Even when a phone is completely powered down, its battery continues to lose charge. The clock‑and‑wake circuit draws a tiny amount of current so the power button remains responsive and the device can sync time immediately after booting. The biggest drain is a chemical self‑discharge; lithium‑ion cells lose about 1‑2 percent of their power each month when turned off.

If stored above room temperature, the loss accelerates. With the phone left on, the screen, radio and background apps draw far more power. I’ve seen Android phones fall from half charge to dead in a week when simply left idle.

Apple recommends keeping a stored phone at about 50 percent and recharging it every six months to avoid a deep‑discharge state that can corrode the battery’s copper current collector. Store it in a cool, dry place below 90°F.

So, powering down does slow drain, but the battery will still be lower after weeks or months. A brief outage, like a 12‑hour flight, may not noticeably affect the meter, whereas months of dormancy will reduce capacity. Keep the phone partially charged and stored properly to preserve life.