Battery, BMS and all the shenanigans

Hi Sam,

I've been doing my best to keep up with Apple's parts pairing, which is only getting worse as the newer generations come out. I'll pass along everything I know about what you need to do to successfully replace your battery.

First of all, the admonishment about spot welding is primarily due to the fact that heat is deadly to Li-Ion batteries. A spot welder will not transfer heat from the terminal tabs to the battery itself, whereas it's easy to transfer heat when soldering. However, that's not to say it can't be done; you'll just want to be as sparing as possible with the heat from the soldering iron to avoid transferring it up the terminal strips to the battery.

Serialized battery

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All iPhone batteries since the beginning have incorporated a Battery Management System, or BMS, into the battery. This small circuit communicates with the phone's processor and contains sensors to measure the battery current, voltage and temperature. It also stores the battery health (0 to 100 percent) and the battery's cycle count; the number of times it's been charged and discharged.

With the release of the iPhone XR and later models, the BMS is now paired to the logic board and will result in an Important Battery Message warning and the loss of battery health information if the battery is replaced. For the first 15 days or so a pop-up message will appear on the phone with the following message.

Important Battery Message

Unable to verify this iPhone has a

genuine Apple battery.

Under the Battery Health section of Settings, a similar message will appear permanently.

Important Battery Message

Unable to verify this iPhone has a genuine Apple

battery. Health information not available for this

battery.

The Maximum Capacity and Peak Performance Capability fields will be blank as well.

Note that the battery will still charge and operate normally. This will occur whether you use a genuine Apple battery or an aftermarket replacement.

The aftermarket industry has created a workaround to avoid that message and the loss of battery information. The procedure is to cut the BMS from the original battery and spot weld it onto a new cell. Manufacturers have started offering batteries with just the cell and not the BMS specifically for this purpose.

Once the BMS has been transferred to the new cell, the battery can be used but the old battery information will still be present. In order to reset it, a device programmer such as the JC V1SE or the QianLi iCopy can be hooked up to the battery and used to reset the battery health back to 100 and the cycle count to zero.

iPhone XS -12 Pro Max Important Battery Message Pop-ups Removing - 100% Fixed - iFixit Repair Guide

(4) Remove Non-Genuine Battery Warning / Important Battery Message On iPhone XS And Above - ALL About iPhone Repair - Quora

This next part is something I have been hearing about but I need more information regarding. Apparently the battery must be "activated" after swapping the BMS by applying approximately 4 volts to it.

It's my understanding that this should be all you need to do in order to have a fully functional aftermarket battery working in your phone. If you'd like to know more about the Tag-On flex device, read on, but it doesn't apply to your phone.

Battery health encrypted (Tag-on flex required)

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With the release of the iPhone 11, Apple changed the way the battery health information is stored, and made it so that it is no longer possible to reprogram battery health information; even with the BMS swapped over, the health data is now stored on the processor rather than the battery and can no longer be rewritten with a device programmer.

Aftermarket manufacturers have developed a device called a tag-on flex circuit that goes between the battery and the logic board and allows the health percentage and cycle counts to be reprogrammed. The tag-on board is permanently installed into the phone.

One UK site seems to be indicating that the tag-on flex alone can allow you to eliminate the service message without needing to swap the BMS.

iPhone 11 Tag-On Flex For QianLi Copy Power Battery Data Corrector — Repair Outlet

iPhone 11 battery replacement tag on flex issues : r/mobilerepair

XCAP - BMS Tag On Flex - No Programming - For iPhone 11 - ReplaceBase

In your particular case the device programmer would not be used to copy anything; rather it is only used to reset the battery health to 100 and the cycle count to zero. Since your new battery cell has the old BMS, if it's not programmed it will show the exact same information it did before you put the new cell on it. Resetting it starts it over the same as a new battery.

By the way, most of iFixit's guides don't go into detail about implications or consequences of replacing paired parts; I'm guessing they don't want to get into the weeds like this answer is when their purpose is to show you how to replace that particular hardware.