Teardown: Swollen Xiaomi Redmi Note 2 BM45 Battery

While I was using my Xiaomi Redmi Note 2 the other day, I realized something strange was happening. It didn’t quite fit in its case as well as it used to, the screen felt a little like it was bowing, and the case itself had blistered a crack along its length.

More interestingly, there was a light leak from the edge of the screen between the phone body and the screen glass which wasn’t there before. Disassembling the phone revealed the culprit.

The battery had slowly swollen, and now no longer sits inside the battery bay correctly, instead being wedged between the rear shell and the body, exerting force onto the back of the screen.

Swelling batteries are nothing new – I’ve had the same experience with Samsung and my LG phone. To me, it seems that the quality of the lithium-ion/lithium-polymer cells that are used may be problematic, but also the way the phones treat the batteries. When fully charged, the charge termination voltages are now pushing 4.35-4.40v to squeeze a little more capacity out of the batteries, and the battery forms an integrated part of the power regulation mechanism resulting in “microcycling” of a cell that is fully charged, quite possibly wearing it out prematurely.

Swollen batteries are a potential safety hazard if they rupture. A high profile case recently is that of the Samsung Galaxy Note 7 where battery swelling (and insufficient clearance to account for it) appears to be a part of the reason why the units failed so spectacularly. In my case, it stopped my phone from really being usable without the risk of physical damage to the unit itself. As a result, I was forced to discontinue using the phone until a replacement battery came in.

The biggest disappointment was that this battery was only used for 11 months before it had failed, right before my upcoming trip, adding more things to worry about. The other cells often lived to 18 months or thereabouts before it failed, so this is a new record in terms of failure. A redeeming factor was that a “claimed” genuine replacement was only about AU$14, thus not breaking the bank. I’d rather that, than ever having to deal with GearBest (where I purchased the phone) ever again.

From the top, the swelling was much more obvious, resulting in the label peeling away from the plastic frame which still held its original shape.

The swelling was more extreme at the edge, and not as steep along the middle, but the gap between the straight edge and the cell confirms it.

In fact, a closer look at the cell shows there is a thin metal plate around the outside, which has lifted up. This suggests the cell is actually a Lithium Polymer cell with a thin metal shell for enhanced safety when handling the battery, required as the unit is user replaceable.

Teardown

Since I have no real reason to keep using the battery, I wanted to get rid of it. But I wasn’t going to just throw it out.

Once the label is off, the internal metal protection plate is visible, and in some ways, it’s constructed similarly to a CF card.

However, this metal plate is not just “loose” – it’s adhered to the li-poly cell pouch with double-sided adhesive. This is not good for disassembly, as being too rough and damaging the pouch can have catastrophic consequences. As a result, I decided to carefully tear the plates off, however, in doing so managed to stretch the pouch slightly so the battery looks more baggy rather than swollen.

The flat side reveals the cell is marked with PGF376279HT FAD97605 B6XFLJ5B1-22-195. The actual capacity is not printed on the pouch, and the actual manufacturer is not known. Maybe its lack of reputable branding is part of the reason it didn’t last as the other phone batteries I had previously used.

No further cell identifications were found on the other side.

The frame seems to be secured well to the cell using some form of potting compound so that it conforms to the shape of the li-poly cell within while sealing out moisture.

The protection board is a black PCB with white screen printing.

All the circuitry is on the opposite side, including a transistor, a polyfuse/fuse/resistor, a few capacitors/resistors, and a controller/MOSFET package under resin.

Conclusion

It’s a shame that just shy of a year of usage has seen my Xiaomi Redmi Note 2’s battery essentially fail. I did notice declining capacity, although that is usually not that unusual. The swelling made it mechanically impossible to continue using the battery without risking damage to the phone, thus I have torn it apart and disposed of it, while ordering a reasonably priced replacement. I suppose it’s a win for the fact this is still user replaceable.

However, I’d have to say that it seems likely the pushing of density and higher charge-termination voltages may have something to do with these failures. All in the name of a little more capacity, a larger number in their product specs, in the hope of drawing in more consumers without much consideration to longevity. Not my idea of sustainability.

About lui_gough

I’m a bit of a nut for electronics, computing, photography, radio, satellite and other technical hobbies. Click for more about me!

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