romanlutsk, there is a slight problem with using any form of adapter from the 5v 0.5A USB socket to supply 12v at over 1A - it just will not work.
As for connecting two batteries on series you have the very real problem of burning out the charging circuit or, if that does not happen, neither of the batteries ever getting a full charge at the rate they are designed for.
One question, you talk of using another PE battery, where did you get it from? If you bought it as a new item pleas let us know where you got it from and the part number for future reference.
As a bit of information, I have and regularly use a 12v 2.5A limited output adapter that plugs into my car lighter socket to charge my EE.
tarvoke, you are talking about two different design ideas when comparing an APC ups to a PE. APC design to a strict specification that allows for upping capacity - we have a rack mounted 2kW unit that now uses a large truck battery and keeps it charged without problems - whereas the PE was designed to a price with a 'this will just do' specification - remember the WSoD caused by pushing the video subsystem too hard.