Battery losing charge while off

[Yioryos Asprobounitis]

One of my XO batteries (~2years old) is loosing about 12% of its charge in 24h period while the XO is off.
This is not related to a specific XO but rather the battery itself.
I did run batman.fth bat-recover just in case but made no difference.
Also tried to clean the contacts of any possible dust and humidity with no result either.
I did not see any relevant info on the wiki about this specific issue and I was wondering if this 'can happen' and if there is anything that can be done about it, other than replacing the battery.
Thx.