That sounds very normal. Although I didn't find anything about the battery sign in the manual, I guess it's fair to assume that it was designed to come up whenever the input voltage drops below a certain threshold. :-) And that's exactly what happens: when you start the engine, the starter motor draws a huge current (up to 100 A on my Diesel) which makes the battery voltage drop by some volts. I see the exactly same behaviour in my car as you in yours.
In most cars that I know of, though, the ACC line (to which the empeg is connected in most cases) is wired so that it goes to 0V when you start the engine (to relief the battery from the additional load of all the accessories), so that the empeg just goes off and comes back on again, without showing the battery sign. That's why most empeg users never see it, I guess. Our cars seem to be wired differently. Mine at least is - it's ignition switch doesn't have an ACC position...
Daniel
---
"My software never has bugs. It just develops random features."
_________________________
---
"I love deadlines. I love the WHOOSHing noise they make as they go by." - Douglas Adams