I never got a flat line when charging. I would rather blame charging instability due to bme been kicked out from i2c by patched bq - one simple question to be sure, do you use kp47? If yes, you should blacklist troublesome module by:

echo "blacklist bq27x00_battery" >>/etc/modprobe.d/blacklist

patched version of this module found in kp47 is 1005 confirmed to NOT work properly, due to its tendency to exclusively occupying i2c, causing charging instabilities/ wrong charging calibration etc.

If that is the case,m blacklist will fix it (battery may need to be charged few time to get proper calibration), then just go to kernel-power v 47 thread and buggy pali about removing new bq, cause he seems to deny accepting facts about it being broken (module, not Pali).

In other case, i would check if cable/charger isn't damaged

No battery should perform like that, be it fake or not - even really "disturbed"* ones don't drop from 50% to 0%

*By disturbed, i got a side-story to tell - before arrival of my scuds, i was disassembling some old samsung li-ion batteries to use them as replacement for my Son's toys (instead of using 3x coin-size batteries, that provide 3V-4,5V, cost much and last short - i modify every toy with putting old phone battery inside, and charging port somewhere). Unfortunately, i screwed taking apart protection PCB part - accidentally crossed battery + and - for a quite long time, until "smoke" started to fly from battery, and i was able to smell lithium. I was my first attempt @ disassembling Li-Ion battery ever, and it's design was far from BL-5J ones described with photo-details by fr_frost, so i thought that just teared part of it's metal case (pcb was inside plastic, that was glued extremely hard to battery itself). Because battery was REALLY hot, i just dropped it into nearby pot with crushed ice (to cool it down - prepared that before starting disassembling, in case oh temperature problems), and when was sure that no lithium is leaking, sealed the pot. I was sure that battery was totally screwed at this point, and I was really tired (about 3 AM o'clock), so went to sleep. Next day morning, battery was swimming happily in pot of water. Further examination revealed that metal case was NOT damaged - just safety thing released Lithium, when internal pressure started getting high (i think that implementing this pressure-safety things made Li-Ion batteries less likely to explode, hitting everything around with teared sharp metal parts). So, literally battery just lost some of it capacity (and, ended up being about 1/4 thicker than before, cause still some expanded lithium was inside, just with too low pressure to open pressure-safety "system").

After all, I use this battery as 3x coin batteries replacement happily for quite long time, and its performing perfectly. Internal resistance didn't increased, turned out to be water-proof (at least during night time storage in pot of water ), temperature and voltages as normal...

Conclusion - I can't imagine how someone was able to "screw" battery during modding process (without literally cutting battery into 2 pieces, injuring himself and nearby people by expanding Lithium explosion), or how Li-Ion battery can be so damaged that it's capacity drop from 50% to 0%.

Like dr_frost I'm waiting for HtheB story - maybe it will warn other people from repeating same mistakes? Or We'll be able to help/advice something?