Why smartphone batteries drain even when powered off
Experts explain the mechanics of self-discharge and clock-and-wake circuits, and outline best practices for long-term device storage to prevent permanent battery damage.
Contrary to popular belief, turning off a smartphone does not place its battery in a state of suspended animation. According to an explanation published by Engadget, lithium-ion cells continue to lose charge even when the device is completely powered down. This phenomenon is driven by two primary factors: a clock-and-wake circuit that maintains timekeeping and power-button responsiveness, and the natural chemical self-discharge inherent to the battery chemistry.
The clock-and-wake circuit draws a minimal amount of current to ensure the device can be turned on instantly and retains the correct date and time upon reboot. However, the more significant factor is the chemical self-discharge of the lithium-ion cells. This process typically results in a power loss of approximately one to two percent per month. The rate of depletion accelerates if the device is stored in environments exceeding room temperature.
While powering off a device is still the most effective way to conserve energy compared to leaving it on, the battery will not remain at its initial charge level indefinitely. When a phone is active, power is consumed by the screen, radio signals, and background applications. In contrast, a powered-off device avoids these drains, though the slow bleed from self-discharge and the clock circuit continues. For instance, leaving an Android phone unused for a week while powered on can drain it from half charge to zero, whereas a powered-off state preserves significantly more energy.
To prevent permanent damage during long-term storage, experts advise against storing devices in a fully charged or fully depleted state. Apple recommends charging an iPhone to approximately 50 percent before extended storage and recharging it every six months. This routine prevents a deep-discharge state, where a battery that has been dead for a prolonged period may suffer from the dissolution and redeposition of its copper current collector. This chemical change can lead to short circuits or a permanent reduction in battery capacity.
The ideal storage conditions for a smartphone are cool and dry, with temperatures kept below 90 degrees Fahrenheit. While anecdotal evidence suggests that devices left off for months or even years, such as a BlackBerry PlayBook unbooted since 2011, can often be revived, their battery life is typically diminished. Ultimately, while powering down a phone does not stop the battery from draining entirely, it remains a prudent measure for preserving charge during short periods of inactivity.
