Most listings give the BN31 battery model name for the Xiaomi Mi A1 and Redmi Note 5A, with a typical capacity of 3080 mAh. Several other older Xiaomi phones are also listed with it, including the Redmi Note 5A Prime, Redmi Y1, Redmi Y2, Redmi S2 and Mi 5X. BN31 is a Xiaomi part, a group covering the Mi and Redmi names.
These are older phones with a metal body, and the battery sits under the back plate, so the code is not visible from outside. Many people type BN31 after reading it on a loose pack, or after a repairer says the phone needs one. Treat the phone list below as a strong guide, then confirm it on a pack you can hold before any money changes hands.
Quick facts
Step by step
Write down the phone name and model number
Open Settings, tap About phone and write down the name and the model number. If you cannot find them, type model in the Settings search bar.
Separate Mi A1 from Mi A3 and Redmi A3
The name on the retail box or in About phone must say A1. The A3 phones are different models with different packs.
Have the old pack read after the back is off
The sticker should show
BN31beside capacity and voltage. Ask for a clear photo of it, because the pack is usually thrown away.Set the new pack against the old one
Put both on the table together. The code, the size, the connector and the weight should look the same as the old pack.
Check print quality before fitting
Look for sharp text, a batch mark and a capacity near 3000 to 3100 mAh. A bold claim of 4000 mAh or more on this size of pack is a red flag.
Phones that use the BN31 battery model name
BN names only the Xiaomi group. BN31's phone list comes from parts listings, which point to a family of older Xiaomi phones launched together. The Mi A1 and Redmi Note 5A are the pair listed most often. The rest are matched by most listings, not all.
| Mi or Redmi model | Status | Note |
|---|---|---|
| Mi A1 | Listed most often | 3080 mAh typical |
| Redmi Note 5A | Listed most often | 3080 mAh typical |
| Redmi Note 5A Prime | Most BN31 listings name it | Check the BN31 label |
| Redmi Y1 | Listed by most BN31 sellers | Check the BN31 label to be sure |
| Redmi Y2 and Redmi S2 | Matched by most listings | Same phone sold under two names in different markets |
| Mi 5X | Matched by most listings | Sold in China; closely related to the Mi A1 |
If your phone is missing from the table, do not force BN31 on it because the size looks close. Every code in the Xiaomi, Redmi and POCO family is gathered on the hub page for Redmi batteries.
Mi A1 is not Mi A3
Searches for "A1" and "A3" get mixed up all the time. The Mi A1 is the older phone that uses BN31. The Mi A3 and the Redmi A3 came much later and are listed with other packs. Do not fit a BN31 pack in either of them, and do not fit their packs in the Mi A1.
The page for Mi A3 and Redmi A3 explains those two phones separately. For a Mi A1 or Note 5A, the About phone model number and the box name decide the match.
A customer brought a dead Mi A1 and asked for "the A3 pack" because his cousin had one. The technician opened the back, read BN31 on the cell and explained that the A3 is a different phone. The Mi A1 got a BN31 pack, matched sticker to sticker.
Capacity of BN31: 3080 mAh typical and what it means
The figure of 3080 mAh is the typical capacity that spare part listings and adverts quote. A BN31 label usually prints a lower rated minimum as well; read yours instead of guessing. Both numbers describe the same cell.
Multiply BN31's 3080 mAh by its printed voltage, then divide by a thousand for watt hours. If your label shows 3.85 V, a 3080 mAh pack equals roughly 11.9 Wh. That is a modest store of energy by today's standards, where many phones carry 5000 mAh or more.
For a phone of this age, a day of use is realistic only when the battery is healthy and the screen is not set bright. A worn pack can drop well below its label value, and your phone's software and signal strength also change the result.
Look-alike codes and phones to keep apart
Many old Xiaomi packs have codes that differ by one digit, and pack sizes look similar on a counter. Each of these has its own phone list, so read it before assuming any share:
- BN36 and BN37 belong to other phones. Do not use either as a substitute for BN31.
- BN4A and BN8 have their own pages and phones.
- A pack for the Redmi 5A or the Redmi 4A may look close in size, but a close size is not a match. Read the code.
- On a worn BN31 label, the 3, 8 and 5 are easy to confuse. Angle the BN31 sticker under a bright lamp.
Capacity alone does not settle anything. Two packs of about 3000 mAh can have different connectors and different codes.
Signs that a BN31 pack is tired
| Mi A1 or Note 5A sign | Cause | BN31 action |
|---|---|---|
| Percentage falls quickly after unplugging | BN31 has lost much capacity | Test Mi A1 health, then book a BN31 |
| Note 5A cuts out at 30 percent or higher | Tired BN31 sags under sudden load | Have BN31 tested at a repair counter |
| Screen lifts or back plate gaps open | Swollen cell pushing outward | Unplug the Mi A1 and go the same day |
| Mi A1 warm while idle | Worn BN31 or an app working hard | Restart the Note 5A, remove its cover, test BN31 |
A Mi A1 this old often drains fast because of aging software and many apps, so read our drain page first. Then use the health check before buying a BN31.
Should BN31 swell, heat up, leak or dent, there is a fire risk. Take the Mi A1 off charge, leave the cell unpressed and away from warmth. Get the phone to a repair counter that very day. BN31 copies with false capacities are common, so buy only a pack printed BN31.
Fitting a new BN31 pack
On these phones the back is part of a metal body, and the cell is usually held down with adhesive. The panel must be warmed and lifted with care, and a slip can damage the antenna lines or the fingerprint sensor cable. Leave the Mi A1 to Xiaomi service or a reliable technician rather than a home attempt.
Give the technician the Mi A1 or Note 5A model number and ask for the new BN31 sticker to be compared with the old. When the new pack arrives, look for these signs:
- It weighs close to the BN31 you removed.
- The print is clean and a batch mark is present.
- The label does not claim a number far above 3100 mAh. No Note 5A sized cell can honestly hold that.
Our replacement guide covers fitting for every phone.
Ask for the removed pack to be shown to you. If it is swollen, you will know the swap was needed, and the sticker photo saves a second visit.
Keeping an older Xiaomi phone going
If your phone is still healthy, small habits help. Avoid charging the Mi A1 in a hot room or on a bed, and take off a thick cover while it charges. Heat ages BN31 faster than almost anything.
Use a charger and cable of normal quality, and replace a cable that gets warm at the plug end. Avoid leaving the Note 5A full for hours every night. Once a reboot and a full charge fail to stop the Mi A1 quitting early, suspect a tired BN31 and get it measured instead of guessing.
Frequently asked questions
Which phone uses the BN31 battery model name?
Most listings match BN31 to the Mi A1 and Redmi Note 5A, with a typical capacity of 3080 mAh. Several Redmi Y and S models and the Mi 5X are also listed, so read your own sticker.
Is BN31 the Redmi Y1 battery model?
Most listings say yes, but a few may differ. Once the back is off, have the technician read the code, and accept only a cell marked BN31.
How many mAh is the BN31 battery?
Listings give 3080 mAh as the typical value. The rated minimum on your label could read slightly under that.
Is the Mi A1 battery the same as the Mi A3 battery?
No. The Mi A2 and Mi A1 are different phones with different cells. A BN31 pack is for the older Mi A1 only, not for the Mi A3.
Can I use BN36 or BN37 in place of BN31?
No, treat them as different packs. The codes differ, and the shape or connector may not match. Read each code on its own page and on your sticker.
How can I tell a fake BN31 battery?
A fake BN31 tends to be light, smudged, missing a batch mark and claims far above 3080 mAh. Compare weight and BN31 code with the old cell.