Xperia LT26 Replacment Battery
Firstly, any non-OEM replacement battery for most early Sony Android phones(those I have tested) do not report temperature.
This is not good for a phone that can often run at high temperatures and where apps/system rely on battery temperature for system controls.
Mugen Power has also confirmed to me that their Sony replacement batteries do not provide temperature to the system controls.
You can replace the battery in the Sony Xperia S, but you do need to take some care, that no ribbon cables are damaged in doing so.
The Xperia S is not water resident and doesn't have any 'seals' to contend with when gaining access to the battery.
I have tested a 'cheap' PISEN BA800 1700mAh battery in an Xperia S and it gave comparable performance to the original BA800 1700mAh battery. The overall long term life of the battery could be another story.
There are many replacement batteries for the Xperia S, but I'd stay will clear of any battery claiming 2000mAh and above.
They're cheap for a reason.
I have long speculated (but never tested) that the Mugen Power BA800 1800mAh battery for the Xperia V (LT25) would work in an Xperia S (LT26).
Please do your own research on this, before any purchase.
I would only recommend an original Sony BA800 replacement battery for the Xperia S (LT26).