"Shallow charge and discharge" is the golden rule for lithium-ion battery maintenance, based on the electrochemical characteristics of lithium-ion batteries. Targeting the aging status of used iPhone batteries, formulating precise "shallow charge and discharge" practical standards, and correcting common maintenance myths can effectively slow down cell degradation and extend the service life of used batteries by 1-2 times.
Electrochemical Principles of "Shallow Charge and Discharge"
(Why It Extends Life)
Lithium-ion batteries used in iPhones operate on the core principle of lithium ions intercalating and deintercalating between the positive and negative electrodes. Battery aging is essentially the reduction of lithium ion activity and damage to the electrode material structure, and the depth of charge and discharge directly affects the intensity of these two processes.
1.Damage Mechanism of Deep Discharge to Battery Cells
When the battery power drops below 10% (deep discharge), the lithium ion concentration inside the cell decreases sharply, leading to "lithium plating" on the negative electrode material (graphite)—lithium ions cannot be normally intercalated into the graphite layers and instead form metallic lithium deposits on the electrode surface. These metallic lithium deposits can pierce the separator inside the battery, causing internal short circuits; at the same time, deep discharge causes irreversible damage to the crystal structure of the electrode material, reducing the amount of active material and leading to permanent loss of battery capacity.
Experimental data shows: Lithium-ion batteries that undergo long-term deep discharge (used until automatic shutdown at 0%) retain only about 65% of their capacity after 500 cycles; while lithium-ion batteries using shallow charge and discharge (not below 20% power) still maintain more than 85% of their capacity after 500 cycles, extending their life by approximately 30%.
2.Hazard Logic of Overcharging to Battery Cells
When the battery is fully charged to 100% and continues to charge (overcharging), the internal voltage of the cell remains at a high voltage of 4.2V. At this time, the electrolyte will decompose rapidly to produce gas, increasing the battery's internal resistance; at the same time, the positive electrode material (lithium cobalt oxide) will collapse structurally, releasing oxygen that reacts with the electrolyte to accelerate cell aging. Long-term overcharging can cause the battery health to drop by 15%-20% within a year, and may also trigger bulging risks.
3.Core Advantages of "Shallow Charge and Discharge"
"Shallow charge and discharge" (charging range 20%-90%) keeps lithium ions in an active and stable concentration range, avoiding extreme electrochemical pressure on the electrode materials: ① The intercalation and deintercalation of lithium ions are gentle, without sudden changes in concentration, reducing "lithium plating" and electrode structure damage; ② The battery voltage is always maintained in a safe range of 3.8V-4.1V, reducing the electrolyte decomposition rate by 80% and significantly slowing down cell aging; ③ Reduces heat generation inside the battery—high temperature is a catalyst for cell aging. The battery temperature during shallow charge and discharge is 5-8℃ lower than that during deep charge and discharge, further reducing the aging rate.
For used iPhone batteries (which already have a certain degree of cell aging), "shallow charge and discharge" can effectively prevent accelerated aging, equivalent to "reducing the load" on damaged cells and extending their effective service life.
Practical Standards for "Shallow Charge and Discharge" of Used iPhone Batteries
(Detailed to Specifics)
Combined with the aging characteristics of used batteries, formulate scenario-specific "shallow charge and discharge" practical standards covering core dimensions such as charging range, charging frequency, and charging method to ensure feasibility and ease of implementation.
1.Core Charging Range Control (Most Critical Indicator)
Optimal charging range: 20% (lower limit) - 90% (upper limit), which is the core standard of "shallow charge and discharge" and must be strictly followed.
Lower limit control: During daily use, charge the battery in a timely manner when the power drops to 20%, and never let it drop below 10%. If it accidentally drops below 10%, stop using it immediately, restart, and charge as soon as possible—do not continue to discharge until automatic shutdown. Allow a maximum of 1 deep discharge (to 5%-10%) per month, only for system calibration of battery capacity, and do not operate frequently.
Upper limit control: Manually disconnect the power when charging to 90% (you can enable "Battery Level Notification" in settings to pop up a reminder at 90%); if the "Optimized Battery Charging" function is enabled (iOS 14.2 and above), the system will automatically slow down charging after 80% and charge to 100% before the user uses it. This method also conforms to the "shallow charge and discharge" logic and can be used with confidence.
Special scenario adjustment: For scenarios requiring long battery life such as long-distance travel, you can temporarily charge to 100%, but reduce the power to below 90% as soon as you reach your destination. Do not exceed 3 such scenarios per month to avoid long-term full charge status.
2.Charging Frequency and Timing Selection (Avoid "Full Charge for Completeness")
Charging frequency: There is no need to wait for the battery to run out before charging—follow the principle of "charge as you use". For example, if the power drops to 60% after morning use, charge to 90%; if it drops to 50% after afternoon use, charge to 90% again. Multiple short-time charges will not damage the battery; on the contrary, they can maintain lithium ion activity.
Charging timing: Priority is given to charging when the battery temperature is close to room temperature (16℃-22℃), avoiding charging in high-temperature (above 35℃) or low-temperature (below 0℃) environments. Charging at high temperatures will accelerate cell aging; charging at low temperatures is inefficient and may cause "false power".
Avoid myths: Do not wait for the battery to fully charge to 100% for "completeness", nor wait for the power to be completely exhausted before charging—both behaviors will increase the battery load.
3.Charging Method and Accessory Selection (Reduce Heat Damage)
Charging power selection: Priority is given to slow charging (5W-10W), reducing the frequency of fast charging (above 20W). The advantage of fast charging is speed, but it generates more heat during charging (the battery temperature during fast charging is 3-5℃ higher than that during slow charging). Long-term use will accelerate cell aging. It is recommended to use a 5W original charger for daily charging and only use 20W fast charging in emergency situations.
Charging accessory requirements: Must use Apple original or MFi-certified chargers and data cables. Non-certified accessories have unstable output voltage and current, which will increase charging heat and even damage the battery management system, affecting the effect of "shallow charge and discharge".
Usage specifications during charging: Avoid running high-power applications (such as large games, video editing) during charging. At this time, the battery must both charge and supply power, leading to a sharp increase in heat; at the same time, do not wrap the phone in clothes or quilts—place it in a well-ventilated place to dissipate heat, keeping the battery temperature below 30℃ during charging.
4.Maintenance Differences for Batteries with Different Health Levels (Precise Adaptation)
For used iPhone batteries with different health levels, the implementation standards of "shallow charge and discharge" need to be appropriately adjusted for more targeted maintenance effects:
Health ≥85% (mild aging): Strictly implement the 20%-90% charging range, perform 1 deep discharge calibration per month, and use fast charging no more than 5 times per month. This can keep the health declining slowly (≤5% per year).
Health 70%-85% (moderate aging): Adjust the charging range to 30%-85% to avoid excessive cell pressure caused by low power; prohibit fast charging, only use slow charging; perform 2 deep discharge calibrations per month, and reduce the use of high-power scenarios to slow down the health decline rate (≤8% per year).
Health <70% (severe aging): Further narrow the charging range to 40%-80% to reduce charge and discharge depth and cell load; completely prohibit fast charging and deep discharge (only perform 1 time when the system prompts calibration); avoid using the phone while charging, and increase the daily charging frequency to 2-3 times to maintain stable power, extend usage time (avoid sudden shutdown due to excessive aging).
Correction of Common Maintenance Myths
(Pitfalls 90% of Users Fall Into)
Myth 1: "Newly replaced used batteries need deep discharge activation"
Wrong reason: This myth originates from the maintenance logic of old nickel-cadmium batteries, which have a "memory effect" and require deep discharge to activate capacity. However, lithium-ion batteries used in iPhones have no memory effect.
Correction: Used batteries (including newly replaced original disassembled batteries) do not need activation. Deep discharge will instead damage the cells and reduce health. After replacing the battery, simply perform 2-3 normal shallow charge and discharge cycles (20%-90%), and the system will automatically calibrate the capacity—no need for deliberate deep discharge.
Myth 2: "Must charge to 100% to be fully charged, otherwise the battery life is insufficient"
Wrong reason: Users mistakenly believe that 100% power is required to meet daily use. In fact, the process of charging the battery from 80% to 100% can only increase the light-use battery life by about 1-2 hours, but it requires enduring the damage of high-voltage charging.
Correction: 80%-90% power can already meet the daily use of most users (6-8 hours of light use). The remaining 10% power has limited improvement in battery life but will accelerate cell aging. Unless required by special scenarios, there is no need to charge to 100%.
Myth 3: "The more charging times, the faster the battery ages"
Wrong reason: Users confuse "number of charging times" with "cycle count", believing that frequent charging will increase the cycle count and accelerate aging.
Correction: Number of charging times ≠ cycle count (only 1 complete charge-discharge cycle counts as 1 cycle). Multiple short-time shallow charges (e.g., charging from 60% to 90%, then discharging to 60%) will not increase the cycle count; on the contrary, they can maintain lithium ion activity. For example, 3 shallow charges per day only accumulate 0.5 cycles per week, which has a negligible impact on battery aging.
Myth 4: "Disabling 'Optimized Battery Charging' makes the battery more durable"
Wrong reason: Users believe that "Optimized Battery Charging" will extend the charging time, causing the battery to be in a charging state for a long time and damaging the cells.
Correction: The core logic of "Optimized Battery Charging" is to avoid the battery being in a high-voltage 100% state for a long time. When enabled, the system will slow down charging after 80% and charge to 100% before the user uses it, reducing the high-voltage charging time and thus slowing down aging. For used batteries, it is recommended to keep this function enabled, especially for overnight charging scenarios.
Myth 5: "Charging to full before long-term storage is safer"
Wrong reason: Users worry that the battery will not wake up if it is undercharged, so they choose to store it fully charged, ignoring the high-voltage damage to the cells in the full charge state.
Correction: For long-term storage (more than 1 month), the optimal battery power is 40%-60%. Storing at full charge keeps the cells in a high-voltage state, accelerating electrolyte decomposition; storing at low power (below 20%) causes "over-discharge" of the cells, resulting in permanent capacity damage. Correct method: Charge the battery to 50% before storage, place it in a dry environment at 16℃-22℃, and supplement charging to 50% every 1-2 months to maintain cell activity.
Auxiliary Measures to Extend the Life of Used iPhone Batteries
(Used with "Shallow Charge and Discharge")
1.Temperature Control: Avoid Extreme Environments
High-temperature protection: Avoid using the phone in high-temperature environments such as direct sunlight and car exposure. When the temperature exceeds 35℃, turn off high-power functions such as 5G and large games to reduce battery heat; choose a well-ventilated place for charging, avoiding stacking the phone and charger.
Low-temperature protection: Keep the phone warm in a pocket when using it outdoors in winter, and avoid using it when the power is below 30% in low-temperature environments; do not charge in low temperatures—allow it to return to room temperature first.
2.Power Optimization: Reduce Battery Load
System setting optimization: Disable unnecessary background app refresh, location services, and push notifications to reduce invalid power consumption; enable "Low Power Mode" (when the power is below 50%) to reduce processor frequency and screen brightness, reducing battery load.
Application usage specifications: Reduce the usage time of high-power scenarios such as large games and 4K video shooting. Such scenarios cause the battery to run at high load continuously, increasing heat and accelerating aging; use Wi-Fi instead of 5G—5G network power consumption is 2-3 times that of Wi-Fi, which can effectively reduce power consumption speed.
3.Regular Calibration: Maintain Data Accuracy
Perform capacity calibration once a month (only for batteries with health ≥70%) as follows: ① Charge to 90% and use normally until 20% (not below 15%); ② Charge to 90% with a slow charger, using the phone normally during the process without shutting down; ③ Restart the phone, and the system will re-read the battery capacity data to ensure accurate health display. The purpose of calibration is to allow the system to accurately identify the battery status, avoiding abnormal charging strategies due to data deviations and indirectly protecting the battery.
4.Regular Detection: Detect Problems in Time
Test key battery indicators every 3 months: ① Health (Settings → Battery → Battery Health & Charging). If it drops by more than 5% within 3 months, check usage habits or potential hardware issues; ② Internal resistance (tested with professional tools). If the internal resistance exceeds 60mΩ, the cell is severely aged and needs to be replaced; ③ Charging efficiency (record the time from 20% to 90% charge). Normal slow charging should take 1.5-2 hours. If it exceeds 2.5 hours, the charging accessories may be aged or the battery is degraded—replace the accessories or battery in a timely manner.
