After replacing battery, and keeping the old BMS, sometimes it says 1%

Hi all DIY folks,

I'm having a odd problem when I'm replacing battery cells on iPhone 11. But first, let me explain to you how I succesfully multiple times have managed to avoid permanently installing the tag-on flex cable between the socket and the battery flex cable.

Maybe you can spot a thing I'm doing wrong here.

1. Take out old battery of iPhone 11
2. Install compatible battery with unoriginal BMS, in order to 'reset' BMS info in the iPhone.
3. Start up the iPhone, and let it recognize the unoriginal battery
4. Shut it down again, remove battery.
5. Remove BMS from original battery, spot weld a new battery cell to the BMS
6. Attach a tag-on flex cable, program the tag-on flex using JCV1SE or other programmer
7. Attach the battery with tag-on flex to iPhone, start it up.
8. Verify that it reads the percentage correctly (100%)
9. Shut down the iphone
10. Remove the Tag-on flex cable
11. Install the battery, start up the Phone.
12. The phone still says genuine battery and 100%

So this has been my preferred procedure when installing replacement batteries, also because I can save money on the relatively pricey tag-on flex.

However, once in a while, my customers return the phone to me saying that it only says 1% and cannot charge. Sometimes I need to do the procedure again. This last time, I chose to leave the tag-on flex inside the phone permanently.

How do you guys do it? Is it mandatory to keep the tag-on flex cable, or is there some other reason why it suddenly says 1%?

One of the culprits could also be that the welding was not succesful, leaving a flimsy connection, and because of the power outage to the phone, it would not be able to read the BMS correctly after regaining power?

Let me hear your ideas or comments.

Thanks,

Carsten