“If we use the screens for an hour or so on the ground with no ground power then the starter motor is very slow, so they do drain quite a bit ( we also need the EIS and Garmin on for the whole system to work). In addition they reset when the starter is engaged. Not sure this is a problem, but thought I'd mention it.”
The reset issue is common on homebuilts. If you put in a backup battery, you do the set up on the backup and then switch in main power after engine start. This prevents all the problems mentioned and the backup will be recharged in flight,
Rod1