You unlock your iPhone and immediately notice something unusual — the battery percentage suddenly drops several percent at once. It may fall from 42% to 38% or lower within seconds, even though the phone was idle just before. Afterward, the battery level often stabilizes and drains normally again. The device usually isn’t hot, and nothing obvious appears to be running.
This behavior typically happens because iOS recalculates battery capacity the moment the screen wakes and background processes resume. The percentage shown while the phone is locked is only an estimate based on low activity. Once the system fully powers components like the display, radios, and processor, the software updates its reading to reflect real-time voltage and load.
Recognize the Symptom
The issue usually looks like this:
- Battery percentage drops immediately after unlocking.
- The drop happens once, not continuously.
- Battery usage afterward feels normal.
- No sudden shutdown occurs.
If these match your experience, the battery itself is often still functional, but calibration or software behavior may be slightly off.
Likely Cause
iPhones estimate battery percentage using voltage, temperature, and usage patterns. When the device is locked, power draw is minimal. Unlocking activates the display, Face ID, network checks, and background refresh tasks at the same time. If the battery’s calibration data isn’t perfectly aligned, iOS corrects the estimate instantly, which appears as a sudden drop.
This is more noticeable when:
- The battery level is below about 40%.
- The battery has aged or experienced many charge cycles.
- iOS was recently updated.
- The phone was charged in short sessions frequently.
Step-by-Step Fixes
Check Battery Health
Go to Settings → Battery → Battery Health & Charging. If Maximum Capacity is below 80%, sudden percentage adjustments become more common because voltage drops faster under load.
Restart the iPhone
A simple restart clears temporary power management glitches.
- Press and hold the side button and volume button.
- Slide to power off.
- Wait about 30 seconds, then turn it back on.
Recalibrate the Battery Indicator
This helps iOS relearn accurate battery levels.
- Use the phone normally until it reaches around 5%.
- Continue using until it shuts down by itself.
- Charge uninterrupted to 100%.
- Keep it plugged in for another 30–60 minutes after reaching full charge.
This process doesn’t repair the battery but often fixes sudden percentage jumps.
Check Background Activity
After unlocking, some apps may briefly spike power usage.
- Open Settings → Battery.
- Look for apps showing high background activity.
- Disable unnecessary Background App Refresh under Settings → General → Background App Refresh.
Update iOS
Battery reporting bugs occasionally appear after updates and are later corrected.
- Go to Settings → General → Software Update.
- Install any available update.
Optional Alternative Solution
If the battery percentage drops are large (more than 10% repeatedly) or the phone sometimes shuts down before reaching low percentages, the battery may be chemically worn even if the phone still works normally. In that case, replacing the battery is the most reliable long-term fix.
Once calibration is corrected or the battery is replaced, the percentage reading typically becomes stable again and no longer drops suddenly after unlocking.