I think it unlikely to run below 8V, as there is probably a low battery cut off.
As I assume you know from the manual, 'High Voltage In' is:
8 - 32V for pins 14 & 15 (of15) of the 35xxAS
8 - 16V for pins 14 & 15 (of 20) for the 36xxAS
This also supports 'Low Voltage' of 2 - 3.2V on pins 1,2,3
They are insistent on connecting all the available power pins, not just one + and one -
8 - 32V for pins 14 & 15 (of 20) for the 45xxAS.
The battery control is on pages 55 & 56, 3600AS only for some reason best known to L3
The 2000AS was the odd one out, powered from 2.5 - 4.0, or 6 - 32V unless you added the 'Power Board' to accept 4 - 32V.
This was probably due to use in Raytheon's own products that ran on 2 x AA cells, and the same for the 3600AS in the X-1
http://www.americaninfrared.com/ProductDetail.asp?ID=43 regards
Bill