My understanding was that the 4 batteries are "unspecific", working in a (n,n<4) four fold symmetrically redundant arrangement ?
or is it as you write two sets of two fold redundancy ? ( theoretical best redudancy/states per bit/ whatnot is e ( 2.71...) being in the integer domain it is either 2 or 3 )
The two sides are basically mirrored down the centre of the aircraft, on the AC side each engine has two variable frequency 230V generators, these feed the 2 normal AC busses on each side. One of these AC busses is connected to the normal DC bus, the other to the emergency AC bus. The emergency AC bus powers the emergency DC bus. Each side has a battery connected to the normal DC bus and another battery connected to the emergency DC bus on that side, so 4 in total. Each side is redundant as AC busses on the same side can be connected, I.e. each engine driven generator can power its own side if the other generator on the same side failed (both the normal and emergency bus)
Then there are bus ties between the two sides, so the left side can be connected to the right (automatically if needed), ie the normal AC or DC can be connected to the normal AC or DC on the other side, and the emergency AC and DC busses can be connected to the other side.
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