Xiaomi smartphone owners often face the need to replace a battery, especially when the device stops holding a full day.One of the most common and popular batteries in the ecosystem of the Chinese brand is the model labeled BN31. This component was installed in many mid-range gadgets released between 2017 and 2019, and is still actively bought by users to restore the health of the equipment.
But there is a lot of confusion about the battery market: sellers can offer BM37 or BM39 analogs, claiming they are interchangeable. It's important to understand that while these batteries may have similar dimensions, their electrical characteristics and plume connectors may differ. Not only will the wrong part fail to work, but it can also damage the smartphone's power controller.
In this article, we will discuss in detail what models the original BN31 battery is designed for, what its key differences are from other popular versions and how to determine whether it is suitable for your device. We will also touch on the topic of safety and quality control of purchased components, as the market is saturated with low-quality copies.
Specifications and features of the BN31
The BN31 battery is a lithium polymer battery developed specifically for the Xiaomi and Redmi line of smartphones, with a capacity of 4,000 mAh (typical) or 3,900 mAh (minimum), which was considered a very high number for mid-range devices at the time of release, with a nominal voltage of 3.85 V and a charge voltage of 4.4 V, which is the standard for modern Li-Pol batteries.
The physical dimensions of the battery are optimized for cases with a non-removable rear cover. An important feature is the location of the contact plume and its width. Unlike older models, which used wide plumes, the BN31 uses a compact connector that requires precise positioning during installation. The dimensions of the battery housing are approximately 73.5 x 69.5 x 4.5 mm, which allows you to use the interior space of the smartphone efficiently.
A key design element is the built-in charge and discharge board (BMS), which prevents overcharging, deep discharge and short circuiting, and the quality of the board is often affected by fake versions, so you should pay attention to not only the capacity, but also the quality of the soldering and the presence of holographic stickers on the battery itself.
- 🔋 Type of chemistry: Lithium-polymer with high energy density.
- ⚡ Nominal capacity: 3900 mAh (min), 4000 mAh (type).
- 🛡️ Protection: Built-in controller against overheating and voltage surges.
It is worth noting that the manufacturer’s stated capacity of 4000 mAh is achieved only with the use of original components. Copies often have real capacity 20-30% lower, which leads to rapid discharge and unstable operation of the phone under load.
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When buying, pay attention to QR-Battery code: scanning it through the Mi Verify app (if available) or simply checking for a hologram can partially verify the authenticity of the hologram.
Compatibility: What Xiaomi models are suitable for BN31
The most important question that users are concerned about is which phones to put the battery into, the BN31 model was created primarily for the Redmi Note series, which was extremely popular due to the ratio of price to quality, the main carrier of this battery was the legendary Redmi Note 4X, which sold millions of copies around the world.
But Xiaomi engineers often use unified solutions for different devices, so the BN31 is also compatible with other models that have similar internal components. It's important to understand that compatibility is determined not only by the physical size, but also by the location of the plug to the motherboard. If the plume is shorter or longer, or has a different configuration of contacts, the phone will either not turn on or show a battery error.
Below is a table of the main models that this battery is designed for. If your device is not on the list, this does not always mean 100% incompatibility, but requires additional verification by the internal code of the old battery.
| Smartphone model | Year of release | Original AKB code | Note |
|---|---|---|---|
| Xiaomi Redmi Note 4X | 2017 | BN31 | Main carrier model |
| Xiaomi Redmi Note 4 (Global) | 2017 | BN31 | Global version (Snapdragon) |
| Xiaomi Mi Max 2 | 2017 | BN31 | Requires a size check. |
| Xiaomi Redmi 4X | 2017 | BN31 | In some audits |
Particular attention should be paid to the difference between the global and Chinese versions of the Redmi Note 4. The Chinese version (on the MediaTek Helio X20 processor) often featured a BN32 or BM46 battery, which are of excellent shape. Installing a BN31 in a device designed for another model can lead to the fact that the battery simply does not stand in the reserved bed or its plume will block other components.
Comparison of BN31 with BM37 and BM39
There is often confusion in the parts market between the BN31, BM37 and BM39 models. Many vendors claim that these batteries are interchangeable, and to some extent they are right, but with significant caveats. All three models have a capacity of about 4,000 mAh and similar dimensions, but their design design plumes differ.
The BM37 was often used in the Redmi Note 4 (the Chinese version on MediaTek) and some Redmi Note 4X modifications. The main difference lies in the length and shape of the plume tongue. The BN31 plume has a certain length, which is ideal for a particular layout of the board. If you try to install the BM37 instead of the BN31, it may turn out that the plume is too short and the connector does not reach the return part on the board, or vice versa, it will break, which is fraught with a breakage of contacts.
The BM39 is also a popular analogue, often found in the Redmi Note 5 and Redmi 5 Plus. It has similar capacity, but differs in the location of contacts and the shape of the protection board. Using a non-original model can lead to incorrect display of percentage of charge or failure of fast charging Quick Charge.
⚠️ Warning: Never try to forcibly plug a plume unless it comes in easily, which can cause a connector on the motherboard to break down, costing more than a new battery to repair.
When choosing a replacement, always compare the visual appearance of your old battery with the new one. If the shapes of the plumes and the location of the screw holes (if they are on the board) do not match, it is better to search for an exact copy by code.
Can I re-sweeten the plume?
Signs of wear and the need for replacement
Lithium polymer batteries have an average lifespan of 2-3 years, or about 500-800 full charge-discharge cycles. After that, the capacity begins to decline irreversibly. You can understand that your BN31 or its analogue has run out of life, based on a number of characteristics that are not recommended to ignore.
The first and most obvious symptom is a quick discharge, and if the phone that used to hold a charge for a day and a half, it now runs out of charge. 3-4 When used in moderation, it's a signal of degradation of chemistry inside the element, and you should also pay attention to the behavior of the charge indicator: if the percentage jumps off the cell. 20% straight away 5% or the phone is turned off when 15-20%, This indicates a memory effect or a controller malfunction.
Visual inspection can also tell you a lot, because if you notice that the back of your smartphone is starting to swell or move away from the body, it's a critical sign that chemical reactions release gases inside the battery, which causes it to swell, and it's strictly forbidden to operate such a device, because a damaged battery can ignite or even explode when mechanically exposed.
- 📉 The phone turns off when the charge is higher 10-15%.
- 🔥 The device heats up strongly in the camera or screen area during charging.
- 🔋 Charging occurs unevenly (quickly gaining 80%, then it costs an hour at 81%).
Another sign could be the system's message of battery temperature, even if the phone is lying at rest: The power controller picks up abnormal internal resistance and blocks work for safety.
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Battery bloating is not just a defect, it's a direct safety hazard. Replace the bloated battery immediately, without waiting for the device to fail completely.
Instructions for self-replacement of the battery
Replacing the battery in Xiaomi smartphones with a fixed lid requires accuracy and a minimum set of tools. You will need: a suction cup, a thin plastic spatula (mediator), a hair dryer (preferably a construction or ordinary household glue for heating) and a cross screwdriver. Before starting work, be sure to turn off your smartphone.
The process begins by heating the perimeter of the back cover. Don't heat the phone too much, 60-70 degrees is enough to make the glue elastic. Then, with a suction cup, lift the edge of the lid and gently walk the shoulder blade around the perimeter, peeling it off. Be careful with the fingerprint scanner plume, if it's on the lid, don't hurt it.
Once you remove the lid, you'll see a metal plate covering the battery and the bottom of the board. Unscrew all the screws holding the plate. Then you need to disconnect the battery plume from the motherboard. Do this with a plastic blade or nail, avoiding using metal tools to keep the contacts from closing.
☑️ Replacement tools
The old BN31 battery is usually glued to double-sided tape. Carefully shovel it with a spatula or use a special hood. Don't puncture or bend the battery. Clean the bed of glue residues, install a new battery, plug the plume before the click and assemble the device in reverse. After assembly, it is recommended to calibrate: discharge the phone to 0% and charge to 100% when turned off.
⚠️ Warning: When working with lithium batteries, avoid damaging their body with sharp objects.Puncture of the battery will cause instant ignition of the electrolyte.
Where to buy and how not to run into a fake
The Xiaomi parts market is crowded with copies that may be indistinguishable from the original, but have cheap, low-quality cells inside. The original BN31 battery has a clear label applied with a laser, rather than printed with paint that can be erased. Also, the original batteries often have a holographic sticker with the logo of Xiaomi or the cell manufacturer (for example, ATL or Desay).
When buying, pay attention to the packaging. Original batteries often (but not always) come in branded boxes with protective films and plugs on the contacts. If the battery came in a simple bag "by weight", the probability that it is a restored or defective specimen is very high. Recovered batteries are old cells that have soldered a new board or replaced the case, but the chemistry inside remains old.
You can check the battery using special applications such as AccuBattery or built-in engineering menu ##6485## dialer. In the menu that opens, find the parameters. MB_06 (the state of battery health; and MF_05 (If the cycles are already several hundred and the seller claims that the battery is new, it is a clear fake or used.
- 🏷️ Availability of a holographic sticker with a logo.
- ✍️ Clear, indelible laser engraving of text.
- 📦 Availability of a protective plug on contacts when buying.
It is best to purchase components from trusted suppliers or authorized service centers, even if the price there will be higher. 200-300 The ruble may require a second replacement in six months.