Introduction
To avoid the dreaded Important Battery Message caused by using an aftermarket battery in your iPhone, you can instead transplant the battery's BMS, or Battery Management System, from the original battery to a replacement battery cell; i.e., a battery sold without the BMS attached.
This guide will show you how to disassemble the old battery and install the BMS onto a new battery cell, restoring the full capacity of the battery while avoiding the error message.
Note that the downside to this procedure is that the BMS carries the health information, so even though the battery will effectively be brand new, your phone will still show the original battery health information. On batteries prior to the iPhone 11, you could use a device programmer to reset the health percentage and cycle count, but Apple decided they didn't want people doing that any more, and started encrypting the information such that it can no longer be reprogrammed.
What you need
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After trimming any excess terminals from the BMS contacts, place the BMS on the battery cell.
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Fold the terminal strips over onto the contacts on the BMS.
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Spot weld the terminals to the BMS.
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Trim the excess terminal strips.
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Place the double-sided tape over the BMS.
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Peel off the protective paper.
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Stick on the plastic cover.
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Flip the battery over and place the sticker on the back as shown.
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Return to the front side and fold the top over the plastic cover.
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Fold up the sides.
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To reassemble your device, follow these instructions in reverse order.
