What is in the 2nd compartment on the back of some Alembics?  It appears too large to be a battery compartment.

the battery compartment for LEDs (4 9v) or for the bridge block on series instruments

4 9v?  I do not get it.  I though LED's consumed very little energy?  Is there an separate Battery for the op-amp?

More voltage does not equal more energy.

When a bass has LED's there is a set of batteries for the LED's and a separate battery for the preamp.
 
Keith

Are you asking about Series basses?  They are quite different than the other Alembics.  
 
The normal compartment has a lot of pots and connectors.  A Series I would have 4 pots, but there are also Q-switches, the 5-pin stereo output jack, and a very complicated 1/4 jack.  
 
On the other side of the body, there's a the preamp and EQ boards.  There's a tunnel between the compartments with a lot of wiring.  
 
There's also a third, smaller plate in the center block that holds a pair of 9V batteries that power the electronics when you're not using the 5-pin external power.  Sometimes it's round, sometimes it's square.
 
There was a period where Series bodies were quite thick, so the electronics were all on one side.  
 
And, as you probably already knew, the Series basses are semi-hollow contstruction - the neck/center block runs the entire length of the instrument and the top and back are 1/4 thick, but they bandsaw most of the center core out.  This is only really visible when the plates are off.
 
David Fung

My Rogue has three compartments. One for the electronics, one 9v compartment and one for the four 9V batteries for the LEDS. I don't know if they are in series or parallel. Will work for batteries!