Has this Li-Ion battery gone bad?

I have an external Li-Ion (flat, not cylindrical) battery for a cell device, rated at 10,000 mAh. It is an aftermarket battery made by ZeroLemon that triples that capacity of the stock 3,000 mAh battery. I got it used, the seller says it has 1 month of runtime on it.

I installed a neat app on the device that monitors the voltage in the status bar down to 2 decimal places. Now I am having a hard time charging this battery to 4.3V. That is, it will come off the charger at 4.3V and then very quickly, seemingly in the first 5 minutes drop to something like 4.18V. then in another 30 minutes to 1 hour quickly drop to 4.08V. With minimal usage. No heavy duty apps.


Then it will take 12 hours to drop to about 3.8V. And overnight while doing nothing it will hang there basically coming down to 3.75V. The default notification will hang at 1% for a long time, now we all know that "percent" is just a dumb gauge that doesn't mean much and geared towards ignorant users who don't deal with volts and mAh.

So why is the discharge curve not linear? Does that indicate it going bad? Or lost capacity? And the fact it doesn't stay at 4.3V or even 4.2V long. I have had several ZeroLemon Li-Ion batteries, they are capable of going for 2 days in the 10,000 mAh configuration. I don't remember the last one dropping so rapidly.
OTOH, it could be the phone. It went through the default 3,000 mAh cell very fast, in 3 hours which is not normal by any measure. I locked down every app, optimized for runtime. It could be the default 3000 mAh cell went bad as the phone, while NIB, is a 2.5 year old model. so the stock 3000 mah cell was likely sitting in storage somewhere.

I realize the battery needs to be calibrated for the phone but I trust the voltage reading it does in an infinite loop.

Possibly the external battery is on its last leg, i.e. is very unhealthy with high IR. This could be caused by much use (many cycles), or by little use (low cycles), but much calendar lifetime spent at high voltages (e.g. often fully charged as a backup/standby), or many other scenarios that cause degradation. Generally the less time the cell spends at extreme voltages and temperatures, the longer its life.

OTOH, possibly the gauges or tools you are using are confused by the setup. How does your ZeroLemon work? Does it charge the internal battery through the normal charging port, or does it replace the internal battery?

ZeroLemon replaces the internal battery of lesser capacity. Out it goes and in goes ZL. It doesn't work with 95% of devices on the market, just a select few. They provide a much larger back cover and the device triples in thickness.

To add to mystery, over the last 12 hours, it went from 3.8V to 3.6V. And it took only about 2 hours to go from 4.2V to 4.0V. So it's definitely a non-linear decline. The seller - I got it used - says it has been used for only a month. If it makes it until midnight, it will have made it 1.5 days, or 36 hours which is in line with what it should do.

Questions - now I know these are not possible to answer. But.

1. Is the voltage reporting accurate by the app? Now I am beginning to doubt it.

2. Why is the voltage decline non-linear, the farther it is in the cycle, the slower it discharges.

3. Perhaps there is some kind of an error somewhere with reporting because the device thinks it's a 3,000 mAh battery while in reality it's 10,000. The phone (incorrectly) has been showing 1% left for the last 10 hours. From what I hear, others report the same thing.

It just won't charge above 4.0V, so I wonder if I am getting a false reading with the voltage or it really does charge to 4.3V or maybe the battery is not good. Not sure.

May need to pull it fully charged and test it independently with a voltmeter.

What is the CPU in the cellphone?
I DIY a few external 18650 case wire parallel the stock battery.
Snapdragon work perfectly :thumbsup:
MTK not, it did not show correct voltage and percentage.

it is normal for the voltage to drop sharply off the charger then level off.
any battery % calculator is only valid for the original pack anyhow.
use the device as you normally would and see if the runtime matches the marked capacity.
ex your original is 1ah and runs device 1 day.
replacement is 3ah and could run device 3 days or a bit more due to peukert effect.

I've explained that I don't rely on the idiot gauge known as the % sign, I go strictly by voltage. I pulled the battery and confirmed it with the voltmeter. Yes, that voltage is real.

I have been running it for 26 hours and it's down to 3.82V.

It has really slowed down in the range between 4.0V - 3.8V. Which is amazing. The discharge is not proportional at all.