[Sony Ericsson XPERIA X10 Mini] Gx3:
The battery used for test is a BST-38 li-ion and should be 1200mah 2€ so I don't expect to be a great battery but this is only for reference.

tear down instructions

Please read all before starting this will help you choose the most convenient acting way.

Removal of original battery from its placing is a pain in the ass, cause it is very well attached to the aluminium cover whith a very sticky double-face tape ...so proceed with caution, after that I preferred to remove that tape: this permits to connect the battery "the rough way (img 10)" and then place the cover whithout sticking.

1

2

3

4

5

This battery was easy to disassemble by removing printed tape and making a small cut on the bottom

...but be aware this may be not so easy with others!!

6

7

8

soldered wires, this is a very complicated step for those who do not know how to use a solder very well, short circuit is round the corner, so first solder positive wire, then cover it with a good thermal tape then proceed with green (again cover it) and finally go for black. (however remember to solder wires in a sequence that avoids overlapping of wires while soldering: if the sequence was black>green>red it would have been easy to melt black wire while soldering green wire... and melt both green and black wire's cover while soldering red)

Note that wires are soldered to run on the opposite direction in comparison wiht the original battery wiring, the reason is to obtain a perfect fit into battery compartment and to obtain a better resistance to wire dragging.

9

Paper tape has been used to insulate battery's body.

Please note that this picture does not show the final battery placing, you should imagine the batteryrotated by 180° (with wires on the right of the current image and then running on the side of the battery up to the hole which exposes the connector).

So make sure the wires are long enough if you are wishing to use this setup

(otherwise simply solder wires to run the opposite way compared to img 8).

(warning: the whole body is the Negative pole of the battery)

10

Wires need to be cut perfectly to make sure they don't move,

green wire should be a little bit shorter than red and black shorter than green.

Wire's edges are stained, flattened with plain edge pliers to fit into connector (a hammer and a flat surface will do the job too) and cut to result fully inserted (this avoids unwanted wire scattering while plugging into connector). All this paying much attention to not short circuit wires (I suggest to ready wires one by one insulating with tape each edge, when done proceed by deinsulating and plugging one by one).

warning : positive (red wire) and middle battery contact (green wire) are both positive so prevent any hazard, notice that the whole battery's body is NEGATIVE so handle all wires paying much attention!!

yeah I know i did'nt use the original connector but this is not meant to be a permanent solution

and still have the original battery usable

Before closing the hole with its rubber cover I placed some folded paper on the wires shown in image 10 (just to make sure the cables don't move).

Thanks to TheCuteCriminal for making public that Bst-38 batteries can be modded to fit