From the Parallax website:
Q. I have read that I must leave my battery connected to my ferro-resonant converter. Why is this important?
A. The ferro-resonant type converter output is not a filtered DC output. Therefore, any DC load circuits that need a “pure” DC voltage will not operate properly unless the battery remains connected. Examples of some items that may require filtered DC would include the 12 VDC TV, radio, computer power ports, and some fluorescent lights. You may add our optional SB100 “Simulated Battery” to the system in the event you wish to take your battery “offline” for extended periods. One or more SB-100 units may be required depending on the rated converter amperage output.
The SB100 is nothing more than two, 50,000 mfd, 16-20 wvdc, electrolytic capacitors wired in parallel. I currently have a pile of old electrolytics wired in place of my battery, and it works fine - The filtering the capacitors provide makes the lights burn brighter as compared to running without anything in place of a battery.