Battery replacement, but the hard way
Back to the blog overview page | Post date: 2024-10-12Objective
My Honor 6 was in good condition. But the battery began to bulge. This became noticeable by the rear cover that slowly got detached from the rest of the phone chassis. If safety were not a concern to me, I could still have used it for a few months more.
Online guides on how to replace the battery did not seem to be too complicated [1]. So I ordered a new battery from AliExpress. The price was surprisingly low, and the exact phone model appeared in its description.
[1]: At 5:28, they forgot to remove a screw covered by a white "warranty void" sticker at the bottom left.
After replacing the battery, the phone should work just fine, right? Haha. No. This is the beginning of the blog post.
Initial bootup
I replaced the battery according to the tutorial and something started to smell fishy [in a figurative sense].
Whats fishy? I made sure that the battery is OK by measuring the voltage at its connector beforehand. According to this graph, the measured 3.88 V would indicate that the battery is charged to at least 50%. Booted up the phone with the old battery - it is apparently still charged to about 60%. Its voltage is also 3.88 V - a lucky coincidence.
The power draw of the USB charger was roughly 1.5 watts. The charging screen periodically appeared for a brief time, then disappeared for a few seconds. The image above had to be timed well to capture the screen.
Does the battery not have enough power yet to power the display? I disconnected the LVDS cable for the display (wide middle connector), which detached the LCD backlight as well. After another hour, the battery voltage was still at 3.88 V. It did not charge.
Deeper analysis
Pin mapping: (measured)
| Pin(s) | Description | Battery old | Battery new |
|---|---|---|---|
| 3/6 → 1/4 | Cell voltage | 3.88 V | 3.88 V |
| 2 → 1/4 | Temperature sense NTC | 10 kOhm (@ 23°C) | 10 kOhm (@ 23 °C) |
| 5 → 1/4 | "ID", battery type | 10 kOhm | 470 kOhm |
| 5 → 2 | Plausibility check [2] | 19.3 kOhm | 480 kOhm |
| A → B | Not connected | Not connected [3] |
- [2]: This ensures that the resistors are independent to each other. They are independent. The difference of resistance is due to thermal changes of the NTC.
- [3]: We will get to these two pins later.
I suspected that a resistance reading of the ID pin is the reason why the battery is not recognised correctly. Unfortunately there is not much information on that topic when searching for smartphone battery pinouts. Even helpful and less helpful comments only give sparse information. Nokia batteries apparently had a BSI (Battery Size Indicator) pin, consisting of a resistor.
I could order a new battery from another seller, but at this point I did not want to waste this battery and considered to repair it myself. If I can make the old battery to look, swim and quack like a duck by replacing the resistor it should work. Right?
Autopsy and repair
Any steps in this chapter are at risk to cause explosions and fire. I am not responsible for any damages caused by failed imitators.
I consulted a few videos to have an idea how smartphone batteries are assembled. Similarly, this and the old battery are equipped with a small PCB which is spot welded to the battery contacts and then packed tightly onto the battery.
So I carefully unfolded the PCB from the battery. Description of the numbered components:
- Unused mounting location(s) for a copper/zinc bridge for spot welding the battery contacts.
- P- pole, spot welded.
- Overcurrent and undervoltage protection circuit
- Temperature sense NTC
- ID resistor
- P+ pole, spot welded.
This seemed doable with the right equipment. Except for one problem: the risk of breaking things. The following options exist:
- Remove the spot welded contacts. Based on previous experience, they are somewhat robust and difficult to remove. They often cannot be separated perfectly. There might be issues reattaching the P+ pole.
- Desolder the bridges. This would heat up the battery, thus damage it very quickly due to the heat transfer through the spot welded contacts.
- Solder without detaching. In case of short-circuits with the P+ side during the operation, this might discharge the battery and cause fire.
- Give up and order another battery.
After examining the available equipment, I decided to for option 3. The image on the right shows how almost every sensible part is covered in temperature resistant Kapton (polyimide) tape. This reduces the risk of causing shorts during the operation. By heating the soldering iron to 380 °C, the protective goo around the resistor adhered less and the opposite pad was kept hot too. By this, I could to repeatedly heat and push the resistor from both sides and eventually remove it from the PCB.
This approx. 0603 sized resistor is now mounted upside down (white, rather than black top). The multimeter shows the new resistance of 10 kOhm at the battery connector nonetheless. Success!
The Aftermath
After connecting the battery and the charger, the battery charged successfully to 100% while drawing a peak of 5 watts from the power outlet. An external fan was used to ensure that the battery would not overheat. The smartphone then started up just fine. So I began to assemble it.
But wait! There's more. (a TF2 clip)
Screws are back in, but the rear cover not yet glued - the phone refuses to start. Connecting it to the charger shows the empty battery symbol. It does not charge. After unscrewing, remounting the battery it works. Screwed back in - stops working. What is going on?
.. remember the pinout table above? It's incomplete. In [3] I analysed A → B but not A/B to any other pin. Further investigation reveals the following:
| Pin(s) | Battery old | Battery new |
|---|---|---|
| A → 1/4 | Not connected | Not connected |
| A → 3/6 | Not connected | Not connected |
| B → 1/4 | Not connected | 4.3 V |
| B → 3/6 | Not connected | Connected |
The socket on the smartphone PCB does appear to connect A and B to GND, hence to P-. Based on various attempts to get the phone to work previously, at least contact B appears to be on the verge of being connected or disconnected depending on how the connector is mounted.
To verify this assumption, I bent the B pin slightly inwards and used a very small piece of electrical tape to insulate this pin from the socket on the PCB side. The phone started up on the first try. Mounted everything back together. It still works. This means I unknowingly short-circuited the battery a few times. Hereby I can say that the overcurrent protection circuit does work properly.
Final words
At the time of writing, the rear cover is yet not glued back in place. It will be done after it has proven itself to be stable enough.
This was an interesting adventure, and a rewarding one on top of it. Lessons learnt:
- Avoid reassembling a system that is yet not tested.
- Measure twice, cut once. Initially I accidentally replaced the wrong resistor (NTC, index 4) and spent too much time on soldering it back in place.
- Hardware advertised as compatible might not be compatible. Buying from official sources yields better chances to success.
Thank you for making it this far. ~ Have a nice day ~