, in some cases, cause an unexpected shutdown.

3. The percentage is an estimate, not a perfect meter
   iOS estimates remaining charge using battery-management data, so the number you see is not a simple linear gauge. If the battery is worn or the estimate has drifted, the last part of the charge can appear to fall faster than the rest.

What you can do:

• Check Battery Health: Go to Settings > Battery > Battery Health & Charging.
• If Maximum Capacity is low, the battery may be worn enough that replacement could help.
• Use Low Power Mode: This reduces background activity and may lessen sudden drops.
• Update iOS: If battery health looks fine but the drops are extreme, software or calibration issues may be involved.
• Watch for shutdowns under load: If the phone turns off while it still shows charge left, that’s a stronger sign the battery is degrading.