I had a new (1 1/2 y.o.) pair of Mi True Wireless 2 earbuds with the right earbud having a very poor battery. I also had the left earbud of another old pair (somehow I lost the right piece) with good battery. So, what I did is to take off the battery of the good left piece and put it on right new piece.
It was quite easy to open the earbud but I had to be very careful with two cable strips and the tiny wires of the speaker.
I hope you'll enjoy.