Why Xiaomi Redmi 9 doesn’t charge to 100% and how to fix it

If your Xiaomi Redmi 9 suddenly stopped charging to 100%, it does not always mean that the battery breaks. 60% of the time, the problem is solved by software methods, from resetting the charging settings to updating the firmware. But you can not ignore the symptom: prolonged work with “undercharge” reduces the battery life by 20-30% due to improper balancing of the cells.

In this article, we’ll look at all the possible causes, from the banal wear of the cable to the hidden MIUI functions that limit the charge. You’ll learn how to distinguish hardware from software when you carry the phone to the service, and when you can do with home methods.

⚠️ Note: If the phone is only charged before 80-90% And it turns off when you plug in to charge, which is a sign of critical battery wear, and if you use it, you can blow up the battery!

1. Problems with the cable or power adapter

Start with the simplest: check the charger. Redmi 9 is sensitive to cable quality — cheap uncertified wires often don’t provide stable current. This is especially true for models that support fast charging 18W, where unstable voltage leads to artificially limited charge.

How to check:

• 🔌 Connect the original cable and power supply from Xiaomi (if any). Charge to 100% must be restored during the 1-2 cycle.
• 📱 Try charging your phone from another device (laptop, power bank) and if the percentage goes higher, the adapter is a problem.
• 🔍 Check the connector. USB-C on the phone: contamination or strain of contacts can interrupt charging on the phone 80-95%.

⚠️ Note: Use of adapters with output current more 2.4A (For example, from tablets, can cause overheating of the power controller Redmi 9. Maximum allowable value for this model — 5V/2A or 9V/2A (fast-charging).

Battery calibration: why the phone "forgets" the real charge

The Redmi 9 battery controller sometimes gets knocked down and misrepresents the charge level. For example, the phone shows 100%, but 5 minutes after disconnecting from the network, the percentage drops sharply to 80%. Or vice versa, the charge gets stuck by 95%. This is treated by calibration.

Calibration instructions:

1. Discharge the phone before automatically switching off (0%).
2. Connect to the original charger and do not turn on for 30 minutes.
3. Turn on the phone, wait for 100% charge, then turn off and reconnect the charge for 1 hour.
4. Repeat the discharge / charge cycle 2-3 times.

If calibration doesn’t help, try resetting the battery settings via the engineering menu:

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In the Battery Information menu, check the Charge Level setting. If it differs from the one displayed on the screen by more than 10%, a deep calibration is required (see spoiler below).

Turn off battery optimization for all applications |Use only the original cable |Use only the original cable |Use at room temperature (18-25°C)|Do not interrupt the charging/discharge cycle-->

3. Hidden MIUI functions that limit charge

MIUI has built-in battery protection mechanisms that can block 90-95% of the charge, for example, the function of "Adaptive Charging" (in some firmware called Battery Saver) artificially limits the maximum charge level if the phone is connected to the network for a long time.

How to disable restrictions:

• 🔋 Go to Settings. → Battery → Battery settings.
• 🔍 Find Optimized Charging or Adaptive Charging and turn them off.
• 📊 In some versions MIUI Restriction hidden in the Settings menu → Special facilities → Additionally. → Battery notifications.

If there are no such items, check the developer mode:

Settings → About the phone → MIUI version (shap 7 times)

Then go to Settings → Additional → For developers and find the Battery Limit option.

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After the adaptive charging is turned off, complete the full discharge/charging cycle so that the battery controller recalculates the capacity.

Battery wear: how to check and what to do

The average life of the Redmi 9 battery is 300-500 full charge cycles, and then the capacity drops by 20-30%, and the phone may not charge up to 100% due to physical wear and tear.

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On the menu. MB_test Select Battery and see the settings:

• Battery Health – Must be Good (if Bad or Replace – Battery to be replaced)
• Battery capacity — compare to passport capacity (5020 mAh for Redmi 9). If the difference is more than 20%, the battery is worn out.

📊 Table: Battery status by capacity

⚠️ Warning: If the battery is swollen (seen by the raised screen or the warping of the case), you can not continue using! disconnect the phone and contact the service. Redmi 9 has protection against overheating, but the risk of short circuit remains.

5. Software failures: resetting settings and flashing

If the hardware checks fail, the problem may lie in the firmware. MIUI sometimes knocks down the power controller settings after the updates.

Method 1: Resetting battery settings

• 🔄 Go to Settings. → System system → Resetting settings.
• 🔋 Choose Reset Battery and Performance Settings.
• 📱 After reset, perform calibration (see Section 2).

Method 2: Reinstallation of firmware

If the reset doesn’t help, search the phone via Fastboot:

1. Download the official firmware for Redmi 9 (codenamed Lancelot) from the Xiaomi Firmware website.


2. Connect your phone to your PC in Fastboot mode (clip Power + Volume down).




3 Run the script flash_all.bat from the firmware archive.

⚠️ Attention: Fastboot Firmware erases all data! pre-reverse through Settings → Additionally. → Backup and reset.

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If after flashing the problem remains – the charging controller is defective (diagnosis is required in the service).

6. Problems with the power controller or connector

If all the previous methods didn't work, it's the hardware part. Redmi 9 most often fails:

• 🔌 Charging controller (microchip) SY6974 Or similar, it's responsible for the distribution of current. If it breaks, the phone can only charge until it breaks. 80-90% or not to respond to the cable connection at all.
• 🔋 Connector. USB-C — oxidation or mechanical damage to the contacts leads to unstable charging.
• 🔄 Battery plume is rare, but there is a break in the chain between the battery and the motherboard.

🔧 Home-based diagnostics:

1. Connect your phone to your PC and check if it is defined as a shortened device (in Windows Device Manager).
2. Try charging your phone when it's off, and if it's going up, it's a problem with the firmware or the apps.
3. Light a flashlight into the USB-C connector: if you see blackening or green plaque, it is oxidation.

💰 Cost of repair (average prices in 2026):

• Replacement of the connector USB-C: 800-1500 ₽
• Repair/replacement of the power controller: 1500-3000 ₽
• Replacement of battery plume: 1000-2000 ₽

External factors: temperature and humidity

The Redmi 9 has overheating and hypothermia protection that can block charging up to 100%.The optimal charging temperature is 10-35°C. When going beyond these limits, the controller limits current.

How to check the effect of temperature:

• 🌡️ Install the app AIDA64 Check the battery temperature in the Sensors section.
• ☀️ If the phone is warmed above 40°C – Remove the case and let it cool before charging.
• ❄️ If the temperature is below 5°C, charge your phone in a warm room (but not near heaters!).

💧 Humidity also affects charging, and if you've been in a high humidity environment, like a bathroom, you can connect it. USB-C It could have oxidized:

1. Take the phone off the grid.
2. Place it in a silica gel bag for 12-24 hours.
3. Clean the connector with alcohol (at least 90%) with a cotton swab.

FAQ: Frequent questions about charging Redmi 9